- Assertion Testing
- Async Hooks
- Buffer
- C++ Addons
- C/C++ Addons - N-API
- Child Processes
- Cluster
- Command Line Options
- Console
- Crypto
- Debugger
- Deprecated APIs
- DNS
- Domain
- Errors
- Events
- File System
- Globals
- HTTP
- HTTPS
- Inspector
- Internationalization
- Modules
- Net
- OS
- Path
- Process
- Punycode
- Query Strings
- Readline
- REPL
- Stream
- String Decoder
- Timers
- TLS/SSL
- Tracing
- TTY
- UDP/Datagram
- URL
- Utilities
- V8
- VM
- ZLIB
Node.js v8.2.1-nightly2017080132b30d519e Documentation
Table of Contents
- About this Documentation
- Usage
- Assert
- assert(value[, message])
- assert.deepEqual(actual, expected[, message])
- assert.deepStrictEqual(actual, expected[, message])
- assert.doesNotThrow(block[, error][, message])
- assert.equal(actual, expected[, message])
- assert.fail(message)
- assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])
- assert.ifError(value)
- assert.notDeepEqual(actual, expected[, message])
- assert.notDeepStrictEqual(actual, expected[, message])
- assert.notEqual(actual, expected[, message])
- assert.notStrictEqual(actual, expected[, message])
- assert.ok(value[, message])
- assert.strictEqual(actual, expected[, message])
- assert.throws(block[, error][, message])
- Caveats
- Async Hooks
- Buffer
Buffer.from()
,Buffer.alloc()
, andBuffer.allocUnsafe()
- Buffers and Character Encodings
- Buffers and TypedArray
- Buffers and ES6 iteration
- Class: Buffer
- new Buffer(array)
- new Buffer(arrayBuffer[, byteOffset [, length]])
- new Buffer(buffer)
- new Buffer(size)
- new Buffer(string[, encoding])
- Class Method: Buffer.alloc(size[, fill[, encoding]])
- Class Method: Buffer.allocUnsafe(size)
- Class Method: Buffer.allocUnsafeSlow(size)
- Class Method: Buffer.byteLength(string[, encoding])
- Class Method: Buffer.compare(buf1, buf2)
- Class Method: Buffer.concat(list[, totalLength])
- Class Method: Buffer.from(array)
- Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])
- Class Method: Buffer.from(buffer)
- Class Method: Buffer.from(string[, encoding])
- Class Method: Buffer.from(object[, offsetOrEncoding[, length]])
- Class Method: Buffer.isBuffer(obj)
- Class Method: Buffer.isEncoding(encoding)
- Class Property: Buffer.poolSize
- buf[index]
- buf.buffer
- buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])
- buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])
- buf.entries()
- buf.equals(otherBuffer)
- buf.fill(value[, offset[, end]][, encoding])
- buf.includes(value[, byteOffset][, encoding])
- buf.indexOf(value[, byteOffset][, encoding])
- buf.keys()
- buf.lastIndexOf(value[, byteOffset][, encoding])
- buf.length
- buf.parent
- buf.readDoubleBE(offset[, noAssert])
- buf.readDoubleLE(offset[, noAssert])
- buf.readFloatBE(offset[, noAssert])
- buf.readFloatLE(offset[, noAssert])
- buf.readInt8(offset[, noAssert])
- buf.readInt16BE(offset[, noAssert])
- buf.readInt16LE(offset[, noAssert])
- buf.readInt32BE(offset[, noAssert])
- buf.readInt32LE(offset[, noAssert])
- buf.readIntBE(offset, byteLength[, noAssert])
- buf.readIntLE(offset, byteLength[, noAssert])
- buf.readUInt8(offset[, noAssert])
- buf.readUInt16BE(offset[, noAssert])
- buf.readUInt16LE(offset[, noAssert])
- buf.readUInt32BE(offset[, noAssert])
- buf.readUInt32LE(offset[, noAssert])
- buf.readUIntBE(offset, byteLength[, noAssert])
- buf.readUIntLE(offset, byteLength[, noAssert])
- buf.slice([start[, end]])
- buf.swap16()
- buf.swap32()
- buf.swap64()
- buf.toJSON()
- buf.toString([encoding[, start[, end]]])
- buf.values()
- buf.write(string[, offset[, length]][, encoding])
- buf.writeDoubleBE(value, offset[, noAssert])
- buf.writeDoubleLE(value, offset[, noAssert])
- buf.writeFloatBE(value, offset[, noAssert])
- buf.writeFloatLE(value, offset[, noAssert])
- buf.writeInt8(value, offset[, noAssert])
- buf.writeInt16BE(value, offset[, noAssert])
- buf.writeInt16LE(value, offset[, noAssert])
- buf.writeInt32BE(value, offset[, noAssert])
- buf.writeInt32LE(value, offset[, noAssert])
- buf.writeIntBE(value, offset, byteLength[, noAssert])
- buf.writeIntLE(value, offset, byteLength[, noAssert])
- buf.writeUInt8(value, offset[, noAssert])
- buf.writeUInt16BE(value, offset[, noAssert])
- buf.writeUInt16LE(value, offset[, noAssert])
- buf.writeUInt32BE(value, offset[, noAssert])
- buf.writeUInt32LE(value, offset[, noAssert])
- buf.writeUIntBE(value, offset, byteLength[, noAssert])
- buf.writeUIntLE(value, offset, byteLength[, noAssert])
- buffer.INSPECT_MAX_BYTES
- buffer.kMaxLength
- buffer.transcode(source, fromEnc, toEnc)
- Class: SlowBuffer
- Buffer Constants
- C++ Addons
- N-API
- Basic N-API Data Types
- Error Handling
- Object Lifetime management
- Module registration
- Working with JavaScript Values
- Enum types
- Object Creation Functions
- Functions to convert from C types to N-API
- Functions to convert from N-API to C types
- napi_get_array_length
- napi_get_arraybuffer_info
- napi_get_buffer_info
- napi_get_prototype
- napi_get_typedarray_info
- napi_get_dataview_info
- napi_get_value_bool
- napi_get_value_double
- napi_get_value_external
- napi_get_value_int32
- napi_get_value_int64
- napi_get_value_string_utf8
- napi_get_value_string_utf16
- napi_get_value_uint32
- Functions to get global instances
- Working with JavaScript Values - Abstract Operations
- Working with JavaScript Properties
- Working with JavaScript Functions
- Object Wrap
- Asynchronous Operations
- Version Management
- Child Process
- Asynchronous Process Creation
- Synchronous Process Creation
- Class: ChildProcess
maxBuffer
and Unicode- Shell Requirements
- Default Windows Shell
- Cluster
- How It Works
- Class: Worker
- Event: 'disconnect'
- Event: 'error'
- Event: 'exit'
- Event: 'listening'
- Event: 'message'
- Event: 'online'
- worker.disconnect()
- worker.exitedAfterDisconnect
- worker.id
- worker.isConnected()
- worker.isDead()
- worker.kill([signal='SIGTERM'])
- worker.process
- worker.send(message[, sendHandle][, callback])
- worker.suicide
- Event: 'disconnect'
- Event: 'exit'
- Event: 'fork'
- Event: 'listening'
- Event: 'message'
- Event: 'online'
- Event: 'setup'
- cluster.disconnect([callback])
- cluster.fork([env])
- cluster.isMaster
- cluster.isWorker
- cluster.schedulingPolicy
- cluster.settings
- cluster.setupMaster([settings])
- cluster.worker
- cluster.workers
- Command Line Options
- Synopsis
- Options
-v
,--version
-h
,--help
-e
,--eval "script"
-p
,--print "script"
-c
,--check
-i
,--interactive
-r
,--require module
--inspect[=[host:]port]
--inspect-brk[=[host:]port]
--inspect-port=[host:]port
--no-deprecation
--trace-deprecation
--throw-deprecation
--pending-deprecation
--no-warnings
--napi-modules
--abort-on-uncaught-exception
--trace-warnings
--redirect-warnings=file
--trace-sync-io
--trace-events-enabled
--trace-event-categories
--zero-fill-buffers
--preserve-symlinks
--track-heap-objects
--prof-process
--v8-options
--tls-cipher-list=list
--enable-fips
--force-fips
--openssl-config=file
--use-openssl-ca
,--use-bundled-ca
--icu-data-dir=file
-
--
- Environment Variables
NODE_DEBUG=module[,…]
NODE_PATH=path[:…]
NODE_DISABLE_COLORS=1
NODE_ICU_DATA=file
NODE_NO_WARNINGS=1
NODE_OPTIONS=options...
NODE_PENDING_DEPRECATION=1
NODE_PRESERVE_SYMLINKS=1
NODE_REPL_HISTORY=file
NODE_EXTRA_CA_CERTS=file
OPENSSL_CONF=file
SSL_CERT_DIR=dir
SSL_CERT_FILE=file
NODE_REDIRECT_WARNINGS=file
- Console
- Class: Console
- new Console(stdout[, stderr])
- console.assert(value[, message][, ...args])
- console.dir(obj[, options])
- console.error([data][, ...args])
- console.info([data][, ...args])
- console.log([data][, ...args])
- console.time(label)
- console.timeEnd(label)
- console.trace([message][, ...args])
- console.warn([data][, ...args])
- Class: Console
- Crypto
- Determining if crypto support is unavailable
- Class: Certificate
- Class: Cipher
- Class: Decipher
- Class: DiffieHellman
- diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
- diffieHellman.generateKeys([encoding])
- diffieHellman.getGenerator([encoding])
- diffieHellman.getPrime([encoding])
- diffieHellman.getPrivateKey([encoding])
- diffieHellman.getPublicKey([encoding])
- diffieHellman.setPrivateKey(privateKey[, encoding])
- diffieHellman.setPublicKey(publicKey[, encoding])
- diffieHellman.verifyError
- Class: ECDH
- Class: Hash
- Class: Hmac
- Class: Sign
- Class: Verify
crypto
module methods and properties- crypto.constants
- crypto.DEFAULT_ENCODING
- crypto.fips
- crypto.createCipher(algorithm, password)
- crypto.createCipheriv(algorithm, key, iv)
- crypto.createCredentials(details)
- crypto.createDecipher(algorithm, password)
- crypto.createDecipheriv(algorithm, key, iv)
- crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])
- crypto.createDiffieHellman(primeLength[, generator])
- crypto.createECDH(curveName)
- crypto.createHash(algorithm)
- crypto.createHmac(algorithm, key)
- crypto.createSign(algorithm)
- crypto.createVerify(algorithm)
- crypto.getCiphers()
- crypto.getCurves()
- crypto.getDiffieHellman(groupName)
- crypto.getHashes()
- crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)
- crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)
- crypto.privateDecrypt(privateKey, buffer)
- crypto.privateEncrypt(privateKey, buffer)
- crypto.publicDecrypt(publicKey, buffer)
- crypto.publicEncrypt(publicKey, buffer)
- crypto.randomBytes(size[, callback])
- crypto.randomFillSync(buffer[, offset][, size])
- crypto.randomFill(buffer[, offset][, size], callback)
- crypto.setEngine(engine[, flags])
- crypto.timingSafeEqual(a, b)
- Notes
- Crypto Constants
- Debugger
- Deprecated APIs
- Un-deprecation
- List of Deprecated APIs
- DEP0001: http.OutgoingMessage.prototype.flush
- DEP0002: require('_linklist')
- DEP0003: _writableState.buffer
- DEP0004: CryptoStream.prototype.readyState
- DEP0005: Buffer() constructor
- DEP0006: child_process options.customFds
- DEP0007: cluster worker.suicide
- DEP0008: require('constants')
- DEP0009: crypto.pbkdf2 without digest
- DEP0010: crypto.createCredentials
- DEP0011: crypto.Credentials
- DEP0012: Domain.dispose
- DEP0013: fs async function without callback
- DEP0014: fs.read legacy String interface
- DEP0015: fs.readSync legacy String interface
- DEP0016: GLOBAL/root
- DEP0017: Intl.v8BreakIterator
- DEP0018: Unhandled promise rejections
- DEP0019: require('.') resolved outside directory
- DEP0020: Server.connections
- DEP0021: Server.listenFD
- DEP0022: os.tmpDir()
- DEP0023: os.getNetworkInterfaces()
- DEP0024: REPLServer.prototype.convertToContext()
- DEP0025: require('sys')
- DEP0026: util.print()
- DEP0027: util.puts()
- DEP0028: util.debug()
- DEP0029: util.error()
- DEP0030: SlowBuffer
- DEP0031: ecdh.setPublicKey()
- DEP0032: domain module
- DEP0033: EventEmitter.listenerCount()
- DEP0034: fs.exists(path, callback)
- DEP0035: fs.lchmod(path, mode, callback)
- DEP0036: fs.lchmodSync(path, mode)
- DEP0037: fs.lchown(path, uid, gid, callback)
- DEP0038: fs.lchownSync(path, uid, gid)
- DEP0039: require.extensions
- DEP0040: punycode module
- DEP0041: NODE_REPL_HISTORY_FILE environment variable
- DEP0042: tls.CryptoStream
- DEP0043: tls.SecurePair
- DEP0044: util.isArray()
- DEP0045: util.isBoolean()
- DEP0046: util.isBuffer()
- DEP0047: util.isDate()
- DEP0048: util.isError()
- DEP0049: util.isFunction()
- DEP0050: util.isNull()
- DEP0051: util.isNullOrUndefined()
- DEP0052: util.isNumber()
- DEP0053 util.isObject()
- DEP0054: util.isPrimitive()
- DEP0055: util.isRegExp()
- DEP0056: util.isString()
- DEP0057: util.isSymbol()
- DEP0058: util.isUndefined()
- DEP0059: util.log()
- DEP0060: util._extend()
- DEP0061: fs.SyncWriteStream
- DEP0062: node --debug
- DEP0063: ServerResponse.prototype.writeHeader()
- DEP0064: tls.createSecurePair()
- DEP0065: repl.REPL_MODE_MAGIC and NODE_REPL_MODE=magic
- DEP0066: outgoingMessage._headers, outgoingMessage._headerNames
- DEP0067: OutgoingMessage.prototype._renderHeaders
- DEP0068: node debug
- DEP0069: vm.runInDebugContext(string)
- DEP0070: async_hooks.currentId()
- DEP0071: async_hooks.triggerId()
- DEP0072: async_hooks.AsyncResource.triggerId()
- DNS
- Class dns.Resolver
- dns.getServers()
- dns.lookup(hostname[, options], callback)
- dns.lookupService(address, port, callback)
- dns.resolve(hostname[, rrtype], callback)
- dns.resolve4(hostname[, options], callback)
- dns.resolve6(hostname[, options], callback)
- dns.resolveCname(hostname, callback)
- dns.resolveMx(hostname, callback)
- dns.resolveNaptr(hostname, callback)
- dns.resolveNs(hostname, callback)
- dns.resolvePtr(hostname, callback)
- dns.resolveSoa(hostname, callback)
- dns.resolveSrv(hostname, callback)
- dns.resolveTxt(hostname, callback)
- dns.resolveAny(hostname, callback)
- dns.reverse(ip, callback)
- dns.setServers(servers)
- Error codes
- Implementation considerations
- Domain
- Errors
- Error Propagation and Interception
- Class: Error
- Class: RangeError
- Class: ReferenceError
- Class: SyntaxError
- Class: TypeError
- Exceptions vs. Errors
- System Errors
- Node.js Error Codes
- ERR_ARG_NOT_ITERABLE
- ERR_FALSY_VALUE_REJECTION
- ERR_INVALID_ARG_TYPE
- ERR_INVALID_CALLBACK
- ERR_INVALID_FILE_URL_HOST
- ERR_INVALID_FILE_URL_PATH
- ERR_INVALID_HANDLE_TYPE
- ERR_INVALID_OPT_VALUE
- ERR_INVALID_SYNC_FORK_INPUT
- ERR_INVALID_THIS
- ERR_INVALID_TUPLE
- ERR_INVALID_URL
- ERR_INVALID_URL_SCHEME
- ERR_IPC_CHANNEL_CLOSED
- ERR_IPC_DISCONNECTED
- ERR_IPC_ONE_PIPE
- ERR_IPC_SYNC_FORK
- ERR_MISSING_ARGS
- ERR_SOCKET_ALREADY_BOUND
- ERR_SOCKET_BAD_PORT
- ERR_SOCKET_BAD_TYPE
- ERR_SOCKET_CANNOT_SEND
- ERR_SOCKET_DGRAM_NOT_RUNNING
- ERR_STDERR_CLOSE
- ERR_STDOUT_CLOSE
- ERR_UNKNOWN_BUILTIN_MODULE
- ERR_UNKNOWN_SIGNAL
- ERR_UNKNOWN_STDIN_TYPE
- ERR_UNKNOWN_STREAM_TYPE
- Events
- Passing arguments and
this
to listeners - Asynchronous vs. Synchronous
- Handling events only once
- Error events
- Class: EventEmitter
- Event: 'newListener'
- Event: 'removeListener'
- EventEmitter.listenerCount(emitter, eventName)
- EventEmitter.defaultMaxListeners
- emitter.addListener(eventName, listener)
- emitter.emit(eventName[, ...args])
- emitter.eventNames()
- emitter.getMaxListeners()
- emitter.listenerCount(eventName)
- emitter.listeners(eventName)
- emitter.on(eventName, listener)
- emitter.once(eventName, listener)
- emitter.prependListener(eventName, listener)
- emitter.prependOnceListener(eventName, listener)
- emitter.removeAllListeners([eventName])
- emitter.removeListener(eventName, listener)
- emitter.setMaxListeners(n)
- Passing arguments and
- File System
- WHATWG URL object support
- Buffer API
- Class: fs.FSWatcher
- Class: fs.ReadStream
- Class: fs.Stats
- Class: fs.WriteStream
- fs.access(path[, mode], callback)
- fs.accessSync(path[, mode])
- fs.appendFile(file, data[, options], callback)
- fs.appendFileSync(file, data[, options])
- fs.chmod(path, mode, callback)
- fs.chmodSync(path, mode)
- fs.chown(path, uid, gid, callback)
- fs.chownSync(path, uid, gid)
- fs.close(fd, callback)
- fs.closeSync(fd)
- fs.constants
- fs.createReadStream(path[, options])
- fs.createWriteStream(path[, options])
- fs.exists(path, callback)
- fs.existsSync(path)
- fs.fchmod(fd, mode, callback)
- fs.fchmodSync(fd, mode)
- fs.fchown(fd, uid, gid, callback)
- fs.fchownSync(fd, uid, gid)
- fs.fdatasync(fd, callback)
- fs.fdatasyncSync(fd)
- fs.fstat(fd, callback)
- fs.fstatSync(fd)
- fs.fsync(fd, callback)
- fs.fsyncSync(fd)
- fs.ftruncate(fd[, len], callback)
- fs.ftruncateSync(fd[, len])
- fs.futimes(fd, atime, mtime, callback)
- fs.futimesSync(fd, atime, mtime)
- fs.lchmod(path, mode, callback)
- fs.lchmodSync(path, mode)
- fs.lchown(path, uid, gid, callback)
- fs.lchownSync(path, uid, gid)
- fs.link(existingPath, newPath, callback)
- fs.linkSync(existingPath, newPath)
- fs.lstat(path, callback)
- fs.lstatSync(path)
- fs.mkdir(path[, mode], callback)
- fs.mkdirSync(path[, mode])
- fs.mkdtemp(prefix[, options], callback)
- fs.mkdtempSync(prefix[, options])
- fs.open(path, flags[, mode], callback)
- fs.openSync(path, flags[, mode])
- fs.read(fd, buffer, offset, length, position, callback)
- fs.readdir(path[, options], callback)
- fs.readdirSync(path[, options])
- fs.readFile(path[, options], callback)
- fs.readFileSync(path[, options])
- fs.readlink(path[, options], callback)
- fs.readlinkSync(path[, options])
- fs.readSync(fd, buffer, offset, length, position)
- fs.realpath(path[, options], callback)
- fs.realpathSync(path[, options])
- fs.rename(oldPath, newPath, callback)
- fs.renameSync(oldPath, newPath)
- fs.rmdir(path, callback)
- fs.rmdirSync(path)
- fs.stat(path, callback)
- fs.statSync(path)
- fs.symlink(target, path[, type], callback)
- fs.symlinkSync(target, path[, type])
- fs.truncate(path[, len], callback)
- fs.truncateSync(path[, len])
- fs.unlink(path, callback)
- fs.unlinkSync(path)
- fs.unwatchFile(filename[, listener])
- fs.utimes(path, atime, mtime, callback)
- fs.utimesSync(path, atime, mtime)
- fs.watch(filename[, options][, listener])
- fs.watchFile(filename[, options], listener)
- fs.write(fd, buffer[, offset[, length[, position]]], callback)
- fs.write(fd, string[, position[, encoding]], callback)
- fs.writeFile(file, data[, options], callback)
- fs.writeFileSync(file, data[, options])
- fs.writeSync(fd, buffer[, offset[, length[, position]]])
- fs.writeSync(fd, string[, position[, encoding]])
- FS Constants
- Global Objects
- HTTP
- Class: http.Agent
- Class: http.ClientRequest
- Event: 'abort'
- Event: 'aborted'
- Event: 'connect'
- Event: 'continue'
- Event: 'response'
- Event: 'socket'
- Event: 'upgrade'
- request.abort()
- request.aborted
- request.connection
- request.end([data[, encoding]][, callback])
- request.flushHeaders()
- request.setNoDelay([noDelay])
- request.setSocketKeepAlive([enable][, initialDelay])
- request.setTimeout(timeout[, callback])
- request.socket
- request.write(chunk[, encoding][, callback])
- Class: http.Server
- Event: 'checkContinue'
- Event: 'checkExpectation'
- Event: 'clientError'
- Event: 'close'
- Event: 'connect'
- Event: 'connection'
- Event: 'request'
- Event: 'upgrade'
- server.close([callback])
- server.listen(handle[, callback])
- server.listen(path[, callback])
- server.listen([port][, hostname][, backlog][, callback])
- server.listening
- server.maxHeadersCount
- server.setTimeout([msecs][, callback])
- server.timeout
- server.keepAliveTimeout
- Class: http.ServerResponse
- Event: 'close'
- Event: 'finish'
- response.addTrailers(headers)
- response.connection
- response.end([data][, encoding][, callback])
- response.finished
- response.getHeader(name)
- response.getHeaderNames()
- response.getHeaders()
- response.hasHeader(name)
- response.headersSent
- response.removeHeader(name)
- response.sendDate
- response.setHeader(name, value)
- response.setTimeout(msecs[, callback])
- response.socket
- response.statusCode
- response.statusMessage
- response.write(chunk[, encoding][, callback])
- response.writeContinue()
- response.writeHead(statusCode[, statusMessage][, headers])
- Class: http.IncomingMessage
- http.METHODS
- http.STATUS_CODES
- http.createServer([requestListener])
- http.get(options[, callback])
- http.globalAgent
- http.request(options[, callback])
- HTTPS
- Inspector
- Internationalization Support
- Modules
- Net
- IPC Support
- Class: net.Server
- new net.Server([options][, connectionListener])
- Class: net.Socket
- new net.Socket([options])
- Event: 'close'
- Event: 'connect'
- Event: 'data'
- Event: 'drain'
- Event: 'end'
- Event: 'error'
- Event: 'lookup'
- Event: 'timeout'
- socket.address()
- socket.bufferSize
- socket.bytesRead
- socket.bytesWritten
- socket.connect()
- socket.connecting
- socket.destroy([exception])
- socket.destroyed
- socket.end([data][, encoding])
- socket.localAddress
- socket.localPort
- socket.pause()
- socket.ref()
- socket.remoteAddress
- socket.remoteFamily
- socket.remotePort
- socket.resume()
- socket.setEncoding([encoding])
- socket.setKeepAlive([enable][, initialDelay])
- socket.setNoDelay([noDelay])
- socket.setTimeout(timeout[, callback])
- socket.unref()
- socket.write(data[, encoding][, callback])
- net.connect()
- net.createConnection()
- net.createServer([options][, connectionListener])
- net.isIP(input)
- net.isIPv4(input)
- net.isIPv6(input)
- OS
- Path
- Process
- Process Events
- process.abort()
- process.arch
- process.argv
- process.argv0
- process.channel
- process.chdir(directory)
- process.config
- process.connected
- process.cpuUsage([previousValue])
- process.cwd()
- process.disconnect()
- process.emitWarning(warning[, options])
- process.emitWarning(warning[, type[, code]][, ctor])
- process.env
- process.execArgv
- process.execPath
- process.exit([code])
- process.exitCode
- process.getegid()
- process.geteuid()
- process.getgid()
- process.getgroups()
- process.getuid()
- process.hrtime([time])
- process.initgroups(user, extra_group)
- process.kill(pid[, signal])
- process.mainModule
- process.memoryUsage()
- process.nextTick(callback[, ...args])
- process.pid
- process.platform
- process.release
- process.send(message[, sendHandle[, options]][, callback])
- process.setegid(id)
- process.seteuid(id)
- process.setgid(id)
- process.setgroups(groups)
- process.setuid(id)
- process.stderr
- process.stdin
- process.stdout
- process.title
- process.umask([mask])
- process.uptime()
- process.version
- process.versions
- Exit Codes
- Punycode
- Query String
- Readline
- REPL
- Stream
- Organization of this Document
- Types of Streams
- API for Stream Consumers
- Writable Streams
- Readable Streams
- Two Modes
- Three States
- Choose One
- Class: stream.Readable
- Event: 'close'
- Event: 'data'
- Event: 'end'
- Event: 'error'
- Event: 'readable'
- readable.isPaused()
- readable.pause()
- readable.pipe(destination[, options])
- readable.read([size])
- readable.resume()
- readable.setEncoding(encoding)
- readable.unpipe([destination])
- readable.unshift(chunk)
- readable.wrap(stream)
- readable.destroy([error])
- Duplex and Transform Streams
- API for Stream Implementers
- Additional Notes
- String Decoder
- Timers
- TLS (SSL)
- TLS/SSL Concepts
- Modifying the Default TLS Cipher suite
- Class: tls.Server
- Event: 'newSession'
- Event: 'OCSPRequest'
- Event: 'resumeSession'
- Event: 'secureConnection'
- Event: 'tlsClientError'
- server.addContext(hostname, context)
- server.address()
- server.close([callback])
- server.connections
- server.getTicketKeys()
- server.listen(port[, hostname][, callback])
- server.setTicketKeys(keys)
- Class: tls.TLSSocket
- new tls.TLSSocket(socket[, options])
- Event: 'OCSPResponse'
- Event: 'secureConnect'
- tlsSocket.address()
- tlsSocket.authorizationError
- tlsSocket.authorized
- tlsSocket.encrypted
- tlsSocket.getCipher()
- tlsSocket.getEphemeralKeyInfo()
- tlsSocket.getPeerCertificate([detailed])
- tlsSocket.getProtocol()
- tlsSocket.getSession()
- tlsSocket.getTLSTicket()
- tlsSocket.localAddress
- tlsSocket.localPort
- tlsSocket.remoteAddress
- tlsSocket.remoteFamily
- tlsSocket.remotePort
- tlsSocket.renegotiate(options, callback)
- tlsSocket.setMaxSendFragment(size)
- tls.connect(options[, callback])
- tls.connect(path[, options][, callback])
- tls.connect(port[, host][, options][, callback])
- tls.createSecureContext(options)
- tls.createServer([options][, secureConnectionListener])
- tls.getCiphers()
- tls.DEFAULT_ECDH_CURVE
- Deprecated APIs
- Tracing
- TTY
- UDP / Datagram Sockets
- Class: dgram.Socket
- Event: 'close'
- Event: 'error'
- Event: 'listening'
- Event: 'message'
- socket.addMembership(multicastAddress[, multicastInterface])
- socket.address()
- socket.bind([port][, address][, callback])
- socket.bind(options[, callback])
- socket.close([callback])
- socket.dropMembership(multicastAddress[, multicastInterface])
- socket.ref()
- socket.send(msg, [offset, length,] port [, address] [, callback])
- socket.setBroadcast(flag)
- socket.setMulticastLoopback(flag)
- socket.setMulticastTTL(ttl)
- socket.setTTL(ttl)
- socket.unref()
- Change to asynchronous
socket.bind()
behavior
dgram
module functions
- Class: dgram.Socket
- URL
- URL Strings and URL Objects
- The WHATWG URL API
- Class: URL
- Class: URLSearchParams
- Constructor: new URLSearchParams()
- Constructor: new URLSearchParams(string)
- Constructor: new URLSearchParams(obj)
- Constructor: new URLSearchParams(iterable)
- urlSearchParams.append(name, value)
- urlSearchParams.delete(name)
- urlSearchParams.entries()
- urlSearchParams.forEach(fn[, thisArg])
- urlSearchParams.get(name)
- urlSearchParams.getAll(name)
- urlSearchParams.has(name)
- urlSearchParams.keys()
- urlSearchParams.set(name, value)
- urlSearchParams.sort()
- urlSearchParams.toString()
- urlSearchParams.values()
- urlSearchParams[@@iterator]()
- url.domainToASCII(domain)
- url.domainToUnicode(domain)
- url.format(URL[, options])
- Legacy URL API
- Percent-Encoding in URLs
- Util
- util.callbackify(original)
- util.debuglog(section)
- util.deprecate(function, string)
- util.format(format[, ...args])
- util.inherits(constructor, superConstructor)
- util.inspect(object[, options])
- util.promisify(original)
- Deprecated APIs
- util._extend(target, source)
- util.debug(string)
- util.error([...strings])
- util.isArray(object)
- util.isBoolean(object)
- util.isBuffer(object)
- util.isDate(object)
- util.isError(object)
- util.isFunction(object)
- util.isNull(object)
- util.isNullOrUndefined(object)
- util.isNumber(object)
- util.isObject(object)
- util.isPrimitive(object)
- util.isRegExp(object)
- util.isString(object)
- util.isSymbol(object)
- util.isUndefined(object)
- util.log(string)
- util.print([...strings])
- util.puts([...strings])
- V8
- v8.cachedDataVersionTag()
- v8.getHeapSpaceStatistics()
- v8.getHeapStatistics()
- v8.setFlagsFromString(string)
- Serialization API
- v8.serialize(value)
- v8.deserialize(buffer)
- class: v8.Serializer
- new Serializer()
- serializer.writeHeader()
- serializer.writeValue(value)
- serializer.releaseBuffer()
- serializer.transferArrayBuffer(id, arrayBuffer)
- serializer.writeUint32(value)
- serializer.writeUint64(hi, lo)
- serializer.writeDouble(value)
- serializer.writeRawBytes(buffer)
- serializer._writeHostObject(object)
- serializer._getDataCloneError(message)
- serializer._getSharedArrayBufferId(sharedArrayBuffer)
- serializer._setTreatArrayBufferViewsAsHostObjects(flag)
- class: v8.Deserializer
- new Deserializer(buffer)
- deserializer.readHeader()
- deserializer.readValue()
- deserializer.transferArrayBuffer(id, arrayBuffer)
- deserializer.getWireFormatVersion()
- deserializer.readUint32()
- deserializer.readUint64()
- deserializer.readDouble()
- deserializer.readRawBytes(length)
- deserializer._readHostObject()
- class: v8.DefaultSerializer
- class: v8.DefaultDeserializer
- VM (Executing JavaScript)
- Class: vm.Script
- vm.createContext([sandbox])
- vm.isContext(sandbox)
- vm.runInContext(code, contextifiedSandbox[, options])
- vm.runInDebugContext(code)
- vm.runInNewContext(code[, sandbox][, options])
- vm.runInThisContext(code[, options])
- Example: Running an HTTP Server within a VM
- What does it mean to "contextify" an object?
- Zlib
- Compressing HTTP requests and responses
- Memory Usage Tuning
- Flushing
- Constants
- Class Options
- Class: zlib.Deflate
- Class: zlib.DeflateRaw
- Class: zlib.Gunzip
- Class: zlib.Gzip
- Class: zlib.Inflate
- Class: zlib.InflateRaw
- Class: zlib.Unzip
- Class: zlib.Zlib
- zlib.constants
- zlib.createDeflate([options])
- zlib.createDeflateRaw([options])
- zlib.createGunzip([options])
- zlib.createGzip([options])
- zlib.createInflate([options])
- zlib.createInflateRaw([options])
- zlib.createUnzip([options])
- Convenience Methods
- zlib.deflate(buffer[, options], callback)
- zlib.deflateSync(buffer[, options])
- zlib.deflateRaw(buffer[, options], callback)
- zlib.deflateRawSync(buffer[, options])
- zlib.gunzip(buffer[, options], callback)
- zlib.gunzipSync(buffer[, options])
- zlib.gzip(buffer[, options], callback)
- zlib.gzipSync(buffer[, options])
- zlib.inflate(buffer[, options], callback)
- zlib.inflateSync(buffer[, options])
- zlib.inflateRaw(buffer[, options], callback)
- zlib.inflateRawSync(buffer[, options])
- zlib.unzip(buffer[, options], callback)
- zlib.unzipSync(buffer[, options])
About this Documentation#
The goal of this documentation is to comprehensively explain the Node.js API, both from a reference as well as a conceptual point of view. Each section describes a built-in module or high-level concept.
Where appropriate, property types, method arguments, and the arguments provided to event handlers are detailed in a list underneath the topic heading.
Every .html
document has a corresponding .json
document presenting
the same information in a structured manner. This feature is
experimental, and added for the benefit of IDEs and other utilities that
wish to do programmatic things with the documentation.
Every .html
and .json
file is generated based on the corresponding
.md
file in the doc/api/
folder in Node.js's source tree. The
documentation is generated using the tools/doc/generate.js
program.
The HTML template is located at doc/template.html
.
If errors are found in this documentation, please submit an issue or see the contributing guide for directions on how to submit a patch.
Stability Index#
Throughout the documentation are indications of a section's stability. The Node.js API is still somewhat changing, and as it matures, certain parts are more reliable than others. Some are so proven, and so relied upon, that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous and in the process of being redesigned.
The stability indices are as follows:
Stability: 0 - Deprecated This feature is known to be problematic, and changes may be planned. Do not rely on it. Use of the feature may cause warnings to be emitted. Backwards compatibility across major versions should not be expected.
Stability: 1 - Experimental This feature is still under active development and subject to non-backwards compatible changes, or even removal, in any future version. Use of the feature is not recommended in production environments. Experimental features are not subject to the Node.js Semantic Versioning model.
Note: Caution must be used when making use of Experimental
features,
particularly within modules that may be used as dependencies (or dependencies
of dependencies) within a Node.js application. End users may not be aware that
experimental features are being used, and therefore may experience unexpected
failures or behavioral changes when changes occur. To help avoid such surprises,
Experimental
features may require a command-line flag to explicitly enable
them, or may cause a process warning to be emitted. By default, such warnings
are printed to stderr
and may be handled by attaching a listener to the
process.on('warning')
event.
Stability: 2 - Stable The API has proven satisfactory. Compatibility with the npm ecosystem is a high priority, and will not be broken unless absolutely necessary.
JSON Output#
Stability: 1 - Experimental
Every HTML file in the markdown has a corresponding JSON file with the same data.
This feature was added in Node.js v0.6.12. It is experimental.
Syscalls and man pages#
System calls like open(2) and read(2) define the interface between user programs
and the underlying operating system. Node functions which simply wrap a syscall,
like fs.open()
, will document that. The docs link to the corresponding man
pages (short for manual pages) which describe how the syscalls work.
Note: some syscalls, like lchown(2), are BSD-specific. That means, for
example, that fs.lchown()
only works on macOS and other BSD-derived systems,
and is not available on Linux.
Most Unix syscalls have Windows equivalents, but behavior may differ on Windows relative to Linux and macOS. For an example of the subtle ways in which it's sometimes impossible to replace Unix syscall semantics on Windows, see Node issue 4760.
Usage#
node [options] [v8 options] [script.js | -e "script" | - ] [arguments]
Please see the Command Line Options document for information about different options and ways to run scripts with Node.js.
Example#
An example of a web server written with Node.js which responds with
'Hello World'
:
const http = require('http');
const hostname = '127.0.0.1';
const port = 3000;
const server = http.createServer((req, res) => {
res.statusCode = 200;
res.setHeader('Content-Type', 'text/plain');
res.end('Hello World\n');
});
server.listen(port, hostname, () => {
console.log(`Server running at http://${hostname}:${port}/`);
});
To run the server, put the code into a file called example.js
and execute
it with Node.js:
$ node example.js
Server running at http://127.0.0.1:3000/
All of the examples in the documentation can be run similarly.
Assert#
Stability: 2 - Stable
The assert
module provides a simple set of assertion tests that can be used to
test invariants.
assert(value[, message])#
value
<any>message
<any>
An alias of assert.ok()
.
assert.deepEqual(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Tests for deep equality between the actual
and expected
parameters.
Primitive values are compared with the Abstract Equality Comparison
( ==
).
Only enumerable "own" properties are considered. The
assert.deepEqual()
implementation does not test the
[[Prototype]]
of objects, attached symbols, or
non-enumerable properties — for such checks, consider using
assert.deepStrictEqual()
instead. This can lead to some
potentially surprising results. For example, the following example does not
throw an AssertionError
because the properties on the Error
object are
not enumerable:
// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));
An exception is made for Map
and Set
. Maps and Sets have their
contained items compared too, as expected.
"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:
const assert = require('assert');
const obj1 = {
a: {
b: 1
}
};
const obj2 = {
a: {
b: 2
}
};
const obj3 = {
a: {
b: 1
}
};
const obj4 = Object.create(obj1);
assert.deepEqual(obj1, obj1);
// OK, object is equal to itself
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different
assert.deepEqual(obj1, obj3);
// OK, objects are equal
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored
If the values are not equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned.
assert.deepStrictEqual(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Generally identical to assert.deepEqual()
with three exceptions:
- Primitive values are compared using the Strict Equality Comparison
(
===
). Set values and Map keys are compared using the SameValueZero comparison. (Which means they are free of the caveats). [[Prototype]]
of objects are compared using the Strict Equality Comparison too.- Type tags of objects should be the same.
const assert = require('assert');
assert.deepEqual({ a: 1 }, { a: '1' });
// OK, because 1 == '1'
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality
// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);
assert.deepEqual(object, fakeDate);
// OK, doesn't check [[Prototype]]
assert.deepStrictEqual(object, fakeDate);
// AssertionError: {} deepStrictEqual Date {}
// Different [[Prototype]]
assert.deepEqual(date, fakeDate);
// OK, doesn't check type tags
assert.deepStrictEqual(date, fakeDate);
// AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {}
// Different type tags
If the values are not equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned.
assert.doesNotThrow(block[, error][, message])#
block
<Function>error
<RegExp> | <Function>message
<any>
Asserts that the function block
does not throw an error. See
assert.throws()
for more details.
When assert.doesNotThrow()
is called, it will immediately call the block
function.
If an error is thrown and it is the same type as that specified by the error
parameter, then an AssertionError
is thrown. If the error is of a different
type, or if the error
parameter is undefined, the error is propagated back
to the caller.
The following, for instance, will throw the TypeError
because there is no
matching error type in the assertion:
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
SyntaxError
);
However, the following will result in an AssertionError
with the message
'Got unwanted exception (TypeError)..':
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
TypeError
);
If an AssertionError
is thrown and a value is provided for the message
parameter, the value of message
will be appended to the AssertionError
message:
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
TypeError,
'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops
assert.equal(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Tests shallow, coercive equality between the actual
and expected
parameters
using the Abstract Equality Comparison ( ==
).
const assert = require('assert');
assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'
assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
//AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
If the values are not equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned.
assert.fail(message)#
assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])#
actual
<any>expected
<any>message
<any>operator
<string> (default: '!=')stackStartFunction
<function> (default:assert.fail
)
Throws an AssertionError
. If message
is falsy, the error message is set as
the values of actual
and expected
separated by the provided operator
.
If just the two actual
and expected
arguments are provided, operator
will
default to '!='
. If message
is provided only it will be used as the error
message, the other arguments will be stored as properties on the thrown object.
If stackStartFunction
is provided, all stack frames above that function will
be removed from stacktrace (see Error.captureStackTrace
).
const assert = require('assert');
assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2
assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail
assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops
Note: Is the last two cases actual
, expected
, and operator
have no
influence on the error message.
assert.fail();
// AssertionError [ERR_ASSERTION]: Failed
assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom
assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'
Example use of stackStartFunction
for truncating the exception's stacktrace:
function suppressFrame() {
assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
// at repl:1:1
// at ContextifyScript.Script.runInThisContext (vm.js:44:33)
// ...
assert.ifError(value)#
value
<any>
Throws value
if value
is truthy. This is useful when testing the error
argument in callbacks.
const assert = require('assert');
assert.ifError(0);
// OK
assert.ifError(1);
// Throws 1
assert.ifError('error');
// Throws 'error'
assert.ifError(new Error());
// Throws Error
assert.notDeepEqual(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Tests for any deep inequality. Opposite of assert.deepEqual()
.
const assert = require('assert');
const obj1 = {
a: {
b: 1
}
};
const obj2 = {
a: {
b: 2
}
};
const obj3 = {
a: {
b: 1
}
};
const obj4 = Object.create(obj1);
assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj2);
// OK, obj1 and obj2 are not deeply equal
assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj4);
// OK, obj1 and obj2 are not deeply equal
If the values are deeply equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned.
assert.notDeepStrictEqual(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Tests for deep strict inequality. Opposite of assert.deepStrictEqual()
.
const assert = require('assert');
assert.notDeepEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } notDeepEqual { a: '1' }
assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK
If the values are deeply and strictly equal, an AssertionError
is thrown
with a message
property set equal to the value of the message
parameter. If
the message
parameter is undefined, a default error message is assigned.
assert.notEqual(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Tests shallow, coercive inequality with the Abstract Equality Comparison
( !=
).
const assert = require('assert');
assert.notEqual(1, 2);
// OK
assert.notEqual(1, 1);
// AssertionError: 1 != 1
assert.notEqual(1, '1');
// AssertionError: 1 != '1'
If the values are equal, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined, a default error message is assigned.
assert.notStrictEqual(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Tests strict inequality as determined by the Strict Equality Comparison
( !==
).
const assert = require('assert');
assert.notStrictEqual(1, 2);
// OK
assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 1
assert.notStrictEqual(1, '1');
// OK
If the values are strictly equal, an AssertionError
is thrown with a
message
property set equal to the value of the message
parameter. If the
message
parameter is undefined, a default error message is assigned.
assert.ok(value[, message])#
value
<any>message
<any>
Tests if value
is truthy. It is equivalent to
assert.equal(!!value, true, message)
.
If value
is not truthy, an AssertionError
is thrown with a message
property set equal to the value of the message
parameter. If the message
parameter is undefined
, a default error message is assigned.
const assert = require('assert');
assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok(false);
// throws "AssertionError: false == true"
assert.ok(0);
// throws "AssertionError: 0 == true"
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"
assert.strictEqual(actual, expected[, message])#
actual
<any>expected
<any>message
<any>
Tests strict equality as determined by the Strict Equality Comparison
( ===
).
const assert = require('assert');
assert.strictEqual(1, 2);
// AssertionError: 1 === 2
assert.strictEqual(1, 1);
// OK
assert.strictEqual(1, '1');
// AssertionError: 1 === '1'
If the values are not strictly equal, an AssertionError
is thrown with a
message
property set equal to the value of the message
parameter. If the
message
parameter is undefined, a default error message is assigned.
assert.throws(block[, error][, message])#
block
<Function>error
<RegExp> | <Function>message
<any>
Expects the function block
to throw an error.
If specified, error
can be a constructor, RegExp
, or validation
function.
If specified, message
will be the message provided by the AssertionError
if
the block fails to throw.
Validate instanceof using constructor:
assert.throws(
() => {
throw new Error('Wrong value');
},
Error
);
Validate error message using RegExp
:
assert.throws(
() => {
throw new Error('Wrong value');
},
/value/
);
Custom error validation:
assert.throws(
() => {
throw new Error('Wrong value');
},
function(err) {
if ((err instanceof Error) && /value/.test(err)) {
return true;
}
},
'unexpected error'
);
Note that error
can not be a string. If a string is provided as the second
argument, then error
is assumed to be omitted and the string will be used for
message
instead. This can lead to easy-to-miss mistakes:
// THIS IS A MISTAKE! DO NOT DO THIS!
assert.throws(myFunction, 'missing foo', 'did not throw with expected message');
// Do this instead.
assert.throws(myFunction, /missing foo/, 'did not throw with expected message');
Caveats#
For the following cases, consider using ES2015 Object.is()
,
which uses the SameValueZero comparison.
const a = 0;
const b = -a;
assert.notStrictEqual(a, b);
// AssertionError: 0 !== -0
// Strict Equality Comparison doesn't distinguish between -0 and +0...
assert(!Object.is(a, b));
// but Object.is() does!
const str1 = 'foo';
const str2 = 'foo';
assert.strictEqual(str1 / 1, str2 / 1);
// AssertionError: NaN === NaN
// Strict Equality Comparison can't be used to check NaN...
assert(Object.is(str1 / 1, str2 / 1));
// but Object.is() can!
For more information, see MDN's guide on equality comparisons and sameness.
Async Hooks#
Stability: 1 - Experimental
The async_hooks
module provides an API to register callbacks tracking the
lifetime of asynchronous resources created inside a Node.js application.
It can be accessed using:
const async_hooks = require('async_hooks');
Terminology#
An asynchronous resource represents an object with an associated callback.
This callback may be called multiple times, for example, the connection
event
in net.createServer
, or just a single time like in fs.open
. A resource
can also be closed before the callback is called. AsyncHook does not
explicitly distinguish between these different cases but will represent them
as the abstract concept that is a resource.
Public API#
Overview#
Following is a simple overview of the public API.
const async_hooks = require('async_hooks');
// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();
// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerAsyncId();
// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook = async_hooks.createHook({ init, before, after, destroy });
// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.
asyncHook.enable();
// Disable listening for new asynchronous events.
asyncHook.disable();
//
// The following are the callbacks that can be passed to createHook().
//
// init is called during object construction. The resource may not have
// completed construction when this callback runs, therefore all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerAsyncId, resource) { }
// before is called just before the resource's callback is called. It can be
// called 0-N times for handles (e.g. TCPWrap), and will be called exactly 1
// time for requests (e.g. FSReqWrap).
function before(asyncId) { }
// after is called just after the resource's callback has finished.
function after(asyncId) { }
// destroy is called when an AsyncWrap instance is destroyed.
function destroy(asyncId) { }
async_hooks.createHook(callbacks)
#
callbacks
<Object> the callbacks to register- Returns:
{AsyncHook}
instance used for disabling and enabling hooks
Registers functions to be called for different lifetime events of each async operation.
The callbacks init()
/before()
/after()
/destroy()
are called for the
respective asynchronous event during a resource's lifetime.
All callbacks are optional. So, for example, if only resource cleanup needs to
be tracked then only the destroy
callback needs to be passed. The
specifics of all functions that can be passed to callbacks
is in the section
Hook Callbacks
.
Error Handling#
If any AsyncHook
callbacks throw, the application will print the stack trace
and exit. The exit path does follow that of an uncaught exception but
all uncaughtException
listeners are removed, thus forcing the process to
exit. The 'exit'
callbacks will still be called unless the application is run
with --abort-on-uncaught-exception
, in which case a stack trace will be
printed and the application exits, leaving a core file.
The reason for this error handling behavior is that these callbacks are running at potentially volatile points in an object's lifetime, for example during class construction and destruction. Because of this, it is deemed necessary to bring down the process quickly in order to prevent an unintentional abort in the future. This is subject to change in the future if a comprehensive analysis is performed to ensure an exception can follow the normal control flow without unintentional side effects.
Printing in AsyncHooks callbacks#
Because printing to the console is an asynchronous operation, console.log()
will cause the AsyncHooks callbacks to be called. Using console.log()
or
similar asynchronous operations inside an AsyncHooks callback function will thus
cause an infinite recursion. An easily solution to this when debugging is
to use a synchronous logging operation such as fs.writeSync(1, msg)
. This
will print to stdout because 1
is the file descriptor for stdout and will
not invoke AsyncHooks recursively because it is synchronous.
const fs = require('fs');
const util = require('util');
function debug(...args) {
// use a function like this one when debugging inside an AsyncHooks callback
fs.writeSync(1, `${util.format(...args)}\n`);
}
If an asynchronous operation is needed for logging, it is possible to keep track of what caused the asynchronous operation using the information provided by AsyncHooks itself. The logging should then be skipped when it was the logging itself that caused AsyncHooks callback to call. By doing this the otherwise infinite recursion is broken.
asyncHook.enable()
#
- Returns <AsyncHook> A reference to
asyncHook
.
Enable the callbacks for a given AsyncHook
instance. If no callbacks are
provided enabling is a noop.
The AsyncHook
instance is by default disabled. If the AsyncHook
instance
should be enabled immediately after creation, the following pattern can be used.
const async_hooks = require('async_hooks');
const hook = async_hooks.createHook(callbacks).enable();
asyncHook.disable()
#
- Returns <AsyncHook> A reference to
asyncHook
.
Disable the callbacks for a given AsyncHook
instance from the global pool of
AsyncHook callbacks to be executed. Once a hook has been disabled it will not
be called again until enabled.
For API consistency disable()
also returns the AsyncHook
instance.
Hook Callbacks#
Key events in the lifetime of asynchronous events have been categorized into four areas: instantiation, before/after the callback is called, and when the instance is destructed.
init(asyncId, type, triggerAsyncId, resource)
#
asyncId
<number> a unique ID for the async resourcetype
<string> the type of the async resourcetriggerAsyncId
<number> the unique ID of the async resource in whose execution context this async resource was createdresource
<Object> reference to the resource representing the async operation, needs to be released during destroy
Called when a class is constructed that has the possibility to emit an
asynchronous event. This does not mean the instance must call
before
/after
before destroy
is called, only that the possibility
exists.
This behavior can be observed by doing something like opening a resource then closing it before the resource can be used. The following snippet demonstrates this.
require('net').createServer().listen(function() { this.close(); });
// OR
clearTimeout(setTimeout(() => {}, 10));
Every new resource is assigned a unique ID.
type
#
The type
is a string that represents the type of resource that caused
init
to be called. Generally, it will correspond to the name of the
resource's constructor.
FSEVENTWRAP, FSREQWRAP, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPPARSER,
JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP, SHUTDOWNWRAP,
SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPWRAP, TIMERWRAP, TTYWRAP,
UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST,
RANDOMBYTESREQUEST, TLSWRAP, Timeout, Immediate, TickObject
There is also the PROMISE
resource type, which is used to track Promise
instances and asynchronous work scheduled by them.
Users are be able to define their own type
when using the public embedder API.
Note: It is possible to have type name collisions. Embedders are encouraged to use a unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks.
triggerId
#
triggerAsyncId
is the asyncId
of the resource that caused (or "triggered") the
new resource to initialize and that caused init
to call. This is different
from async_hooks.executionAsyncId()
that only shows when a resource was
created, while triggerAsyncId
shows why a resource was created.
The following is a simple demonstration of triggerAsyncId
:
async_hooks.createHook({
init(asyncId, type, triggerAsyncId) {
const eid = async_hooks.executionAsyncId();
fs.writeSync(
1, `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
}
}).enable();
require('net').createServer((conn) => {}).listen(8080);
Output when hitting the server with nc localhost 8080
:
TCPWRAP(2): trigger: 1 execution: 1
TCPWRAP(4): trigger: 2 execution: 0
The first TCPWRAP
is the server which receives the connections.
The second TCPWRAP
is the new connection from the client. When a new
connection is made the TCPWrap
instance is immediately constructed. This
happens outside of any JavaScript stack (side note: a executionAsyncId()
of 0
means it's being executed from C++, with no JavaScript stack above it).
With only that information it would be impossible to link resources together in
terms of what caused them to be created, so triggerAsyncId
is given the task of
propagating what resource is responsible for the new resource's existence.
resource
#
resource
is an object that represents the actual resource. This can contain
useful information such as the hostname for the GETADDRINFOREQWRAP
resource
type, which will be used when looking up the ip for the hostname in
net.Server.listen
. The API for getting this information is currently not
considered public, but using the Embedder API users can provide and document
their own resource objects. Such as resource object could for example contain
the SQL query being executed.
In the case of Promises, the resource
object will have promise
property
that refers to the Promise that is being initialized, and a parentId
property
that equals the asyncId
of a parent Promise, if there is one, and
undefined
otherwise. For example, in the case of b = a.then(handler)
,
a
is considered a parent Promise of b
.
Note: In some cases the resource object is reused for performance reasons,
it is thus not safe to use it as a key in a WeakMap
or add properties to it.
asynchronous context example#
Below is another example with additional information about the calls to
init
between the before
and after
calls, specifically what the
callback to listen()
will look like. The output formatting is slightly more
elaborate to make calling context easier to see.
let indent = 0;
async_hooks.createHook({
init(asyncId, type, triggerAsyncId) {
const eid = async_hooks.executionAsyncId();
const indentStr = ' '.repeat(indent);
fs.writeSync(
1,
`${indentStr}${type}(${asyncId}):` +
` trigger: ${triggerAsyncId} execution: ${eid}\n`);
},
before(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeSync(1, `${indentStr}before: ${asyncId}\n`);
indent += 2;
},
after(asyncId) {
indent -= 2;
const indentStr = ' '.repeat(indent);
fs.writeSync(1, `${indentStr}after: ${asyncId}\n`);
},
destroy(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeSync(1, `${indentStr}destroy: ${asyncId}\n`);
},
}).enable();
require('net').createServer(() => {}).listen(8080, () => {
// Let's wait 10ms before logging the server started.
setTimeout(() => {
console.log('>>>', async_hooks.executionAsyncId());
}, 10);
});
Output from only starting the server:
TCPWRAP(2): trigger: 1 execution: 1
TickObject(3): trigger: 2 execution: 1
before: 3
Timeout(4): trigger: 3 execution: 3
TIMERWRAP(5): trigger: 3 execution: 3
after: 3
destroy: 3
before: 5
before: 4
TTYWRAP(6): trigger: 4 execution: 4
SIGNALWRAP(7): trigger: 4 execution: 4
TTYWRAP(8): trigger: 4 execution: 4
>>> 4
TickObject(9): trigger: 4 execution: 4
after: 4
after: 5
before: 9
after: 9
destroy: 4
destroy: 9
destroy: 5
Note: As illustrated in the example, executionAsyncId()
and execution
each specify the value of the current execution context; which is delineated by
calls to before
and after
.
Only using execution
to graph resource allocation results in the following:
TTYWRAP(6) -> Timeout(4) -> TIMERWRAP(5) -> TickObject(3) -> root(1)
The TCPWRAP
isn't part of this graph; even though it was the reason for
console.log()
being called. This is because binding to a port without a
hostname is actually synchronous, but to maintain a completely asynchronous API
the user's callback is placed in a process.nextTick()
.
The graph only shows when a resource was created, not why, so to track
the why use triggerAsyncId
.
before(asyncId)
#
asyncId
<number>
When an asynchronous operation is initiated (such as a TCP server receiving a
new connection) or completes (such as writing data to disk) a callback is
called to notify the user. The before
callback is called just before said
callback is executed. asyncId
is the unique identifier assigned to the
resource about to execute the callback.
The before
callback will be called 0 to N times. The before
callback
will typically be called 0 times if the asynchronous operation was cancelled
or for example if no connections are received by a TCP server. Asynchronous
like the TCP server will typically call the before
callback multiple times,
while other operations like fs.open()
will only call it once.
after(asyncId)
#
asyncId
<number>
Called immediately after the callback specified in before
is completed.
Note: If an uncaught exception occurs during execution of the callback then
after
will run after the 'uncaughtException'
event is emitted or a
domain
's handler runs.
destroy(asyncId)
#
asyncId
<number>
Called after the resource corresponding to asyncId
is destroyed. It is also called
asynchronously from the embedder API emitDestroy()
.
Note: Some resources depend on GC for cleanup, so if a reference is made to
the resource
object passed to init
it's possible that destroy
is
never called, causing a memory leak in the application. Of course if
the resource doesn't depend on GC then this isn't an issue.
async_hooks.executionAsyncId()
#
- Returns <number> the
asyncId
of the current execution context. Useful to track when something calls.
For example:
console.log(async_hooks.executionAsyncId()); // 1 - bootstrap
fs.open(path, 'r', (err, fd) => {
console.log(async_hooks.executionAsyncId()); // 6 - open()
});
It is important to note that the ID returned fom executionAsyncId()
is related
to execution timing, not causality (which is covered by triggerAsyncId()
). For
example:
const server = net.createServer(function onConnection(conn) {
// Returns the ID of the server, not of the new connection, because the
// onConnection callback runs in the execution scope of the server's
// MakeCallback().
async_hooks.executionAsyncId();
}).listen(port, function onListening() {
// Returns the ID of a TickObject (i.e. process.nextTick()) because all
// callbacks passed to .listen() are wrapped in a nextTick().
async_hooks.executionAsyncId();
});
async_hooks.triggerAsyncId()
#
- Returns <number> the ID of the resource responsible for calling the callback that is currently being executed.
For example:
const server = net.createServer((conn) => {
// The resource that caused (or triggered) this callback to be called
// was that of the new connection. Thus the return value of triggerAsyncId()
// is the asyncId of "conn".
async_hooks.triggerAsyncId();
}).listen(port, () => {
// Even though all callbacks passed to .listen() are wrapped in a nextTick()
// the callback itself exists because the call to the server's .listen()
// was made. So the return value would be the ID of the server.
async_hooks.triggerAsyncId();
});
JavaScript Embedder API#
Library developers that handle their own I/O, a connection pool, or callback queues will need to hook into the AsyncWrap API so that all the appropriate callbacks are called. To accommodate this a JavaScript API is provided.
class AsyncResource()
#
The class AsyncResource
was designed to be extended by the embedder's async
resources. Using this users can easily trigger the lifetime events of their
own resources.
The init
hook will trigger when an AsyncResource
is instantiated.
It is important that before
/after
calls are unwound
in the same order they are called. Otherwise an unrecoverable exception
will occur and node will abort.
The following is an overview of the AsyncResource
API.
const { AsyncResource } = require('async_hooks');
// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(type, triggerAsyncId);
// Call AsyncHooks before callbacks.
asyncResource.emitBefore();
// Call AsyncHooks after callbacks.
asyncResource.emitAfter();
// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();
// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();
// Return the trigger ID for the AsyncResource instance.
asyncResource.triggerAsyncId();
AsyncResource(type[, triggerAsyncId])
#
Example usage:
class DBQuery extends AsyncResource {
constructor(db) {
super('DBQuery');
this.db = db;
}
getInfo(query, callback) {
this.db.get(query, (err, data) => {
this.emitBefore();
callback(err, data);
this.emitAfter();
});
}
close() {
this.db = null;
this.emitDestroy();
}
}
asyncResource.emitBefore()
#
- Returns <undefined>
Call all before
callbacks and let them know a new asynchronous execution
context is being entered. If nested calls to emitBefore()
are made, the stack
of asyncId
s will be tracked and properly unwound.
asyncResource.emitAfter()
#
- Returns <undefined>
Call all after
callbacks. If nested calls to emitBefore()
were made, then
make sure the stack is unwound properly. Otherwise an error will be thrown.
If the user's callback throws an exception then emitAfter()
will
automatically be called for all asyncId
s on the stack if the error is handled by
a domain or 'uncaughtException'
handler.
asyncResource.emitDestroy()
#
- Returns <undefined>
Call all destroy
hooks. This should only ever be called once. An error will
be thrown if it is called more than once. This must be manually called. If
the resource is left to be collected by the GC then the destroy
hooks will
never be called.
asyncResource.asyncId()
#
- Returns <number> the unique
asyncId
assigned to the resource.
asyncResource.triggerAsyncId()
#
- Returns <number> the same
triggerAsyncId
that is passed to theAsyncResource
constructor.
Buffer#
Stability: 2 - Stable
Prior to the introduction of TypedArray
in ECMAScript 2015 (ES6), the
JavaScript language had no mechanism for reading or manipulating streams
of binary data. The Buffer
class was introduced as part of the Node.js
API to make it possible to interact with octet streams in the context of things
like TCP streams and file system operations.
Now that TypedArray
has been added in ES6, the Buffer
class implements the
Uint8Array
API in a manner that is more optimized and suitable for Node.js'
use cases.
Instances of the Buffer
class are similar to arrays of integers but
correspond to fixed-sized, raw memory allocations outside the V8 heap.
The size of the Buffer
is established when it is created and cannot be
resized.
The Buffer
class is a global within Node.js, making it unlikely that one
would need to ever use require('buffer').Buffer
.
Examples:
// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);
// Creates a Buffer of length 10, filled with 0x1.
const buf2 = Buffer.alloc(10, 1);
// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using either fill() or write().
const buf3 = Buffer.allocUnsafe(10);
// Creates a Buffer containing [0x1, 0x2, 0x3].
const buf4 = Buffer.from([1, 2, 3]);
// Creates a Buffer containing UTF-8 bytes [0x74, 0xc3, 0xa9, 0x73, 0x74].
const buf5 = Buffer.from('tést');
// Creates a Buffer containing Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf6 = Buffer.from('tést', 'latin1');
Buffer.from()
, Buffer.alloc()
, and Buffer.allocUnsafe()
#
In versions of Node.js prior to v6, Buffer
instances were created using the
Buffer
constructor function, which allocates the returned Buffer
differently based on what arguments are provided:
- Passing a number as the first argument to
Buffer()
(e.g.new Buffer(10)
), allocates a newBuffer
object of the specified size. Prior to Node.js 8.0.0, the memory allocated for suchBuffer
instances is not initialized and can contain sensitive data. SuchBuffer
instances must be subsequently initialized by using eitherbuf.fill(0)
or by writing to theBuffer
completely. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitializedBuffer
versus creating a slower-but-saferBuffer
. Starting in Node.js 8.0.0,Buffer(num)
andnew Buffer(num)
will return aBuffer
with initialized memory. - Passing a string, array, or
Buffer
as the first argument copies the passed object's data into theBuffer
. - Passing an
ArrayBuffer
returns aBuffer
that shares allocated memory with the givenArrayBuffer
.
Because the behavior of new Buffer()
changes significantly based on the type
of value passed as the first argument, applications that do not properly
validate the input arguments passed to new Buffer()
, or that fail to
appropriately initialize newly allocated Buffer
content, can inadvertently
introduce security and reliability issues into their code.
To make the creation of Buffer
instances more reliable and less error prone,
the various forms of the new Buffer()
constructor have been deprecated
and replaced by separate Buffer.from()
, Buffer.alloc()
, and
Buffer.allocUnsafe()
methods.
Developers should migrate all existing uses of the new Buffer()
constructors
to one of these new APIs.
Buffer.from(array)
returns a newBuffer
containing a copy of the provided octets.Buffer.from(arrayBuffer[, byteOffset [, length]])
returns a newBuffer
that shares the same allocated memory as the givenArrayBuffer
.Buffer.from(buffer)
returns a newBuffer
containing a copy of the contents of the givenBuffer
.Buffer.from(string[, encoding])
returns a newBuffer
containing a copy of the provided string.Buffer.alloc(size[, fill[, encoding]])
returns a "filled"Buffer
instance of the specified size. This method can be significantly slower thanBuffer.allocUnsafe(size)
but ensures that newly createdBuffer
instances never contain old and potentially sensitive data.Buffer.allocUnsafe(size)
andBuffer.allocUnsafeSlow(size)
each return a newBuffer
of the specifiedsize
whose content must be initialized using eitherbuf.fill(0)
or written to completely.
Buffer
instances returned by Buffer.allocUnsafe()
may be allocated off
a shared internal memory pool if size
is less than or equal to half
Buffer.poolSize
. Instances returned by Buffer.allocUnsafeSlow()
never
use the shared internal memory pool.
The --zero-fill-buffers
command line option#
Node.js can be started using the --zero-fill-buffers
command line option to
force all newly allocated Buffer
instances created using either
new Buffer(size)
, Buffer.allocUnsafe()
, Buffer.allocUnsafeSlow()
or
new SlowBuffer(size)
to be automatically zero-filled upon creation. Use of
this flag changes the default behavior of these methods and can have a significant
impact on performance. Use of the --zero-fill-buffers
option is recommended
only when necessary to enforce that newly allocated Buffer
instances cannot
contain potentially sensitive data.
Example:
$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>
What makes Buffer.allocUnsafe()
and Buffer.allocUnsafeSlow()
"unsafe"?#
When calling Buffer.allocUnsafe()
and Buffer.allocUnsafeSlow()
, the
segment of allocated memory is uninitialized (it is not zeroed-out). While
this design makes the allocation of memory quite fast, the allocated segment of
memory might contain old data that is potentially sensitive. Using a Buffer
created by Buffer.allocUnsafe()
without completely overwriting the memory
can allow this old data to be leaked when the Buffer
memory is read.
While there are clear performance advantages to using Buffer.allocUnsafe()
,
extra care must be taken in order to avoid introducing security
vulnerabilities into an application.
Buffers and Character Encodings#
Buffer
instances are commonly used to represent sequences of encoded characters
such as UTF-8, UCS2, Base64 or even Hex-encoded data. It is possible to
convert back and forth between Buffer
instances and ordinary JavaScript strings
by using an explicit character encoding.
Example:
const buf = Buffer.from('hello world', 'ascii');
// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('hex'));
// Prints: aGVsbG8gd29ybGQ=
console.log(buf.toString('base64'));
The character encodings currently supported by Node.js include:
'ascii'
- for 7-bit ASCII data only. This encoding is fast and will strip the high bit if set.'utf8'
- Multibyte encoded Unicode characters. Many web pages and other document formats use UTF-8.'utf16le'
- 2 or 4 bytes, little-endian encoded Unicode characters. Surrogate pairs (U+10000 to U+10FFFF) are supported.'ucs2'
- Alias of'utf16le'
.'base64'
- Base64 encoding. When creating aBuffer
from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC4648, Section 5.'latin1'
- A way of encoding theBuffer
into a one-byte encoded string (as defined by the IANA in RFC1345, page 63, to be the Latin-1 supplement block and C0/C1 control codes).'binary'
- Alias for'latin1'
.'hex'
- Encode each byte as two hexadecimal characters.
Note: Today's browsers follow the WHATWG spec which aliases both 'latin1'
and ISO-8859-1 to win-1252. This means that while doing something like
http.get()
, if the returned charset is one of those listed in the WHATWG spec
it's possible that the server actually returned win-1252-encoded data, and
using 'latin1'
encoding may incorrectly decode the characters.
Buffers and TypedArray#
Buffer
instances are also Uint8Array
instances. However, there are subtle
incompatibilities with the TypedArray specification in ECMAScript 2015.
For example, while ArrayBuffer#slice()
creates a copy of the slice, the
implementation of Buffer#slice()
creates a view over the
existing Buffer
without copying, making Buffer#slice()
far
more efficient.
It is also possible to create new TypedArray
instances from a Buffer
with
the following caveats:
The
Buffer
object's memory is copied to theTypedArray
, not shared.The
Buffer
object's memory is interpreted as an array of distinct elements, and not as a byte array of the target type. That is,new Uint32Array(Buffer.from([1, 2, 3, 4]))
creates a 4-elementUint32Array
with elements[1, 2, 3, 4]
, not aUint32Array
with a single element[0x1020304]
or[0x4030201]
.
It is possible to create a new Buffer
that shares the same allocated memory as
a TypedArray
instance by using the TypeArray object's .buffer
property.
Example:
const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;
// Copies the contents of `arr`
const buf1 = Buffer.from(arr);
// Shares memory with `arr`
const buf2 = Buffer.from(arr.buffer);
// Prints: <Buffer 88 a0>
console.log(buf1);
// Prints: <Buffer 88 13 a0 0f>
console.log(buf2);
arr[1] = 6000;
// Prints: <Buffer 88 a0>
console.log(buf1);
// Prints: <Buffer 88 13 70 17>
console.log(buf2);
Note that when creating a Buffer
using a TypedArray
's .buffer
, it is
possible to use only a portion of the underlying ArrayBuffer
by passing in
byteOffset
and length
parameters.
Example:
const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);
// Prints: 16
console.log(buf.length);
The Buffer.from()
and TypedArray.from()
have different signatures and
implementations. Specifically, the TypedArray
variants accept a second
argument that is a mapping function that is invoked on every element of the
typed array:
TypedArray.from(source[, mapFn[, thisArg]])
The Buffer.from()
method, however, does not support the use of a mapping
function:
Buffer.from(array)
Buffer.from(buffer)
Buffer.from(arrayBuffer[, byteOffset [, length]])
Buffer.from(string[, encoding])
Buffers and ES6 iteration#
Buffer
instances can be iterated over using the ECMAScript 2015 (ES6) for..of
syntax.
Example:
const buf = Buffer.from([1, 2, 3]);
// Prints:
// 1
// 2
// 3
for (const b of buf) {
console.log(b);
}
Additionally, the buf.values()
, buf.keys()
, and
buf.entries()
methods can be used to create iterators.
Class: Buffer#
The Buffer
class is a global type for dealing with binary data directly.
It can be constructed in a variety of ways.
new Buffer(array)#
Stability: 0 - Deprecated: Use Buffer.from(array)
instead.
array
<Array> An array of bytes to copy from
Allocates a new Buffer
using an array
of octets.
Example:
// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'
const buf = new Buffer([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
new Buffer(arrayBuffer[, byteOffset [, length]])#
Stability: 0 - Deprecated: Use Buffer.from(arrayBuffer[, byteOffset [, length]])
instead.
arrayBuffer
<ArrayBuffer> AnArrayBuffer
or the.buffer
property of aTypedArray
.byteOffset
<integer> Index of first byte to expose. Default:0
length
<integer> Number of bytes to expose. Default:arrayBuffer.length - byteOffset
This creates a view of the ArrayBuffer
without copying the underlying
memory. For example, when passed a reference to the .buffer
property of a
TypedArray
instance, the newly created Buffer
will share the same
allocated memory as the TypedArray
.
The optional byteOffset
and length
arguments specify a memory range within
the arrayBuffer
that will be shared by the Buffer
.
Example:
const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;
// Shares memory with `arr`
const buf = new Buffer(arr.buffer);
// Prints: <Buffer 88 13 a0 0f>
console.log(buf);
// Changing the original Uint16Array changes the Buffer also
arr[1] = 6000;
// Prints: <Buffer 88 13 70 17>
console.log(buf);
new Buffer(buffer)#
Stability: 0 - Deprecated: Use Buffer.from(buffer)
instead.
buffer
<Buffer> An existingBuffer
to copy data from
Copies the passed buffer
data onto a new Buffer
instance.
Example:
const buf1 = new Buffer('buffer');
const buf2 = new Buffer(buf1);
buf1[0] = 0x61;
// Prints: auffer
console.log(buf1.toString());
// Prints: buffer
console.log(buf2.toString());
new Buffer(size)#
Stability: 0 - Deprecated: UseBuffer.alloc()
instead (also seeBuffer.allocUnsafe()
).
size
<integer> The desired length of the newBuffer
Allocates a new Buffer
of size
bytes. If the size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, a RangeError
will be
thrown. A zero-length Buffer
will be created if size
is 0.
Prior to Node.js 8.0.0, the underlying memory for Buffer
instances
created in this way is not initialized. The contents of a newly created
Buffer
are unknown and may contain sensitive data. Use
Buffer.alloc(size)
instead to initialize a Buffer
to zeroes.
Example:
const buf = new Buffer(10);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
console.log(buf);
new Buffer(string[, encoding])#
Stability: 0 - Deprecated: Use Buffer.from(string[, encoding])
instead.
Creates a new Buffer
containing the given JavaScript string string
. If
provided, the encoding
parameter identifies the character encoding of string
.
Examples:
const buf1 = new Buffer('this is a tést');
// Prints: this is a tést
console.log(buf1.toString());
// Prints: this is a tC)st
console.log(buf1.toString('ascii'));
const buf2 = new Buffer('7468697320697320612074c3a97374', 'hex');
// Prints: this is a tést
console.log(buf2.toString());
Class Method: Buffer.alloc(size[, fill[, encoding]])#
size
<integer> The desired length of the newBuffer
fill
<string> | <Buffer> | <integer> A value to pre-fill the newBuffer
with. Default:0
encoding
<string> Iffill
is a string, this is its encoding. Default:'utf8'
Allocates a new Buffer
of size
bytes. If fill
is undefined
, the
Buffer
will be zero-filled.
Example:
const buf = Buffer.alloc(5);
// Prints: <Buffer 00 00 00 00 00>
console.log(buf);
Allocates a new Buffer
of size
bytes. If the size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, a RangeError
will be
thrown. A zero-length Buffer
will be created if size
is 0.
If fill
is specified, the allocated Buffer
will be initialized by calling
buf.fill(fill)
.
Example:
const buf = Buffer.alloc(5, 'a');
// Prints: <Buffer 61 61 61 61 61>
console.log(buf);
If both fill
and encoding
are specified, the allocated Buffer
will be
initialized by calling buf.fill(fill, encoding)
.
Example:
const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>
console.log(buf);
Calling Buffer.alloc()
can be significantly slower than the alternative
Buffer.allocUnsafe()
but ensures that the newly created Buffer
instance
contents will never contain sensitive data.
A TypeError
will be thrown if size
is not a number.
Class Method: Buffer.allocUnsafe(size)#
size
<integer> The desired length of the newBuffer
Allocates a new Buffer
of size
bytes. If the size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, a RangeError
will be
thrown. A zero-length Buffer
will be created if size
is 0.
The underlying memory for Buffer
instances created in this way is not
initialized. The contents of the newly created Buffer
are unknown and
may contain sensitive data. Use Buffer.alloc()
instead to initialize
Buffer
instances to zeroes.
Example:
const buf = Buffer.allocUnsafe(10);
// Prints: (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>
console.log(buf);
buf.fill(0);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
console.log(buf);
A TypeError
will be thrown if size
is not a number.
Note that the Buffer
module pre-allocates an internal Buffer
instance of
size Buffer.poolSize
that is used as a pool for the fast allocation of new
Buffer
instances created using Buffer.allocUnsafe()
and the deprecated
new Buffer(size)
constructor only when size
is less than or equal to
Buffer.poolSize >> 1
(floor of Buffer.poolSize
divided by two).
Use of this pre-allocated internal memory pool is a key difference between
calling Buffer.alloc(size, fill)
vs. Buffer.allocUnsafe(size).fill(fill)
.
Specifically, Buffer.alloc(size, fill)
will never use the internal Buffer
pool, while Buffer.allocUnsafe(size).fill(fill)
will use the internal
Buffer
pool if size
is less than or equal to half Buffer.poolSize
. The
difference is subtle but can be important when an application requires the
additional performance that Buffer.allocUnsafe()
provides.
Class Method: Buffer.allocUnsafeSlow(size)#
size
<integer> The desired length of the newBuffer
Allocates a new Buffer
of size
bytes. If the size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, a RangeError
will be
thrown. A zero-length Buffer
will be created if size
is 0.
The underlying memory for Buffer
instances created in this way is not
initialized. The contents of the newly created Buffer
are unknown and
may contain sensitive data. Use buf.fill(0)
to initialize such
Buffer
instances to zeroes.
When using Buffer.allocUnsafe()
to allocate new Buffer
instances,
allocations under 4KB are, by default, sliced from a single pre-allocated
Buffer
. This allows applications to avoid the garbage collection overhead of
creating many individually allocated Buffer
instances. This approach improves
both performance and memory usage by eliminating the need to track and cleanup as
many Persistent
objects.
However, in the case where a developer may need to retain a small chunk of
memory from a pool for an indeterminate amount of time, it may be appropriate
to create an un-pooled Buffer
instance using Buffer.allocUnsafeSlow()
then
copy out the relevant bits.
Example:
// Need to keep around a few small chunks of memory
const store = [];
socket.on('readable', () => {
const data = socket.read();
// Allocate for retained data
const sb = Buffer.allocUnsafeSlow(10);
// Copy the data into the new allocation
data.copy(sb, 0, 0, 10);
store.push(sb);
});
Use of Buffer.allocUnsafeSlow()
should be used only as a last resort after
a developer has observed undue memory retention in their applications.
A TypeError
will be thrown if size
is not a number.
Class Method: Buffer.byteLength(string[, encoding])#
string
<string> | <Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> A value to calculate the length ofencoding
<string> Ifstring
is a string, this is its encoding. Default:'utf8'
- Returns: <integer> The number of bytes contained within
string
Returns the actual byte length of a string. This is not the same as
String.prototype.length
since that returns the number of characters in
a string.
Note: For 'base64'
and 'hex'
, this function assumes valid input. For
strings that contain non-Base64/Hex-encoded data (e.g. whitespace), the return
value might be greater than the length of a Buffer
created from the string.
Example:
const str = '\u00bd + \u00bc = \u00be';
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes
console.log(`${str}: ${str.length} characters, ` +
`${Buffer.byteLength(str, 'utf8')} bytes`);
When string
is a Buffer
/DataView
/TypedArray
/ArrayBuffer
, the
actual byte length is returned.
Class Method: Buffer.compare(buf1, buf2)#
buf1
<Buffer> | <Uint8Array>buf2
<Buffer> | <Uint8Array>- Returns: <integer>
Compares buf1
to buf2
typically for the purpose of sorting arrays of
Buffer
instances. This is equivalent to calling
buf1.compare(buf2)
.
Example:
const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1])
console.log(arr.sort(Buffer.compare));
Class Method: Buffer.concat(list[, totalLength])#
list
<Array> List ofBuffer
orUint8Array
instances to concattotalLength
<integer> Total length of theBuffer
instances inlist
when concatenated- Returns: <Buffer>
Returns a new Buffer
which is the result of concatenating all the Buffer
instances in the list
together.
If the list has no items, or if the totalLength
is 0, then a new zero-length
Buffer
is returned.
If totalLength
is not provided, it is calculated from the Buffer
instances
in list
. This however causes an additional loop to be executed in order to
calculate the totalLength
, so it is faster to provide the length explicitly if
it is already known.
If totalLength
is provided, it is coerced to an unsigned integer. If the
combined length of the Buffer
s in list
exceeds totalLength
, the result is
truncated to totalLength
.
Example: Create a single Buffer
from a list of three Buffer
instances
const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;
// Prints: 42
console.log(totalLength);
const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);
// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA);
// Prints: 42
console.log(bufA.length);
Class Method: Buffer.from(array)#
array
<Array>
Allocates a new Buffer
using an array
of octets.
Example:
// Creates a new Buffer containing UTF-8 bytes of the string 'buffer'
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
A TypeError
will be thrown if array
is not an Array
.
Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])#
arrayBuffer
<ArrayBuffer> AnArrayBuffer
or the.buffer
property of aTypedArray
.byteOffset
<integer> Index of first byte to expose. Default:0
length
<integer> Number of bytes to expose. Default:arrayBuffer.length - byteOffset
This creates a view of the ArrayBuffer
without copying the underlying
memory. For example, when passed a reference to the .buffer
property of a
TypedArray
instance, the newly created Buffer
will share the same
allocated memory as the TypedArray
.
Example:
const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;
// Shares memory with `arr`
const buf = Buffer.from(arr.buffer);
// Prints: <Buffer 88 13 a0 0f>
console.log(buf);
// Changing the original Uint16Array changes the Buffer also
arr[1] = 6000;
// Prints: <Buffer 88 13 70 17>
console.log(buf);
The optional byteOffset
and length
arguments specify a memory range within
the arrayBuffer
that will be shared by the Buffer
.
Example:
const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);
// Prints: 2
console.log(buf.length);
A TypeError
will be thrown if arrayBuffer
is not an ArrayBuffer
.
Class Method: Buffer.from(buffer)#
buffer
<Buffer> An existingBuffer
to copy data from
Copies the passed buffer
data onto a new Buffer
instance.
Example:
const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);
buf1[0] = 0x61;
// Prints: auffer
console.log(buf1.toString());
// Prints: buffer
console.log(buf2.toString());
A TypeError
will be thrown if buffer
is not a Buffer
.
Class Method: Buffer.from(string[, encoding])#
Creates a new Buffer
containing the given JavaScript string string
. If
provided, the encoding
parameter identifies the character encoding of string
.
Examples:
const buf1 = Buffer.from('this is a tést');
// Prints: this is a tést
console.log(buf1.toString());
// Prints: this is a tC)st
console.log(buf1.toString('ascii'));
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');
// Prints: this is a tést
console.log(buf2.toString());
A TypeError
will be thrown if str
is not a string.
Class Method: Buffer.from(object[, offsetOrEncoding[, length]])#
object
<Object> An object supportingSymbol.toPrimitive
orvalueOf()
offsetOrEncoding
<number> | <string> A byte-offset or encoding, depending on the value returned either byobject.valueOf()
orobject[Symbol.toPrimitive]()
.length
<number> A length, depending on the value returned either byobject.valueOf()
orobject[Symbol.toPrimitive]()
.
For objects whose valueOf()
function returns a value not strictly equal to
object
, returns Buffer.from(object.valueOf(), offsetOrEncoding, length)
.
For example:
const buf = Buffer.from(new String('this is a test'));
// <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
For objects that support Symbol.toPrimitive
, returns
Buffer.from(object[Symbol.toPrimitive](), offsetOrEncoding, length)
.
For example:
class Foo {
[Symbol.toPrimitive]() {
return 'this is a test';
}
}
const buf = Buffer.from(new Foo(), 'utf8');
// <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
Class Method: Buffer.isBuffer(obj)#
Returns true
if obj
is a Buffer
, false
otherwise.
Class Method: Buffer.isEncoding(encoding)#
Returns true
if encoding
contains a supported character encoding, or false
otherwise.
Class Property: Buffer.poolSize#
- <integer> Default:
8192
This is the number of bytes used to determine the size of pre-allocated, internal
Buffer
instances used for pooling. This value may be modified.
buf[index]#
The index operator [index]
can be used to get and set the octet at position
index
in buf
. The values refer to individual bytes, so the legal value
range is between 0x00
and 0xFF
(hex) or 0
and 255
(decimal).
This operator is inherited from Uint8Array
, so its behavior on out-of-bounds
access is the same as UInt8Array
- that is, getting returns undefined
and
setting does nothing.
Example: Copy an ASCII string into a Buffer
, one byte at a time
const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);
for (let i = 0; i < str.length; i++) {
buf[i] = str.charCodeAt(i);
}
// Prints: Node.js
console.log(buf.toString('ascii'));
buf.buffer#
The buffer
property references the underlying ArrayBuffer
object based on
which this Buffer object is created.
const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);
console.log(buffer.buffer === arrayBuffer);
// Prints: true
buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])#
target
<Buffer> | <Uint8Array> ABuffer
orUint8Array
to compare totargetStart
<integer> The offset withintarget
at which to begin comparison. Default:0
targetEnd
<integer> The offset withtarget
at which to end comparison (not inclusive). Ignored whentargetStart
isundefined
. Default:target.length
sourceStart
<integer> The offset withinbuf
at which to begin comparison. Ignored whentargetStart
isundefined
. Default:0
sourceEnd
<integer> The offset withinbuf
at which to end comparison (not inclusive). Ignored whentargetStart
isundefined
. Default:buf.length
- Returns: <integer>
Compares buf
with target
and returns a number indicating whether buf
comes before, after, or is the same as target
in sort order.
Comparison is based on the actual sequence of bytes in each Buffer
.
0
is returned iftarget
is the same asbuf
1
is returned iftarget
should come beforebuf
when sorted.-1
is returned iftarget
should come afterbuf
when sorted.
Examples:
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');
// Prints: 0
console.log(buf1.compare(buf1));
// Prints: -1
console.log(buf1.compare(buf2));
// Prints: -1
console.log(buf1.compare(buf3));
// Prints: 1
console.log(buf2.compare(buf1));
// Prints: 1
console.log(buf2.compare(buf3));
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2])
console.log([buf1, buf2, buf3].sort(Buffer.compare));
The optional targetStart
, targetEnd
, sourceStart
, and sourceEnd
arguments can be used to limit the comparison to specific ranges within target
and buf
respectively.
Examples:
const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);
// Prints: 0
console.log(buf1.compare(buf2, 5, 9, 0, 4));
// Prints: -1
console.log(buf1.compare(buf2, 0, 6, 4));
// Prints: 1
console.log(buf1.compare(buf2, 5, 6, 5));
A RangeError
will be thrown if: targetStart < 0
, sourceStart < 0
,
targetEnd > target.byteLength
or sourceEnd > source.byteLength
.
buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])#
target
<Buffer> | <Uint8Array> ABuffer
orUint8Array
to copy into.targetStart
<integer> The offset withintarget
at which to begin copying to. Default:0
sourceStart
<integer> The offset withinbuf
at which to begin copying from. Ignored whentargetStart
isundefined
. Default:0
sourceEnd
<integer> The offset withinbuf
at which to stop copying (not inclusive). Ignored whensourceStart
isundefined
. Default:buf.length
- Returns: <integer> The number of bytes copied.
Copies data from a region of buf
to a region in target
even if the target
memory region overlaps with buf
.
Example: Create two Buffer
instances, buf1
and buf2
, and copy buf1
from
byte 16 through byte 19 into buf2
, starting at the 8th byte in buf2
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'
buf1[i] = i + 97;
}
buf1.copy(buf2, 8, 16, 20);
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!
console.log(buf2.toString('ascii', 0, 25));
Example: Create a single Buffer
and copy data from one region to an
overlapping region within the same Buffer
const buf = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'
buf[i] = i + 97;
}
buf.copy(buf, 0, 4, 10);
// Prints: efghijghijklmnopqrstuvwxyz
console.log(buf.toString());
buf.entries()#
- Returns: <Iterator>
Creates and returns an iterator of [index, byte]
pairs from the contents of
buf
.
Example: Log the entire contents of a Buffer
const buf = Buffer.from('buffer');
// Prints:
// [0, 98]
// [1, 117]
// [2, 102]
// [3, 102]
// [4, 101]
// [5, 114]
for (const pair of buf.entries()) {
console.log(pair);
}
buf.equals(otherBuffer)#
otherBuffer
<Buffer> ABuffer
orUint8Array
to compare to- Returns: <boolean>
Returns true
if both buf
and otherBuffer
have exactly the same bytes,
false
otherwise.
Examples:
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');
// Prints: true
console.log(buf1.equals(buf2));
// Prints: false
console.log(buf1.equals(buf3));
buf.fill(value[, offset[, end]][, encoding])#
value
<string> | <Buffer> | <integer> The value to fillbuf
withoffset
<integer> Where to start fillingbuf
. Default:0
end
<integer> Where to stop fillingbuf
(not inclusive). Default:buf.length
encoding
<string> Ifvalue
is a string, this is its encoding. Default:'utf8'
- Returns: <Buffer> A reference to
buf
Fills buf
with the specified value
. If the offset
and end
are not given,
the entire buf
will be filled. This is meant to be a small simplification to
allow the creation and filling of a Buffer
to be done on a single line.
Example: Fill a Buffer
with the ASCII character 'h'
const b = Buffer.allocUnsafe(50).fill('h');
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
console.log(b.toString());
value
is coerced to a uint32
value if it is not a String or Integer.
If the final write of a fill()
operation falls on a multi-byte character,
then only the first bytes of that character that fit into buf
are written.
Example: Fill a Buffer
with a two-byte character
// Prints: <Buffer c8 a2 c8>
console.log(Buffer.allocUnsafe(3).fill('\u0222'));
buf.includes(value[, byteOffset][, encoding])#
value
<string> | <Buffer> | <integer> What to search forbyteOffset
<integer> Where to begin searching inbuf
. Default:0
encoding
<string> Ifvalue
is a string, this is its encoding. Default:'utf8'
- Returns: <boolean>
true
ifvalue
was found inbuf
,false
otherwise
Equivalent to buf.indexOf() !== -1
.
Examples:
const buf = Buffer.from('this is a buffer');
// Prints: true
console.log(buf.includes('this'));
// Prints: true
console.log(buf.includes('is'));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));
// Prints: true
// (97 is the decimal ASCII value for 'a')
console.log(buf.includes(97));
// Prints: false
console.log(buf.includes(Buffer.from('a buffer example')));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
// Prints: false
console.log(buf.includes('this', 4));
buf.indexOf(value[, byteOffset][, encoding])#
value
<string> | <Buffer> | <Uint8Array> | <integer> What to search forbyteOffset
<integer> Where to begin searching inbuf
. Default:0
encoding
<string> Ifvalue
is a string, this is its encoding. Default:'utf8'
- Returns: <integer> The index of the first occurrence of
value
inbuf
or-1
ifbuf
does not containvalue
If value
is:
- a string,
value
is interpreted according to the character encoding inencoding
. - a
Buffer
orUint8Array
,value
will be used in its entirety. To compare a partialBuffer
, usebuf.slice()
. - a number,
value
will be interpreted as an unsigned 8-bit integer value between0
and255
.
Examples:
const buf = Buffer.from('this is a buffer');
// Prints: 0
console.log(buf.indexOf('this'));
// Prints: 2
console.log(buf.indexOf('is'));
// Prints: 8
console.log(buf.indexOf(Buffer.from('a buffer')));
// Prints: 8
// (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(97));
// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example')));
// Prints: 8
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');
// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', 0, 'ucs2'));
// Prints: 6
console.log(utf16Buffer.indexOf('\u03a3', -4, 'ucs2'));
If value
is not a string, number, or Buffer
, this method will throw a
TypeError
. If value
is a number, it will be coerced to a valid byte value,
an integer between 0 and 255.
If byteOffset
is not a number, it will be coerced to a number. Any arguments
that coerce to NaN
or 0, like {}
, []
, null
or undefined
, will search
the whole buffer. This behavior matches String#indexOf()
.
const b = Buffer.from('abcdef');
// Passing a value that's a number, but not a valid byte
// Prints: 2, equivalent to searching for 99 or 'c'
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));
// Passing a byteOffset that coerces to NaN or 0
// Prints: 1, searching the whole buffer
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));
If value
is an empty string or empty Buffer
and byteOffset
is less
than buf.length
, byteOffset
will be returned. If value
is empty and
byteOffset
is at least buf.length
, buf.length
will be returned.
buf.keys()#
- Returns: <Iterator>
Creates and returns an iterator of buf
keys (indices).
Example:
const buf = Buffer.from('buffer');
// Prints:
// 0
// 1
// 2
// 3
// 4
// 5
for (const key of buf.keys()) {
console.log(key);
}
buf.lastIndexOf(value[, byteOffset][, encoding])#
value
<string> | <Buffer> | <Uint8Array> | <integer> What to search forbyteOffset
<integer> Where to begin searching inbuf
. Default:buf.length
- 1
encoding
<string> Ifvalue
is a string, this is its encoding. Default:'utf8'
- Returns: <integer> The index of the last occurrence of
value
inbuf
or-1
ifbuf
does not containvalue
Identical to buf.indexOf()
, except buf
is searched from back to front
instead of front to back.
Examples:
const buf = Buffer.from('this buffer is a buffer');
// Prints: 0
console.log(buf.lastIndexOf('this'));
// Prints: 17
console.log(buf.lastIndexOf('buffer'));
// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));
// Prints: 15
// (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(97));
// Prints: -1
console.log(buf.lastIndexOf(Buffer.from('yolo')));
// Prints: 5
console.log(buf.lastIndexOf('buffer', 5));
// Prints: -1
console.log(buf.lastIndexOf('buffer', 4));
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');
// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'ucs2'));
// Prints: 4
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'ucs2'));
If value
is not a string, number, or Buffer
, this method will throw a
TypeError
. If value
is a number, it will be coerced to a valid byte value,
an integer between 0 and 255.
If byteOffset
is not a number, it will be coerced to a number. Any arguments
that coerce to NaN
, like {}
or undefined
, will search the whole buffer.
This behavior matches String#lastIndexOf()
.
const b = Buffer.from('abcdef');
// Passing a value that's a number, but not a valid byte
// Prints: 2, equivalent to searching for 99 or 'c'
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));
// Passing a byteOffset that coerces to NaN
// Prints: 1, searching the whole buffer
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));
// Passing a byteOffset that coerces to 0
// Prints: -1, equivalent to passing 0
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));
If value
is an empty string or empty Buffer
, byteOffset
will be returned.
buf.length#
Returns the amount of memory allocated for buf
in bytes. Note that this
does not necessarily reflect the amount of "usable" data within buf
.
Example: Create a Buffer
and write a shorter ASCII string to it
const buf = Buffer.alloc(1234);
// Prints: 1234
console.log(buf.length);
buf.write('some string', 0, 'ascii');
// Prints: 1234
console.log(buf.length);
While the length
property is not immutable, changing the value of length
can result in undefined and inconsistent behavior. Applications that wish to
modify the length of a Buffer
should therefore treat length
as read-only and
use buf.slice()
to create a new Buffer
.
Examples:
let buf = Buffer.allocUnsafe(10);
buf.write('abcdefghj', 0, 'ascii');
// Prints: 10
console.log(buf.length);
buf = buf.slice(0, 5);
// Prints: 5
console.log(buf.length);
buf.parent#
Stability: 0 - Deprecated: Use buf.buffer
instead.
The buf.parent
property is a deprecated alias for buf.buffer
.
buf.readDoubleBE(offset[, noAssert])#
buf.readDoubleLE(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 8
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <number>
Reads a 64-bit double from buf
at the specified offset
with specified
endian format (readDoubleBE()
returns big endian, readDoubleLE()
returns
little endian).
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Examples:
const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);
// Prints: 8.20788039913184e-304
console.log(buf.readDoubleBE());
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE());
// Throws an exception: RangeError: Index out of range
console.log(buf.readDoubleLE(1));
// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!
console.log(buf.readDoubleLE(1, true));
buf.readFloatBE(offset[, noAssert])#
buf.readFloatLE(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 4
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <number>
Reads a 32-bit float from buf
at the specified offset
with specified
endian format (readFloatBE()
returns big endian, readFloatLE()
returns
little endian).
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Examples:
const buf = Buffer.from([1, 2, 3, 4]);
// Prints: 2.387939260590663e-38
console.log(buf.readFloatBE());
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE());
// Throws an exception: RangeError: Index out of range
console.log(buf.readFloatLE(1));
// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!
console.log(buf.readFloatLE(1, true));
buf.readInt8(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 1
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <integer>
Reads a signed 8-bit integer from buf
at the specified offset
.
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Integers read from a Buffer
are interpreted as two's complement signed values.
Examples:
const buf = Buffer.from([-1, 5]);
// Prints: -1
console.log(buf.readInt8(0));
// Prints: 5
console.log(buf.readInt8(1));
// Throws an exception: RangeError: Index out of range
console.log(buf.readInt8(2));
buf.readInt16BE(offset[, noAssert])#
buf.readInt16LE(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 2
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <integer>
Reads a signed 16-bit integer from buf
at the specified offset
with
the specified endian format (readInt16BE()
returns big endian,
readInt16LE()
returns little endian).
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Integers read from a Buffer
are interpreted as two's complement signed values.
Examples:
const buf = Buffer.from([0, 5]);
// Prints: 5
console.log(buf.readInt16BE());
// Prints: 1280
console.log(buf.readInt16LE());
// Throws an exception: RangeError: Index out of range
console.log(buf.readInt16LE(1));
buf.readInt32BE(offset[, noAssert])#
buf.readInt32LE(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 4
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <integer>
Reads a signed 32-bit integer from buf
at the specified offset
with
the specified endian format (readInt32BE()
returns big endian,
readInt32LE()
returns little endian).
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Integers read from a Buffer
are interpreted as two's complement signed values.
Examples:
const buf = Buffer.from([0, 0, 0, 5]);
// Prints: 5
console.log(buf.readInt32BE());
// Prints: 83886080
console.log(buf.readInt32LE());
// Throws an exception: RangeError: Index out of range
console.log(buf.readInt32LE(1));
buf.readIntBE(offset, byteLength[, noAssert])#
buf.readIntLE(offset, byteLength[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - byteLength
byteLength
<integer> How many bytes to read. Must satisfy:0 < byteLength <= 6
noAssert
<boolean> Skipoffset
andbyteLength
validation? Default:false
- Returns: <integer>
Reads byteLength
number of bytes from buf
at the specified offset
and interprets the result as a two's complement signed value. Supports up to 48
bits of accuracy.
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
// Prints: -546f87a9cbee
console.log(buf.readIntLE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(0, 6).toString(16));
// Throws an exception: RangeError: Index out of range
console.log(buf.readIntBE(1, 6).toString(16));
buf.readUInt8(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 1
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <integer>
Reads an unsigned 8-bit integer from buf
at the specified offset
.
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Examples:
const buf = Buffer.from([1, -2]);
// Prints: 1
console.log(buf.readUInt8(0));
// Prints: 254
console.log(buf.readUInt8(1));
// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt8(2));
buf.readUInt16BE(offset[, noAssert])#
buf.readUInt16LE(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 2
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <integer>
Reads an unsigned 16-bit integer from buf
at the specified offset
with
specified endian format (readUInt16BE()
returns big endian, readUInt16LE()
returns little endian).
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56]);
// Prints: 1234
console.log(buf.readUInt16BE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3456
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(1).toString(16));
// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt16LE(2).toString(16));
buf.readUInt32BE(offset[, noAssert])#
buf.readUInt32LE(offset[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - 4
noAssert
<boolean> Skipoffset
validation? Default:false
- Returns: <integer>
Reads an unsigned 32-bit integer from buf
at the specified offset
with
specified endian format (readUInt32BE()
returns big endian,
readUInt32LE()
returns little endian).
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);
// Prints: 12345678
console.log(buf.readUInt32BE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(0).toString(16));
// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt32LE(1).toString(16));
buf.readUIntBE(offset, byteLength[, noAssert])#
buf.readUIntLE(offset, byteLength[, noAssert])#
offset
<integer> Where to start reading. Must satisfy:0 <= offset <= buf.length - byteLength
byteLength
<integer> How many bytes to read. Must satisfy:0 < byteLength <= 6
noAssert
<boolean> Skipoffset
andbyteLength
validation? Default:false
- Returns: <integer>
Reads byteLength
number of bytes from buf
at the specified offset
and interprets the result as an unsigned integer. Supports up to 48
bits of accuracy.
Setting noAssert
to true
allows offset
to be beyond the end of buf
, but
the result should be considered undefined behavior.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);
// Prints: 1234567890ab
console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: ab9078563412
console.log(buf.readUIntLE(0, 6).toString(16));
// Throws an exception: RangeError: Index out of range
console.log(buf.readUIntBE(1, 6).toString(16));
buf.slice([start[, end]])#
start
<integer> Where the newBuffer
will start. Default:0
end
<integer> Where the newBuffer
will end (not inclusive). Default:buf.length
- Returns: <Buffer>
Returns a new Buffer
that references the same memory as the original, but
offset and cropped by the start
and end
indices.
Note: Modifying the new Buffer
slice will modify the memory in the
original Buffer
because the allocated memory of the two objects overlap.
Example: Create a Buffer
with the ASCII alphabet, take a slice, and then modify
one byte from the original Buffer
const buf1 = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'
buf1[i] = i + 97;
}
const buf2 = buf1.slice(0, 3);
// Prints: abc
console.log(buf2.toString('ascii', 0, buf2.length));
buf1[0] = 33;
// Prints: !bc
console.log(buf2.toString('ascii', 0, buf2.length));
Specifying negative indexes causes the slice to be generated relative to the
end of buf
rather than the beginning.
Examples:
const buf = Buffer.from('buffer');
// Prints: buffe
// (Equivalent to buf.slice(0, 5))
console.log(buf.slice(-6, -1).toString());
// Prints: buff
// (Equivalent to buf.slice(0, 4))
console.log(buf.slice(-6, -2).toString());
// Prints: uff
// (Equivalent to buf.slice(1, 4))
console.log(buf.slice(-5, -2).toString());
buf.swap16()#
- Returns: <Buffer> A reference to
buf
Interprets buf
as an array of unsigned 16-bit integers and swaps the byte-order
in-place. Throws a RangeError
if buf.length
is not a multiple of 2.
Examples:
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);
buf1.swap16();
// Prints: <Buffer 02 01 04 03 06 05 08 07>
console.log(buf1);
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
// Throws an exception: RangeError: Buffer size must be a multiple of 16-bits
buf2.swap16();
buf.swap32()#
- Returns: <Buffer> A reference to
buf
Interprets buf
as an array of unsigned 32-bit integers and swaps the byte-order
in-place. Throws a RangeError
if buf.length
is not a multiple of 4.
Examples:
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);
buf1.swap32();
// Prints: <Buffer 04 03 02 01 08 07 06 05>
console.log(buf1);
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
// Throws an exception: RangeError: Buffer size must be a multiple of 32-bits
buf2.swap32();
buf.swap64()#
- Returns: <Buffer> A reference to
buf
Interprets buf
as an array of 64-bit numbers and swaps the byte-order in-place.
Throws a RangeError
if buf.length
is not a multiple of 8.
Examples:
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);
// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);
buf1.swap64();
// Prints: <Buffer 08 07 06 05 04 03 02 01>
console.log(buf1);
const buf2 = Buffer.from([0x1, 0x2, 0x3]);
// Throws an exception: RangeError: Buffer size must be a multiple of 64-bits
buf2.swap64();
Note that JavaScript cannot encode 64-bit integers. This method is intended for working with 64-bit floats.
buf.toJSON()#
- Returns: <Object>
Returns a JSON representation of buf
. JSON.stringify()
implicitly calls
this function when stringifying a Buffer
instance.
Example:
const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);
// Prints: {"type":"Buffer","data":[1,2,3,4,5]}
console.log(json);
const copy = JSON.parse(json, (key, value) => {
return value && value.type === 'Buffer' ?
Buffer.from(value.data) :
value;
});
// Prints: <Buffer 01 02 03 04 05>
console.log(copy);
buf.toString([encoding[, start[, end]]])#
encoding
<string> The character encoding to decode to. Default:'utf8'
start
<integer> The byte offset to start decoding at. Default:0
end
<integer> The byte offset to stop decoding at (not inclusive). Default:buf.length
- Returns: <string>
Decodes buf
to a string according to the specified character encoding in
encoding
. start
and end
may be passed to decode only a subset of buf
.
The maximum length of a string instance (in UTF-16 code units) is available
as buffer.constants.MAX_STRING_LENGTH
.
Examples:
const buf1 = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
// 97 is the decimal ASCII value for 'a'
buf1[i] = i + 97;
}
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('ascii'));
// Prints: abcde
console.log(buf1.toString('ascii', 0, 5));
const buf2 = Buffer.from('tést');
// Prints: 74c3a97374
console.log(buf2.toString('hex'));
// Prints: té
console.log(buf2.toString('utf8', 0, 3));
// Prints: té
console.log(buf2.toString(undefined, 0, 3));
buf.values()#
- Returns: <Iterator>
Creates and returns an iterator for buf
values (bytes). This function is
called automatically when a Buffer
is used in a for..of
statement.
Examples:
const buf = Buffer.from('buffer');
// Prints:
// 98
// 117
// 102
// 102
// 101
// 114
for (const value of buf.values()) {
console.log(value);
}
// Prints:
// 98
// 117
// 102
// 102
// 101
// 114
for (const value of buf) {
console.log(value);
}
buf.write(string[, offset[, length]][, encoding])#
string
<string> String to be written tobuf
offset
<integer> Where to start writingstring
. Default:0
length
<integer> How many bytes to write. Default:buf.length - offset
encoding
<string> The character encoding ofstring
. Default:'utf8'
- Returns: <integer> Number of bytes written
Writes string
to buf
at offset
according to the character encoding in encoding
.
The length
parameter is the number of bytes to write. If buf
did not contain
enough space to fit the entire string, only a partial amount of string
will
be written. However, partially encoded characters will not be written.
Example:
const buf = Buffer.allocUnsafe(256);
const len = buf.write('\u00bd + \u00bc = \u00be', 0);
// Prints: 12 bytes: ½ + ¼ = ¾
console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
buf.writeDoubleBE(value, offset[, noAssert])#
buf.writeDoubleLE(value, offset[, noAssert])#
value
<number> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 8
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
with specified endian
format (writeDoubleBE()
writes big endian, writeDoubleLE()
writes little
endian). value
should be a valid 64-bit double. Behavior is undefined when
value
is anything other than a 64-bit double.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
Examples:
const buf = Buffer.allocUnsafe(8);
buf.writeDoubleBE(0xdeadbeefcafebabe, 0);
// Prints: <Buffer 43 eb d5 b7 dd f9 5f d7>
console.log(buf);
buf.writeDoubleLE(0xdeadbeefcafebabe, 0);
// Prints: <Buffer d7 5f f9 dd b7 d5 eb 43>
console.log(buf);
buf.writeFloatBE(value, offset[, noAssert])#
buf.writeFloatLE(value, offset[, noAssert])#
value
<number> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 4
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
with specified endian
format (writeFloatBE()
writes big endian, writeFloatLE()
writes little
endian). value
should be a valid 32-bit float. Behavior is undefined when
value
is anything other than a 32-bit float.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
Examples:
const buf = Buffer.allocUnsafe(4);
buf.writeFloatBE(0xcafebabe, 0);
// Prints: <Buffer 4f 4a fe bb>
console.log(buf);
buf.writeFloatLE(0xcafebabe, 0);
// Prints: <Buffer bb fe 4a 4f>
console.log(buf);
buf.writeInt8(value, offset[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 1
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
. value
should be a valid
signed 8-bit integer. Behavior is undefined when value
is anything other than
a signed 8-bit integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
value
is interpreted and written as a two's complement signed integer.
Examples:
const buf = Buffer.allocUnsafe(2);
buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);
// Prints: <Buffer 02 fe>
console.log(buf);
buf.writeInt16BE(value, offset[, noAssert])#
buf.writeInt16LE(value, offset[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 2
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
with specified endian
format (writeInt16BE()
writes big endian, writeInt16LE()
writes little
endian). value
should be a valid signed 16-bit integer. Behavior is undefined
when value
is anything other than a signed 16-bit integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
value
is interpreted and written as a two's complement signed integer.
Examples:
const buf = Buffer.allocUnsafe(4);
buf.writeInt16BE(0x0102, 0);
buf.writeInt16LE(0x0304, 2);
// Prints: <Buffer 01 02 04 03>
console.log(buf);
buf.writeInt32BE(value, offset[, noAssert])#
buf.writeInt32LE(value, offset[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 4
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
with specified endian
format (writeInt32BE()
writes big endian, writeInt32LE()
writes little
endian). value
should be a valid signed 32-bit integer. Behavior is undefined
when value
is anything other than a signed 32-bit integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
value
is interpreted and written as a two's complement signed integer.
Examples:
const buf = Buffer.allocUnsafe(8);
buf.writeInt32BE(0x01020304, 0);
buf.writeInt32LE(0x05060708, 4);
// Prints: <Buffer 01 02 03 04 08 07 06 05>
console.log(buf);
buf.writeIntBE(value, offset, byteLength[, noAssert])#
buf.writeIntLE(value, offset, byteLength[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - byteLength
byteLength
<integer> How many bytes to write. Must satisfy:0 < byteLength <= 6
noAssert
<boolean> Skipvalue
,offset
, andbyteLength
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes byteLength
bytes of value
to buf
at the specified offset
.
Supports up to 48 bits of accuracy. Behavior is undefined when value
is
anything other than a signed integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
Examples:
const buf = Buffer.allocUnsafe(6);
buf.writeUIntBE(0x1234567890ab, 0, 6);
// Prints: <Buffer 12 34 56 78 90 ab>
console.log(buf);
buf.writeUIntLE(0x1234567890ab, 0, 6);
// Prints: <Buffer ab 90 78 56 34 12>
console.log(buf);
buf.writeUInt8(value, offset[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 1
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
. value
should be a
valid unsigned 8-bit integer. Behavior is undefined when value
is anything
other than an unsigned 8-bit integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
Examples:
const buf = Buffer.allocUnsafe(4);
buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);
// Prints: <Buffer 03 04 23 42>
console.log(buf);
buf.writeUInt16BE(value, offset[, noAssert])#
buf.writeUInt16LE(value, offset[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 2
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
with specified endian
format (writeUInt16BE()
writes big endian, writeUInt16LE()
writes little
endian). value
should be a valid unsigned 16-bit integer. Behavior is
undefined when value
is anything other than an unsigned 16-bit integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
Examples:
const buf = Buffer.allocUnsafe(4);
buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);
// Prints: <Buffer de ad be ef>
console.log(buf);
buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);
// Prints: <Buffer ad de ef be>
console.log(buf);
buf.writeUInt32BE(value, offset[, noAssert])#
buf.writeUInt32LE(value, offset[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - 4
noAssert
<boolean> Skipvalue
andoffset
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes value
to buf
at the specified offset
with specified endian
format (writeUInt32BE()
writes big endian, writeUInt32LE()
writes little
endian). value
should be a valid unsigned 32-bit integer. Behavior is
undefined when value
is anything other than an unsigned 32-bit integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
Examples:
const buf = Buffer.allocUnsafe(4);
buf.writeUInt32BE(0xfeedface, 0);
// Prints: <Buffer fe ed fa ce>
console.log(buf);
buf.writeUInt32LE(0xfeedface, 0);
// Prints: <Buffer ce fa ed fe>
console.log(buf);
buf.writeUIntBE(value, offset, byteLength[, noAssert])#
buf.writeUIntLE(value, offset, byteLength[, noAssert])#
value
<integer> Number to be written tobuf
offset
<integer> Where to start writing. Must satisfy:0 <= offset <= buf.length - byteLength
byteLength
<integer> How many bytes to write. Must satisfy:0 < byteLength <= 6
noAssert
<boolean> Skipvalue
,offset
, andbyteLength
validation? Default:false
- Returns: <integer>
offset
plus the number of bytes written
Writes byteLength
bytes of value
to buf
at the specified offset
.
Supports up to 48 bits of accuracy. Behavior is undefined when value
is
anything other than an unsigned integer.
Setting noAssert
to true
allows the encoded form of value
to extend beyond
the end of buf
, but the result should be considered undefined behavior.
Examples:
const buf = Buffer.allocUnsafe(6);
buf.writeUIntBE(0x1234567890ab, 0, 6);
// Prints: <Buffer 12 34 56 78 90 ab>
console.log(buf);
buf.writeUIntLE(0x1234567890ab, 0, 6);
// Prints: <Buffer ab 90 78 56 34 12>
console.log(buf);
buffer.INSPECT_MAX_BYTES#
- <integer> Default:
50
Returns the maximum number of bytes that will be returned when
buf.inspect()
is called. This can be overridden by user modules. See
util.inspect()
for more details on buf.inspect()
behavior.
Note that this is a property on the buffer
module returned by
require('buffer')
, not on the Buffer
global or a Buffer
instance.
buffer.kMaxLength#
- <integer> The largest size allowed for a single
Buffer
instance
An alias for buffer.constants.MAX_LENGTH
Note that this is a property on the buffer
module returned by
require('buffer')
, not on the Buffer
global or a Buffer
instance.
buffer.transcode(source, fromEnc, toEnc)#
source
<Buffer> | <Uint8Array> ABuffer
orUint8Array
instancefromEnc
<string> The current encodingtoEnc
<string> To target encoding
Re-encodes the given Buffer
or Uint8Array
instance from one character
encoding to another. Returns a new Buffer
instance.
Throws if the fromEnc
or toEnc
specify invalid character encodings or if
conversion from fromEnc
to toEnc
is not permitted.
The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:
const buffer = require('buffer');
const newBuf = buffer.transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'
Because the Euro (€
) sign is not representable in US-ASCII, it is replaced
with ?
in the transcoded Buffer
.
Note that this is a property on the buffer
module returned by
require('buffer')
, not on the Buffer
global or a Buffer
instance.
Class: SlowBuffer#
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow()
instead.
Returns an un-pooled Buffer
.
In order to avoid the garbage collection overhead of creating many individually
allocated Buffer
instances, by default allocations under 4KB are sliced from a
single larger allocated object. This approach improves both performance and memory
usage since v8 does not need to track and cleanup as many Persistent
objects.
In the case where a developer may need to retain a small chunk of memory from a
pool for an indeterminate amount of time, it may be appropriate to create an
un-pooled Buffer
instance using SlowBuffer
then copy out the relevant bits.
Example:
// Need to keep around a few small chunks of memory
const store = [];
socket.on('readable', () => {
const data = socket.read();
// Allocate for retained data
const sb = SlowBuffer(10);
// Copy the data into the new allocation
data.copy(sb, 0, 0, 10);
store.push(sb);
});
Use of SlowBuffer
should be used only as a last resort after a developer
has observed undue memory retention in their applications.
new SlowBuffer(size)#
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow()
instead.
size
<integer> The desired length of the newSlowBuffer
Allocates a new Buffer
of size
bytes. If the size
is larger than
buffer.constants.MAX_LENGTH
or smaller than 0, a RangeError
will be
thrown. A zero-length Buffer
will be created if size
is 0.
The underlying memory for SlowBuffer
instances is not initialized. The
contents of a newly created SlowBuffer
are unknown and may contain
sensitive data. Use buf.fill(0)
to initialize a SlowBuffer
to zeroes.
Example:
const { SlowBuffer } = require('buffer');
const buf = new SlowBuffer(5);
// Prints: (contents may vary): <Buffer 78 e0 82 02 01>
console.log(buf);
buf.fill(0);
// Prints: <Buffer 00 00 00 00 00>
console.log(buf);
Buffer Constants#
Note that buffer.constants
is a property on the buffer
module returned by
require('buffer')
, not on the Buffer
global or a Buffer
instance.
buffer.constants.MAX_LENGTH#
- <integer> The largest size allowed for a single
Buffer
instance
On 32-bit architectures, this value is (2^30)-1
(~1GB).
On 64-bit architectures, this value is (2^31)-1
(~2GB).
This value is also available as buffer.kMaxLength
.
buffer.constants.MAX_STRING_LENGTH#
- <integer> The largest length allowed for a single
string
instance
Represents the largest length
that a string
primitive can have, counted
in UTF-16 code units.
This value may depend on the JS engine that is being used.
C++ Addons#
Node.js Addons are dynamically-linked shared objects, written in C++, that
can be loaded into Node.js using the require()
function, and used
just as if they were an ordinary Node.js module. They are used primarily to
provide an interface between JavaScript running in Node.js and C/C++ libraries.
At the moment, the method for implementing Addons is rather complicated, involving knowledge of several components and APIs :
V8: the C++ library Node.js currently uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the
v8.h
header file (deps/v8/include/v8.h
in the Node.js source tree), which is also available online.libuv: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the filesystem, sockets, timers and system events. libuv also provides a pthreads-like threading abstraction that may be used to power more sophisticated asynchronous Addons that need to move beyond the standard event loop. Addon authors are encouraged to think about how to avoid blocking the event loop with I/O or other time-intensive tasks by off-loading work via libuv to non-blocking system operations, worker threads or a custom use of libuv's threads.
Internal Node.js libraries. Node.js itself exports a number of C++ APIs that Addons can use — the most important of which is the
node::ObjectWrap
class.Node.js includes a number of other statically linked libraries including OpenSSL. These other libraries are located in the
deps/
directory in the Node.js source tree. Only the V8 and OpenSSL symbols are purposefully re-exported by Node.js and may be used to various extents by Addons. See Linking to Node.js' own dependencies for additional information.
All of the following examples are available for download and may be used as the starting-point for an Addon.
Hello world#
This "Hello world" example is a simple Addon, written in C++, that is the equivalent of the following JavaScript code:
module.exports.hello = () => 'world';
First, create the file hello.cc
:
// hello.cc
#include <node.h>
namespace demo {
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void Method(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
args.GetReturnValue().Set(String::NewFromUtf8(isolate, "world"));
}
void init(Local<Object> exports) {
NODE_SET_METHOD(exports, "hello", Method);
}
NODE_MODULE(addon, init)
} // namespace demo
Note that all Node.js Addons must export an initialization function following the pattern:
void Initialize(Local<Object> exports);
NODE_MODULE(module_name, Initialize)
There is no semi-colon after NODE_MODULE
as it's not a function (see
node.h
).
The module_name
must match the filename of the final binary (excluding
the .node suffix).
In the hello.cc
example, then, the initialization function is init
and the
Addon module name is addon
.
Building#
Once the source code has been written, it must be compiled into the binary
addon.node
file. To do so, create a file called binding.gyp
in the
top-level of the project describing the build configuration of the module
using a JSON-like format. This file is used by node-gyp -- a tool written
specifically to compile Node.js Addons.
{
"targets": [
{
"target_name": "addon",
"sources": [ "hello.cc" ]
}
]
}
Note: A version of the node-gyp
utility is bundled and distributed with
Node.js as part of npm
. This version is not made directly available for
developers to use and is intended only to support the ability to use the
npm install
command to compile and install Addons. Developers who wish to
use node-gyp
directly can install it using the command
npm install -g node-gyp
. See the node-gyp
installation instructions for
more information, including platform-specific requirements.
Once the binding.gyp
file has been created, use node-gyp configure
to
generate the appropriate project build files for the current platform. This
will generate either a Makefile
(on Unix platforms) or a vcxproj
file
(on Windows) in the build/
directory.
Next, invoke the node-gyp build
command to generate the compiled addon.node
file. This will be put into the build/Release/
directory.
When using npm install
to install a Node.js Addon, npm uses its own bundled
version of node-gyp
to perform this same set of actions, generating a
compiled version of the Addon for the user's platform on demand.
Once built, the binary Addon can be used from within Node.js by pointing
require()
to the built addon.node
module:
// hello.js
const addon = require('./build/Release/addon');
console.log(addon.hello());
// Prints: 'world'
Please see the examples below for further information or https://github.com/arturadib/node-qt for an example in production.
Because the exact path to the compiled Addon binary can vary depending on how
it is compiled (i.e. sometimes it may be in ./build/Debug/
), Addons can use
the bindings package to load the compiled module.
Note that while the bindings
package implementation is more sophisticated
in how it locates Addon modules, it is essentially using a try-catch pattern
similar to:
try {
return require('./build/Release/addon.node');
} catch (err) {
return require('./build/Debug/addon.node');
}
Linking to Node.js' own dependencies#
Node.js uses a number of statically linked libraries such as V8, libuv and
OpenSSL. All Addons are required to link to V8 and may link to any of the
other dependencies as well. Typically, this is as simple as including
the appropriate #include <...>
statements (e.g. #include <v8.h>
) and
node-gyp
will locate the appropriate headers automatically. However, there
are a few caveats to be aware of:
When
node-gyp
runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, Addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available.node-gyp
can be run using the--nodedir
flag pointing at a local Node.js source image. Using this option, the Addon will have access to the full set of dependencies.
Loading Addons using require()#
The filename extension of the compiled Addon binary is .node
(as opposed
to .dll
or .so
). The require()
function is written to look for
files with the .node
file extension and initialize those as dynamically-linked
libraries.
When calling require()
, the .node
extension can usually be
omitted and Node.js will still find and initialize the Addon. One caveat,
however, is that Node.js will first attempt to locate and load modules or
JavaScript files that happen to share the same base name. For instance, if
there is a file addon.js
in the same directory as the binary addon.node
,
then require('addon')
will give precedence to the addon.js
file
and load it instead.
Native Abstractions for Node.js#
Each of the examples illustrated in this document make direct use of the Node.js and V8 APIs for implementing Addons. It is important to understand that the V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, Addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do currently to ensure stability of the V8 APIs.
The Native Abstractions for Node.js (or nan
) provide a set of tools that
Addon developers are recommended to use to keep compatibility between past and
future releases of V8 and Node.js. See the nan
examples for an
illustration of how it can be used.
N-API#
Stability: 1 - Experimental
N-API is an API for building native Addons. It is independent from the underlying JavaScript runtime (ex V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across version of Node.js. It is intended to insulate Addons from changes in the underlying JavaScript engine and allow modules compiled for one version to run on later versions of Node.js without recompilation. Addons are built/packaged with the same approach/tools outlined in this document (node-gyp, etc.). The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the N-API are used.
The functions available and how to use them are documented in the section titled C/C++ Addons - N-API.
Addon examples#
Following are some example Addons intended to help developers get started. The examples make use of the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.
Each of these examples using the following binding.gyp
file:
{
"targets": [
{
"target_name": "addon",
"sources": [ "addon.cc" ]
}
]
}
In cases where there is more than one .cc
file, simply add the additional
filename to the sources
array. For example:
"sources": ["addon.cc", "myexample.cc"]
Once the binding.gyp
file is ready, the example Addons can be configured and
built using node-gyp
:
$ node-gyp configure build
Function arguments#
Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.
The following example illustrates how to read function arguments passed from JavaScript and how to return a result:
// addon.cc
#include <node.h>
namespace demo {
using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;
// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
// Check the number of arguments passed.
if (args.Length() < 2) {
// Throw an Error that is passed back to JavaScript
isolate->ThrowException(Exception::TypeError(
String::NewFromUtf8(isolate, "Wrong number of arguments")));
return;
}
// Check the argument types
if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
isolate->ThrowException(Exception::TypeError(
String::NewFromUtf8(isolate, "Wrong arguments")));
return;
}
// Perform the operation
double value = args[0]->NumberValue() + args[1]->NumberValue();
Local<Number> num = Number::New(isolate, value);
// Set the return value (using the passed in
// FunctionCallbackInfo<Value>&)
args.GetReturnValue().Set(num);
}
void Init(Local<Object> exports) {
NODE_SET_METHOD(exports, "add", Add);
}
NODE_MODULE(addon, Init)
} // namespace demo
Once compiled, the example Addon can be required and used from within Node.js:
// test.js
const addon = require('./build/Release/addon');
console.log('This should be eight:', addon.add(3, 5));
Callbacks#
It is common practice within Addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:
// addon.cc
#include <node.h>
namespace demo {
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Null;
using v8::Object;
using v8::String;
using v8::Value;
void RunCallback(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Function> cb = Local<Function>::Cast(args[0]);
const unsigned argc = 1;
Local<Value> argv[argc] = { String::NewFromUtf8(isolate, "hello world") };
cb->Call(Null(isolate), argc, argv);
}
void Init(Local<Object> exports, Local<Object> module) {
NODE_SET_METHOD(module, "exports", RunCallback);
}
NODE_MODULE(addon, Init)
} // namespace demo
Note that this example uses a two-argument form of Init()
that receives
the full module
object as the second argument. This allows the Addon
to completely overwrite exports
with a single function instead of
adding the function as a property of exports
.
To test it, run the following JavaScript:
// test.js
const addon = require('./build/Release/addon');
addon((msg) => {
console.log(msg);
// Prints: 'hello world'
});
Note that, in this example, the callback function is invoked synchronously.
Object factory#
Addons can create and return new objects from within a C++ function as
illustrated in the following example. An object is created and returned with a
property msg
that echoes the string passed to createObject()
:
// addon.cc
#include <node.h>
namespace demo {
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void CreateObject(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<Object> obj = Object::New(isolate);
obj->Set(String::NewFromUtf8(isolate, "msg"), args[0]->ToString());
args.GetReturnValue().Set(obj);
}
void Init(Local<Object> exports, Local<Object> module) {
NODE_SET_METHOD(module, "exports", CreateObject);
}
NODE_MODULE(addon, Init)
} // namespace demo
To test it in JavaScript:
// test.js
const addon = require('./build/Release/addon');
const obj1 = addon('hello');
const obj2 = addon('world');
console.log(obj1.msg, obj2.msg);
// Prints: 'hello world'
Function factory#
Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:
// addon.cc
#include <node.h>
namespace demo {
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void MyFunction(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
args.GetReturnValue().Set(String::NewFromUtf8(isolate, "hello world"));
}
void CreateFunction(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
Local<Function> fn = tpl->GetFunction();
// omit this to make it anonymous
fn->SetName(String::NewFromUtf8(isolate, "theFunction"));
args.GetReturnValue().Set(fn);
}
void Init(Local<Object> exports, Local<Object> module) {
NODE_SET_METHOD(module, "exports", CreateFunction);
}
NODE_MODULE(addon, Init)
} // namespace demo
To test:
// test.js
const addon = require('./build/Release/addon');
const fn = addon();
console.log(fn());
// Prints: 'hello world'
Wrapping C++ objects#
It is also possible to wrap C++ objects/classes in a way that allows new
instances to be created using the JavaScript new
operator:
// addon.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using v8::Local;
using v8::Object;
void InitAll(Local<Object> exports) {
MyObject::Init(exports);
}
NODE_MODULE(addon, InitAll)
} // namespace demo
Then, in myobject.h
, the wrapper class inherits from node::ObjectWrap
:
// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H
#include <node.h>
#include <node_object_wrap.h>
namespace demo {
class MyObject : public node::ObjectWrap {
public:
static void Init(v8::Local<v8::Object> exports);
private:
explicit MyObject(double value = 0);
~MyObject();
static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
static v8::Persistent<v8::Function> constructor;
double value_;
};
} // namespace demo
#endif
In myobject.cc
, implement the various methods that are to be exposed.
Below, the method plusOne()
is exposed by adding it to the constructor's
prototype:
// myobject.cc
#include "myobject.h"
namespace demo {
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;
Persistent<Function> MyObject::constructor;
MyObject::MyObject(double value) : value_(value) {
}
MyObject::~MyObject() {
}
void MyObject::Init(Local<Object> exports) {
Isolate* isolate = exports->GetIsolate();
// Prepare constructor template
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
tpl->InstanceTemplate()->SetInternalFieldCount(1);
// Prototype
NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);
constructor.Reset(isolate, tpl->GetFunction());
exports->Set(String::NewFromUtf8(isolate, "MyObject"),
tpl->GetFunction());
}
void MyObject::New(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
if (args.IsConstructCall()) {
// Invoked as constructor: `new MyObject(...)`
double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
MyObject* obj = new MyObject(value);
obj->Wrap(args.This());
args.GetReturnValue().Set(args.This());
} else {
// Invoked as plain function `MyObject(...)`, turn into construct call.
const int argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Context> context = isolate->GetCurrentContext();
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Object> result =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(result);
}
}
void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
obj->value_ += 1;
args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}
} // namespace demo
To build this example, the myobject.cc
file must be added to the
binding.gyp
:
{
"targets": [
{
"target_name": "addon",
"sources": [
"addon.cc",
"myobject.cc"
]
}
]
}
Test it with:
// test.js
const addon = require('./build/Release/addon');
const obj = new addon.MyObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13
Factory of wrapped objects#
Alternatively, it is possible to use a factory pattern to avoid explicitly
creating object instances using the JavaScript new
operator:
const obj = addon.createObject();
// instead of:
// const obj = new addon.Object();
First, the createObject()
method is implemented in addon.cc
:
// addon.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;
void CreateObject(const FunctionCallbackInfo<Value>& args) {
MyObject::NewInstance(args);
}
void InitAll(Local<Object> exports, Local<Object> module) {
MyObject::Init(exports->GetIsolate());
NODE_SET_METHOD(module, "exports", CreateObject);
}
NODE_MODULE(addon, InitAll)
} // namespace demo
In myobject.h
, the static method NewInstance()
is added to handle
instantiating the object. This method takes the place of using new
in
JavaScript:
// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H
#include <node.h>
#include <node_object_wrap.h>
namespace demo {
class MyObject : public node::ObjectWrap {
public:
static void Init(v8::Isolate* isolate);
static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
private:
explicit MyObject(double value = 0);
~MyObject();
static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
static v8::Persistent<v8::Function> constructor;
double value_;
};
} // namespace demo
#endif
The implementation in myobject.cc
is similar to the previous example:
// myobject.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;
Persistent<Function> MyObject::constructor;
MyObject::MyObject(double value) : value_(value) {
}
MyObject::~MyObject() {
}
void MyObject::Init(Isolate* isolate) {
// Prepare constructor template
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
tpl->InstanceTemplate()->SetInternalFieldCount(1);
// Prototype
NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);
constructor.Reset(isolate, tpl->GetFunction());
}
void MyObject::New(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
if (args.IsConstructCall()) {
// Invoked as constructor: `new MyObject(...)`
double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
MyObject* obj = new MyObject(value);
obj->Wrap(args.This());
args.GetReturnValue().Set(args.This());
} else {
// Invoked as plain function `MyObject(...)`, turn into construct call.
const int argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Context> context = isolate->GetCurrentContext();
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
}
void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
const unsigned argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Context> context = isolate->GetCurrentContext();
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
obj->value_ += 1;
args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}
} // namespace demo
Once again, to build this example, the myobject.cc
file must be added to the
binding.gyp
:
{
"targets": [
{
"target_name": "addon",
"sources": [
"addon.cc",
"myobject.cc"
]
}
]
}
Test it with:
// test.js
const createObject = require('./build/Release/addon');
const obj = createObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13
const obj2 = createObject(20);
console.log(obj2.plusOne());
// Prints: 21
console.log(obj2.plusOne());
// Prints: 22
console.log(obj2.plusOne());
// Prints: 23
Passing wrapped objects around#
In addition to wrapping and returning C++ objects, it is possible to pass
wrapped objects around by unwrapping them with the Node.js helper function
node::ObjectWrap::Unwrap
. The following examples shows a function add()
that can take two MyObject
objects as input arguments:
// addon.cc
#include <node.h>
#include <node_object_wrap.h>
#include "myobject.h"
namespace demo {
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;
void CreateObject(const FunctionCallbackInfo<Value>& args) {
MyObject::NewInstance(args);
}
void Add(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
args[0]->ToObject());
MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
args[1]->ToObject());
double sum = obj1->value() + obj2->value();
args.GetReturnValue().Set(Number::New(isolate, sum));
}
void InitAll(Local<Object> exports) {
MyObject::Init(exports->GetIsolate());
NODE_SET_METHOD(exports, "createObject", CreateObject);
NODE_SET_METHOD(exports, "add", Add);
}
NODE_MODULE(addon, InitAll)
} // namespace demo
In myobject.h
, a new public method is added to allow access to private values
after unwrapping the object.
// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H
#include <node.h>
#include <node_object_wrap.h>
namespace demo {
class MyObject : public node::ObjectWrap {
public:
static void Init(v8::Isolate* isolate);
static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
inline double value() const { return value_; }
private:
explicit MyObject(double value = 0);
~MyObject();
static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
static v8::Persistent<v8::Function> constructor;
double value_;
};
} // namespace demo
#endif
The implementation of myobject.cc
is similar to before:
// myobject.cc
#include <node.h>
#include "myobject.h"
namespace demo {
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;
Persistent<Function> MyObject::constructor;
MyObject::MyObject(double value) : value_(value) {
}
MyObject::~MyObject() {
}
void MyObject::Init(Isolate* isolate) {
// Prepare constructor template
Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
tpl->InstanceTemplate()->SetInternalFieldCount(1);
constructor.Reset(isolate, tpl->GetFunction());
}
void MyObject::New(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
if (args.IsConstructCall()) {
// Invoked as constructor: `new MyObject(...)`
double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
MyObject* obj = new MyObject(value);
obj->Wrap(args.This());
args.GetReturnValue().Set(args.This());
} else {
// Invoked as plain function `MyObject(...)`, turn into construct call.
const int argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Context> context = isolate->GetCurrentContext();
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
}
void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
const unsigned argc = 1;
Local<Value> argv[argc] = { args[0] };
Local<Function> cons = Local<Function>::New(isolate, constructor);
Local<Context> context = isolate->GetCurrentContext();
Local<Object> instance =
cons->NewInstance(context, argc, argv).ToLocalChecked();
args.GetReturnValue().Set(instance);
}
} // namespace demo
Test it with:
// test.js
const addon = require('./build/Release/addon');
const obj1 = addon.createObject(10);
const obj2 = addon.createObject(20);
const result = addon.add(obj1, obj2);
console.log(result);
// Prints: 30
AtExit hooks#
An "AtExit" hook is a function that is invoked after the Node.js event loop
has ended but before the JavaScript VM is terminated and Node.js shuts down.
"AtExit" hooks are registered using the node::AtExit
API.
void AtExit(callback, args)#
callback
:void (*)(void*)
- A pointer to the function to call at exit.args
:void*
- A pointer to pass to the callback at exit.
Registers exit hooks that run after the event loop has ended but before the VM is killed.
AtExit takes two parameters: a pointer to a callback function to run at exit, and a pointer to untyped context data to be passed to that callback.
Callbacks are run in last-in first-out order.
The following addon.cc
implements AtExit:
// addon.cc
#undef NDEBUG
#include <assert.h>
#include <stdlib.h>
#include <node.h>
namespace demo {
using node::AtExit;
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Object;
static char cookie[] = "yum yum";
static int at_exit_cb1_called = 0;
static int at_exit_cb2_called = 0;
static void at_exit_cb1(void* arg) {
Isolate* isolate = static_cast<Isolate*>(arg);
HandleScope scope(isolate);
Local<Object> obj = Object::New(isolate);
assert(!obj.IsEmpty()); // assert VM is still alive
assert(obj->IsObject());
at_exit_cb1_called++;
}
static void at_exit_cb2(void* arg) {
assert(arg == static_cast<void*>(cookie));
at_exit_cb2_called++;
}
static void sanity_check(void*) {
assert(at_exit_cb1_called == 1);
assert(at_exit_cb2_called == 2);
}
void init(Local<Object> exports) {
AtExit(sanity_check);
AtExit(at_exit_cb2, cookie);
AtExit(at_exit_cb2, cookie);
AtExit(at_exit_cb1, exports->GetIsolate());
}
NODE_MODULE(addon, init)
} // namespace demo
Test in JavaScript by running:
// test.js
const addon = require('./build/Release/addon');
N-API#
Stability: 1 - Experimental
N-API (pronounced N as in the letter, followed by API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (ex V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate Addons from changes in the underlying JavaScript engine and allow modules compiled for one version to run on later versions of Node.js without recompilation.
Addons are built/packaged with the same approach/tools outlined in the section titled C++ Addons. The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the N-API are used.
APIs exposed by N-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA262 Language Specification. The APIs have the following properties:
- All N-API calls return a status code of type
napi_status
. This status indicates whether the API call succeeded or failed. - The API's return value is passed via an out parameter.
- All JavaScript values are abstracted behind an opaque type named
napi_value
. - In case of an error status code, additional information can be obtained
using
napi_get_last_error_info
. More information can be found in the error handling section Error Handling.
The documentation for N-API is structured as follows:
- Basic N-API Data Types
- Error Handling
- Object Lifetime Management
- Module Registration
- Working with JavaScript Values
- Working with JavaScript Values - Abstract Operations
- Working with JavaScript Properties
- Working with JavaScript Functions
- Object Wrap
- Asynchronous Operations
The N-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. However, we also understand that a C++ API can be easier to use in many cases. To support these cases we expect there to be one or more C++ wrapper modules that provide an inlineable C++ API. Binaries built with these wrapper modules will depend on the symbols for the N-API C based functions exported by Node.js. These wrappers are not part of N-API, nor will they be maintained as part of Node.js. One such example is: node-api.
In order to use the N-API functions, include the file node_api.h which is located in the src directory in the node development tree. For example:
#include <node_api.h>
As the feature is experimental it must be enabled with the following command line option:
--napi-modules
Basic N-API Data Types#
N-API exposes the following fundamental datatypes as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other N-API calls.
napi_status#
Integral status code indicating the success or failure of a N-API call. Currently, the following status codes are supported.
typedef enum {
napi_ok,
napi_invalid_arg,
napi_object_expected,
napi_string_expected,
napi_name_expected,
napi_function_expected,
napi_number_expected,
napi_boolean_expected,
napi_array_expected,
napi_generic_failure,
napi_pending_exception,
napi_cancelled,
napi_status_last
} napi_status;
If additional information is required upon an API returning a failed status,
it can be obtained by calling napi_get_last_error_info
.
napi_extended_error_info#
typedef struct {
const char* error_message;
void* engine_reserved;
uint32_t engine_error_code;
napi_status error_code;
} napi_extended_error_info;
error_message
: UTF8-encoded string containing a VM-neutral description of the error.engine_reserved
: Reserved for VM-specific error details. This is currently not implemented for any VM.engine_error_code
: VM-specific error code. This is currently not implemented for any VM.error_code
: The N-API status code that originated with the last error.
See the Error Handling section for additional information.
napi_env#
napi_env
is used to represent a context that the underlying N-API
implementation can use to persist VM-specific state. This structure is passed
to native functions when they're invoked, and it must be passed back when
making N-API calls. Specifically, the same napi_env
that was passed in when
the initial native function was called must be passed to any subsequent
nested N-API calls. Caching the napi_env
for the purpose of general reuse is
not allowed.
napi_value#
This is an opaque pointer that is used to represent a JavaScript value.
N-API Memory Management types#
napi_handle_scope#
This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, N-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, N-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values.
Handle scopes are created using napi_open_handle_scope
and are destroyed
using napi_close_handle_scope
. Closing the scope can indicate to the GC that
all napi_value
s created during the lifetime of the handle scope are no longer
referenced from the current stack frame.
For more details, review the Object Lifetime Management.
napi_escapable_handle_scope#
Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope.
napi_ref#
This is the abstraction to use to reference a napi_value
. This allows for
users to manage the lifetimes of JavaScript values, including defining their
minimum lifetimes explicitly.
For more details, review the Object Lifetime Management.
N-API Callback types#
napi_callback_info#
Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked.
napi_callback#
Function pointer type for user-provided native functions which are to be exposed to JavaScript via N-API. Callback functions should satisfy the following signature:
typedef napi_value (*napi_callback)(napi_env, napi_callback_info);
napi_finalize#
Function pointer type for add-on provided functions that allow the user to be
notified when externally-owned data is ready to be cleaned up because the
object with which it was associated with, has been garbage-collected. The user
must provide a function satisfying the following signature which would get
called upon the object's collection. Currently, napi_finalize
can be used for
finding out when objects that have external data are collected.
typedef void (*napi_finalize)(napi_env env,
void* finalize_data,
void* finalize_hint);
napi_async_execute_callback#
Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature:
typedef void (*napi_async_execute_callback)(napi_env env, void* data);
napi_async_complete_callback#
Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature:
typedef void (*napi_async_complete_callback)(napi_env env,
napi_status status,
void* data);
Error Handling#
N-API uses both return values and Javascript exceptions for error handling. The following sections explain the approach for each case.
Return values#
All of the N-API functions share the same error handling pattern. The
return type of all API functions is napi_status
.
The return value will be napi_ok
if the request was successful and
no uncaught JavaScript exception was thrown. If an error occurred AND
an exception was thrown, the napi_status
value for the error
will be returned. If an exception was thrown, and no error occurred,
napi_pending_exception
will be returned.
In cases where a return value other than napi_ok
or
napi_pending_exception
is returned, napi_is_exception_pending
must be called to check if an exception is pending.
See the section on exceptions for more details.
The full set of possible napi_status values is defined
in napi_api_types.h
.
The napi_status
return value provides a VM-independent representation of
the error which occurred. In some cases it is useful to be able to get
more detailed information, including a string representing the error as well as
VM (engine)-specific information.
In order to retrieve this information napi_get_last_error_info
is provided which returns a napi_extended_error_info
structure.
The format of the napi_extended_error_info
structure is as follows:
typedef struct napi_extended_error_info {
const char* error_message;
void* engine_reserved;
uint32_t engine_error_code;
napi_status error_code;
};
error_message
: Textual representation of the error that occurred.engine_reserved
: Opaque handle reserved for engine use only.engine_error_code
: VM specific error code.error_code
: n-api status code for the last error.
napi_get_last_error_info
returns the information for the last
N-API call that was made.
Note: Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.
napi_get_last_error_info#
NAPI_EXTERN napi_status
napi_get_last_error_info(napi_env env,
const napi_extended_error_info** result);
[in] env
: The environment that the API is invoked under.[out] result
: Thenapi_extended_error_info
structure with more information about the error.
Returns napi_ok
if the API succeeded.
This API retrieves a napi_extended_error_info
structure with information
about the last error that occurred.
Note: The content of the napi_extended_error_info
returned is only
valid up until an n-api function is called on the same env
.
Note: Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.
Exceptions#
Any N-API function call may result in a pending JavaScript exception. This is obviously the case for any function that may cause the execution of JavaScript, but N-API specifies that an exception may be pending on return from any of the API functions.
If the napi_status
returned by a function is napi_ok
then no
exception is pending and no additional action is required. If the
napi_status
returned is anything other than napi_ok
or
napi_pending_exception
, in order to try to recover and continue
instead of simply returning immediately, napi_is_exception_pending
must be called in order to determine if an exception is pending or not.
When an exception is pending one of two approaches can be employed.
The first appoach is to do any appropriate cleanup and then return so that
execution will return to JavaScript. As part of the transition back to
JavaScript the exception will be thrown at the point in the JavaScript
code where the native method was invoked. The behavior of most N-API calls
is unspecified while an exception is pending, and many will simply return
napi_pending_exception
, so it is important to do as little as possible
and then return to JavaScript where the exception can be handled.
The second approach is to try to handle the exception. There will be cases
where the native code can catch the exception, take the appropriate action,
and then continue. This is only recommended in specific cases
where it is known that the exception can be safely handled. In these
cases napi_get_and_clear_last_exception
can be used to get and
clear the exception. On success, result will contain the handle to
the last JavaScript Object thrown. If it is determined, after
retrieving the exception, the exception cannot be handled after all
it can be re-thrown it with napi_throw
where error is the
JavaScript Error object to be thrown.
The following utility functions are also available in case native code
needs to throw an exception or determine if a napi_value
is an instance
of a JavaScript Error
object: napi_throw_error
,
napi_throw_type_error
, napi_throw_range_error
and
napi_is_error
.
The following utility functions are also available in case native
code needs to create an Error object: napi_create_error
,
napi_create_type_error
, and napi_create_range_error
.
where result is the napi_value that refers to the newly created
JavaScript Error object.
The Node.js project is adding error codes to all of the errors
generated internally. The goal is for applications to use these
error codes for all error checking. The associated error messages
will remain, but will only be meant to be used for logging and
display with the expectation that the message can change without
SemVer applying. In order to support this model with N-API, both
in internal functionality and for module specific functionality
(as its good practice), the throw_
and create_
functions
take an optional code parameter which is the string for the code
to be added to the error object. If the optional parameter is NULL
then no code will be associated with the error. If a code is provided,
the name associated with the error is also updated to be:
originalName [code]
where originalName is the original name associated with the error and code is the code that was provided. For example if the code is 'ERR_ERROR_1' and a TypeError is being created the name will be:
TypeError [ERR_ERROR_1]
napi_throw#
NODE_EXTERN napi_status napi_throw(napi_env env, napi_value error);
[in] env
: The environment that the API is invoked under.[in] error
: Thenapi_value
for the Error to be thrown.
Returns napi_ok
if the API succeeded.
This API throws the JavaScript Error provided.
napi_throw_error#
NODE_EXTERN napi_status napi_throw_error(napi_env env,
const char* code,
const char* msg);
[in] env
: The environment that the API is invoked under.[in] code
: Optional error code to be set on the error.[in] msg
: C string representing the text to be associated with the error.
Returns napi_ok
if the API succeeded.
This API throws a JavaScript Error with the text provided.
napi_throw_type_error#
NODE_EXTERN napi_status napi_throw_type_error(napi_env env,
const char* code,
const char* msg);
[in] env
: The environment that the API is invoked under.[in] code
: Optional error code to be set on the error.[in] msg
: C string representing the text to be associated with the error.
Returns napi_ok
if the API succeeded.
This API throws a JavaScript TypeError with the text provided.
napi_throw_range_error#
NODE_EXTERN napi_status napi_throw_range_error(napi_env env,
const char* code,
const char* msg);
[in] env
: The environment that the API is invoked under.[in] code
: Optional error code to be set on the error.[in] msg
: C string representing the text to be associated with the error.
Returns napi_ok
if the API succeeded.
This API throws a JavaScript RangeError with the text provided.
napi_is_error#
NODE_EXTERN napi_status napi_is_error(napi_env env,
napi_value value,
bool* result);
[in] env
: The environment that the API is invoked under.[in] msg
: Thenapi_value
to be checked.[out] result
: Boolean value that is set to true ifnapi_value
represents an error, false otherwise.
Returns napi_ok
if the API succeeded.
This API queries a napi_value
to check if it represents an error object.
napi_create_error#
NODE_EXTERN napi_status napi_create_error(napi_env env,
napi_value code,
napi_value msg,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] code
: Optionalnapi_value
with the string for the error code tobe associated with the error.
[in] msg
: napi_value that references a JavaScript String to be used as the message for the Error.[out] result
:napi_value
representing the error created.
Returns napi_ok
if the API succeeded.
This API returns a JavaScript Error with the text provided.
napi_create_type_error#
NODE_EXTERN napi_status napi_create_type_error(napi_env env,
napi_value code,
napi_value msg,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] code
: Optionalnapi_value
with the string for the error code tobe associated with the error.
[in] msg
: napi_value that references a JavaScript String to be used as the message for the Error.[out] result
:napi_value
representing the error created.
Returns napi_ok
if the API succeeded.
This API returns a JavaScript TypeError with the text provided.
napi_create_range_error#
NODE_EXTERN napi_status napi_create_range_error(napi_env env,
napi_value code,
const char* msg,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] code
: Optionalnapi_value
with the string for the error code tobe associated with the error.
[in] msg
: napi_value that references a JavaScript String to be used as the message for the Error.[out] result
:napi_value
representing the error created.
Returns napi_ok
if the API succeeded.
This API returns a JavaScript RangeError with the text provided.
napi_get_and_clear_last_exception#
NAPI_EXTERN napi_status napi_get_and_clear_last_exception(napi_env env,
napi_value* result);
[in] env
: The environment that the API is invoked under.[out] result
: The exception if one is pending, NULL otherwise.
Returns napi_ok
if the API succeeded.
This API returns true if an exception is pending.
napi_is_exception_pending#
NAPI_EXTERN napi_status napi_is_exception_pending(napi_env env, bool* result);
[in] env
: The environment that the API is invoked under.[out] result
: Boolean value that is set to true if an exception is pending.
Returns napi_ok
if the API succeeded.
This API returns true if an exception is pending.
Fatal Errors#
In the event of an unrecoverable error in a native module, a fatal error can be thrown to immediately terminate the process.
napi_fatal_error#
NAPI_EXTERN NAPI_NO_RETURN void napi_fatal_error(const char* location, const char* message);
[in] location
: Optional location at which the error occurred.[in] message
: The message associated with the error.
The function call does not return, the process will be terminated.
Object Lifetime management#
As N-API calls are made, handles to objects in the heap for the underlying
VM may be returned as napi_values
. These handles must hold the
objects 'live' until they are no longer required by the native code,
otherwise the objects could be collected before the native code was
finished using them.
As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.
In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the N-API functions than can be used to change the handle lifespan from the default.
Making handle lifespan shorter than that of the native method#
It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array:
for (int i = 0; i < 1000000; i++) {
napi_value result;
napi_status status = napi_get_element(e object, i, &result);
if (status != napi_ok) {
break;
}
// do something with element
}
This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope.
To handle this case, N-API provides the ability to establish a new 'scope' to
which newly created handles will be associated. Once those handles
are no longer required, the scope can be 'closed' and any handles associated
with the scope are invalidated. The methods available to open/close scopes are
napi_open_handle_scope
and napi_close_handle_scope
.
N-API only supports a single nested hiearchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method.
Taking the earlier example, adding calls to napi_open_handle_scope
and
napi_close_handle_scope
would ensure that at most a single handle
is valid throughout the execution of the loop:
for (int i = 0; i < 1000000; i++) {
napi_handle_scope scope;
napi_status status = napi_open_handle_scope(env, &scope);
if (status != napi_ok) {
break;
}
napi_value result;
status = napi_get_element(e object, i, &result);
if (status != napi_ok) {
break;
}
// do something with element
status = napi_close_handle_scope(env, scope);
if (status != napi_ok) {
break;
}
}
When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. N-API supports an 'escapable scope' in order to support this case. An escapable scope allows one handle to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope.
The methods available to open/close escapable scopes are
napi_open_escapable_handle_scope
and napi_close_escapable_handle_scope
.
The request to promote a handle is made through napi_escape_handle
which
can only be called once.
napi_open_handle_scope#
NODE_EXTERN napi_status napi_open_handle_scope(napi_env env,
napi_handle_scope* result);
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing the new scope.
Returns napi_ok
if the API succeeded.
This API open a new scope.
napi_close_handle_scope#
NODE_EXTERN napi_status napi_close_handle_scope(napi_env env,
napi_handle_scope scope);
[in] env
: The environment that the API is invoked under.[in] scope
:napi_value
representing the scope to be closed.
Returns napi_ok
if the API succeeded.
This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.
napi_open_escapable_handle_scope#
NODE_EXTERN napi_status
napi_open_escapable_handle_scope(napi_env env,
napi_handle_scope* result);
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing the new scope.
Returns napi_ok
if the API succeeded.
This API open a new scope from which one object can be promoted to the outer scope.
napi_close_escapable_handle_scope#
NODE_EXTERN napi_status
napi_close_escapable_handle_scope(napi_env env,
napi_handle_scope scope);
[in] env
: The environment that the API is invoked under.[in] scope
:napi_value
representing the scope to be closed.
Returns napi_ok
if the API succeeded.
This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.
napi_escape_handle#
NAPI_EXTERN napi_status napi_escape_handle(napi_env env,
napi_escapable_handle_scope scope,
napi_value escapee,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] scope
:napi_value
representing the current scope.[in] escapee
:napi_value
representing the JavaScript Object to be escaped.[out] result
:napi_value
representing the handle to the escaped Object in the outer scope.
Returns napi_ok
if the API succeeded.
This API promotes the handle to the JavaScript object so that it is valid for the lifetime of the outer scope. It can only be called once per scope. If it is called more than once an error will be returned.
References to objects with a lifespan longer than that of the native method#
In some cases an addon will need to be able to create and reference objects
with a lifespan longer than that of a single native method invocation. For
example, to create a constructor and later use that constructor
in a request to creates instances, it must be possible to reference
the constructor object across many different instance creation requests. This
would not be possible with a normal handle returned as a napi_value
as
described in the earlier section. The lifespan of a normal handle is
managed by scopes and all scopes must be closed before the end of a native
method.
N-API provides methods to create persistent references to an object. Each persistent reference has an associated count with a value of 0 or higher. The count determines if the reference will keep the corresponding object live. References with a count of 0 do not prevent the object from being collected and are often called 'weak' references. Any count greater than 0 will prevent the object from being collected.
References can be created with an initial reference count. The count can
then be modified through napi_reference_ref
and
napi_reference_unref
. If an object is collected while the count
for a reference is 0, all subsequent calls to
get the object associated with the reference napi_get_reference_value
will return NULL for the returned napi_value
. An attempt to call
napi_reference_ref
for a reference whose object has been collected
will result in an error.
References must be deleted once they are no longer required by the addon. When a reference is deleted it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference will result in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.
There can be multiple persistent references created which refer to the same object, each of which will either keep the object live or not based on its individual count.
napi_create_reference#
NODE_EXTERN napi_status napi_create_reference(napi_env env,
napi_value value,
int initial_refcount,
napi_ref* result);
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing the Object to which we want a reference to.[in] initial_refcount
: Initial reference count for the new reference.[out] result
:napi_ref
pointing to the new reference.
Returns napi_ok
if the API succeeded.
This API create a new reference with the specified reference count to the Object passed in.
napi_delete_reference#
NODE_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref);
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
to be deleted.
Returns napi_ok
if the API succeeded.
This API deletes the reference passed in.
napi_reference_ref#
NODE_EXTERN napi_status napi_reference_ref(napi_env env,
napi_ref ref,
int* result);
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
for which the reference count will be incremented.[out] result
: The new reference count.
Returns napi_ok
if the API succeeded.
This API increments the reference count for the reference passed in and returns the resulting reference count.
napi_reference_unref#
NODE_EXTERN napi_status napi_reference_unref(napi_env env,
napi_ref ref,
int* result);
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
for which the reference count will be decremented.[out] result
: The new reference count.
Returns napi_ok
if the API succeeded.
This API decrements the reference count for the reference passed in and returns the resulting reference count.
napi_get_reference_value#
NODE_EXTERN napi_status napi_get_reference_value(napi_env env,
napi_ref ref,
napi_value* result);
the napi_value passed
in or out of these methods is a handle to the
object to which the reference is related.
[in] env
: The environment that the API is invoked under.[in] ref
:napi_ref
for which we requesting the corresponding Object.[out] result
: Thenapi_value
for the Object referenced by thenapi_ref
.
Returns napi_ok
if the API succeeded.
If still valid, this API returns the napi_value
representing the
JavaScript Object associated with the napi_ref
. Otherise, result
will be NULL.
Module registration#
N-API modules are registered in the same manner as other modules
except that instead of using the NODE_MODULE
macro the following
is used:
NAPI_MODULE(addon, Init)
The next difference is the signature for the Init
method. For a N-API
module it is as follows:
void Init(napi_env env, napi_value exports, napi_value module, void* priv);
As with any other module, functions are exported by either adding them to
the exports
or module
objects passed to the Init
method.
For example, to add the method hello
as a function so that it can be called
as a method provided by the addon:
void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
napi_status status;
napi_property_descriptor desc =
{"hello", Method, 0, 0, 0, napi_default, 0};
status = napi_define_properties(env, exports, 1, &desc);
}
For example, to set a function to be returned by the require()
for the addon:
void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
napi_status status;
napi_property_descriptor desc =
{"exports", Method, 0, 0, 0, napi_default, 0};
status = napi_define_properties(env, module, 1, &desc);
}
For example, to define a class so that new instances can be created (often used with Object Wrap):
// NOTE: partial example, not all referenced code is included
napi_status status;
napi_property_descriptor properties[] = {
{ "value", nullptr, GetValue, SetValue, 0, napi_default, 0 },
DECLARE_NAPI_METHOD("plusOne", PlusOne),
DECLARE_NAPI_METHOD("multiply", Multiply),
};
napi_value cons;
status =
napi_define_class(env, "MyObject", New, nullptr, 3, properties, &cons);
if (status != napi_ok) return;
status = napi_create_reference(env, cons, 1, &constructor);
if (status != napi_ok) return;
status = napi_set_named_property(env, exports, "MyObject", cons);
if (status != napi_ok) return;
For more details on setting properties on either the exports
or module
objects, see the section on
Working with JavaScript Properties.
For more details on building addon modules in general, refer to the existing API
Working with JavaScript Values#
N-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section 6 of the ECMAScript Language Specification.
Fundamentally, these APIs are used to do one of the following:
- Create a new JavaScript object
- Convert from a primitive C type to an N-API value
- Convert from N-API value to a primitive C type
- Get global instances including
undefined
andnull
N-API values are represented by the type napi_value
.
Any N-API call that requires a JavaScript value takes in a napi_value
.
In some cases, the API does check the type of the napi_value
up-front.
However, for better performance, it's better for the caller to make sure that
the napi_value
in question is of the JavaScript type expected by the API.
Enum types#
napi_valuetype#
typedef enum {
// ES6 types (corresponds to typeof)
napi_undefined,
napi_null,
napi_boolean,
napi_number,
napi_string,
napi_symbol,
napi_object,
napi_function,
napi_external,
} napi_valuetype;
Describes the type of a napi_value
. This generally corresponds to the types
described in
Section 6.1 of
the ECMAScript Language Specification.
In addition to types in that section, napi_valuetype
can also represent
Functions and Objects with external data.
napi_typedarray_type#
typedef enum {
napi_int8_array,
napi_uint8_array,
napi_uint8_clamped_array,
napi_int16_array,
napi_uint16_array,
napi_int32_array,
napi_uint32_array,
napi_float32_array,
napi_float64_array,
} napi_typedarray_type;
This represents the underlying binary scalar datatype of the TypedArray. Elements of this enum correspond to Section 22.2 of the ECMAScript Language Specification.
Object Creation Functions#
napi_create_array#
napi_status napi_create_array(napi_env env, napi_value* result)
[in] env
: The environment that the N-API call is invoked under.[out] result
: Anapi_value
representing a JavaScript Array.
Returns napi_ok
if the API succeeded.
This API returns an N-API value corresponding to a JavaScript Array type. JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.
napi_create_array_with_length#
napi_status napi_create_array_with_length(napi_env env,
size_t length,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: The initial length of the Array.[out] result
: Anapi_value
representing a JavaScript Array.
Returns napi_ok
if the API succeeded.
This API returns an N-API value corresponding to a JavaScript Array type.
The Array's length property is set to the passed-in length parameter.
However, the underlying buffer is not guaranteed to be pre-allocated by the VM
when the array is created - that behavior is left to the underlying VM
implementation.
If the buffer must be a contiguous block of memory that can be
directly read and/or written via C, consider using
napi_create_external_arraybuffer
.
JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.
napi_create_arraybuffer#
napi_status napi_create_arraybuffer(napi_env env,
size_t byte_length,
void** data,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: The length in bytes of the array buffer to create.[out] data
: Pointer to the underlying byte buffer of the ArrayBuffer.[out] result
: Anapi_value
representing a JavaScript ArrayBuffer.
Returns napi_ok
if the API succeeded.
This API returns an N-API value corresponding to a JavaScript ArrayBuffer.
ArrayBuffers are used to represent fixed-length binary data buffers. They are
normally used as a backing-buffer for TypedArray objects.
The ArrayBuffer allocated will have an underlying byte buffer whose size is
determined by the length
parameter that's passed in.
The underlying buffer is optionally returned back to the caller in case the
caller wants to directly manipulate the buffer. This buffer can only be
written to directly from native code. To write to this buffer from JavaScript,
a typed array or DataView object would need to be created.
JavaScript ArrayBuffer objects are described in Section 24.1 of the ECMAScript Language Specification.
napi_create_buffer#
napi_status napi_create_buffer(napi_env env,
size_t size,
void** data,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] size
: Size in bytes of the underlying buffer.[out] data
: Raw pointer to the underlying buffer.[out] result
: Anapi_value
representing anode::Buffer
.
Returns napi_ok
if the API succeeded.
This API allocates a node::Buffer
object. While this is still a
fully-supported data structure, in most cases using a TypedArray will suffice.
napi_create_buffer_copy#
napi_status napi_create_buffer_copy(napi_env env,
size_t length,
const void* data,
void** result_data,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] size
: Size in bytes of the input buffer (should be the same as the size of the new buffer).[in] data
: Raw pointer to the underlying buffer to copy from.[out] result_data
: Pointer to the new Buffer's underlying data buffer.[out] result
: Anapi_value
representing anode::Buffer
.
Returns napi_ok
if the API succeeded.
This API allocates a node::Buffer
object and initializes it with data copied
from the passed-in buffer. While this is still a fully-supported data
structure, in most cases using a TypedArray will suffice.
napi_create_external#
napi_status napi_create_external(napi_env env,
void* data,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] data
: Raw pointer to the external data.[in] finalize_cb
: Optional callback to call when the external value is being collected.[in] finalize_hint
: Optional hint to pass to the finalize callback during collection.[out] result
: Anapi_value
representing an external value.
Returns napi_ok
if the API succeeded.
This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code. The API allows the caller to pass in a finalize callback, in case the underlying native resource needs to be cleaned up when the external JavaScript value gets collected.
Note: The created value is not an object, and therefore does not support
additional properties. It is considered a distinct value type: calling
napi_typeof()
with an external value yields napi_external
.
napi_create_external_arraybuffer#
napi_status
napi_create_external_arraybuffer(napi_env env,
void* external_data,
size_t byte_length,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] external_data
: Pointer to the underlying byte buffer of the ArrayBuffer.[in] byte_length
: The length in bytes of the underlying buffer.[in] finalize_cb
: Optional callback to call when the ArrayBuffer is being collected.[in] finalize_hint
: Optional hint to pass to the finalize callback during collection.[out] result
: Anapi_value
representing a JavaScript ArrayBuffer.
Returns napi_ok
if the API succeeded.
This API returns an N-API value corresponding to a JavaScript ArrayBuffer. The underlying byte buffer of the ArrayBuffer is externally allocated and managed. The caller must ensure that the byte buffer remains valid until the finalize callback is called.
JavaScript ArrayBuffers are described in Section 24.1 of the ECMAScript Language Specification.
napi_create_external_buffer#
napi_status napi_create_external_buffer(napi_env env,
size_t length,
void* data,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: Size in bytes of the input buffer (should be the same as the size of the new buffer).[in] data
: Raw pointer to the underlying buffer to copy from.[in] finalize_cb
: Optional callback to call when the ArrayBuffer is being collected.[in] finalize_hint
: Optional hint to pass to the finalize callback during collection.[out] result
: Anapi_value
representing anode::Buffer
.
Returns napi_ok
if the API succeeded.
This API allocates a node::Buffer
object and initializes it with data
backed by the passed in buffer. While this is still a fully-supported data
structure, in most cases using a TypedArray will suffice.
Note: For Node.js >=4 Buffers
are Uint8Arrays.
napi_create_function#
napi_status napi_create_function(napi_env env,
const char* utf8name,
napi_callback cb,
void* data,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] utf8name
: A string representing the name of the function encoded as UTF8.[in] cb
: A function pointer to the native function to be invoked when the created function is invoked from JavaScript.[in] data
: Optional arbitrary context data to be passed into the native function when it is invoked.[out] result
: Anapi_value
representing a JavaScript function.
Returns napi_ok
if the API succeeded.
This API returns an N-API value corresponding to a JavaScript Function object. It's used to wrap native functions so that they can be invoked from JavaScript.
JavaScript Functions are described in Section 19.2 of the ECMAScript Language Specification.
napi_create_object#
napi_status napi_create_object(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
: Anapi_value
representing a JavaScript Object.
Returns napi_ok
if the API succeeded.
This API allocates a default JavaScript Object.
It is the equivalent of doing new Object()
in JavaScript.
The JavaScript Object type is described in Section 6.1.7 of the ECMAScript Language Specification.
napi_create_symbol#
napi_status napi_create_symbol(napi_env env,
napi_value description,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] description
: Optional napi_value which refers to a JavaScript String to be set as the description for the symbol.[out] result
: Anapi_value
representing a JavaScript Symbol.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript Symbol object from a UTF8-encoded C string
The JavaScript Symbol type is described in Section 19.4 of the ECMAScript Language Specification.
napi_create_typedarray#
napi_status napi_create_typedarray(napi_env env,
napi_typedarray_type type,
size_t length,
napi_value arraybuffer,
size_t byte_offset,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] type
: Scalar datatype of the elements within the TypedArray.[in] length
: Number of elements in the TypedArray.[in] arraybuffer
: ArrayBuffer underlying the typed array.[in] byte_offset
: The byte offset within the ArrayBuffer from which to start projecting the TypedArray.[out] result
: Anapi_value
representing a JavaScript TypedArray.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript TypedArray object over an existing ArrayBuffer. TypedArray objects provide an array-like view over an underlying data buffer where each element has the same underlying binary scalar datatype.
It's required that (length * size_of_element) + byte_offset should be <= the size in bytes of the array passed in. If not, a RangeError exception is raised.
JavaScript TypedArray Objects are described in Section 22.2 of the ECMAScript Language Specification.
napi_create_dataview#
napi_status napi_create_dataview(napi_env env,
size_t byte_length,
napi_value arraybuffer,
size_t byte_offset,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] length
: Number of elements in the DataView.[in] arraybuffer
: ArrayBuffer underlying the DataView.[in] byte_offset
: The byte offset within the ArrayBuffer from which to start projecting the DataView.[out] result
: Anapi_value
representing a JavaScript DataView.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript DataView object over an existing ArrayBuffer. DataView objects provide an array-like view over an underlying data buffer, but one which allows items of different size and type in the ArrayBuffer.
It is required that byte_length + byte_offset
is less than or equal to the
size in bytes of the array passed in. If not, a RangeError exception is raised.
JavaScript DataView Objects are described in Section 24.3 of the ECMAScript Language Specification.
Functions to convert from C types to N-API#
napi_create_number#
napi_status napi_create_number(napi_env env, double value, napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: Double-precision value to be represented in JavaScript.[out] result
: Anapi_value
representing a JavaScript Number.
Returns napi_ok
if the API succeeded.
This API is used to convert from the C double type to the JavaScript Number type.
The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.
napi_create_string_utf16#
napi_status napi_create_string_utf16(napi_env env,
const char16_t* str,
size_t length,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] str
: Character buffer representing a UTF16-LE-encoded string.[in] length
: The length of the string in two-byte code units, or -1 if it is null-terminated.[out] result
: Anapi_value
representing a JavaScript String.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript String object from a UTF16-LE-encoded C string
The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.
napi_create_string_latin1#
NAPI_EXTERN napi_status napi_create_string_latin1(napi_env env,
const char* str,
size_t length,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] str
: Character buffer representing a latin1-encoded string.[in] length
: The length of the string in bytes, or -1 if it is null-terminated.[out] result
: Anapi_value
representing a JavaScript String.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript String object from a latin1-encoded C string.
The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.
napi_create_string_utf8#
napi_status napi_create_string_utf8(napi_env env,
const char* str,
size_t length,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] str
: Character buffer representing a UTF8-encoded string.[in] length
: The length of the string in bytes, or -1 if it is null-terminated.[out] result
: Anapi_value
representing a JavaScript String.
Returns napi_ok
if the API succeeded.
This API creates a JavaScript String object from a UTF8-encoded C string
The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.
Functions to convert from N-API to C types#
napi_get_array_length#
napi_status napi_get_array_length(napi_env env,
napi_value value,
uint32_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing the JavaScript Array whose length is being queried.[out] result
:uint32
representing length of the array.
Returns napi_ok
if the API succeeded.
This API returns the length of an array.
Array length is described in Section 22.1.4.1 of the ECMAScript Language Specification.
napi_get_arraybuffer_info#
napi_status napi_get_arraybuffer_info(napi_env env,
napi_value arraybuffer,
void** data,
size_t* byte_length)
[in] env
: The environment that the API is invoked under.[in] arraybuffer
:napi_value
representing the ArrayBuffer being queried.[out] data
: The underlying data buffer of the ArrayBuffer.[out] byte_length
: Length in bytes of the underlying data buffer.
Returns napi_ok
if the API succeeded.
This API is used to retrieve the underlying data buffer of an ArrayBuffer and its length.
WARNING: Use caution while using this API. The lifetime of the underlying data
buffer is managed by the ArrayBuffer even after it's returned. A
possible safe way to use this API is in conjunction with napi_create_reference
,
which can be used to guarantee control over the lifetime of the
ArrayBuffer. It's also safe to use the returned data buffer within the same
callback as long as there are no calls to other APIs that might trigger a GC.
napi_get_buffer_info#
napi_status napi_get_buffer_info(napi_env env,
napi_value value,
void** data,
size_t* length)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing thenode::Buffer
being queried.[out] data
: The underlying data buffer of thenode::Buffer
.[out] length
: Length in bytes of the underlying data buffer.
Returns napi_ok
if the API succeeded.
This API is used to retrieve the underlying data buffer of a node::Buffer
and it's length.
Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM.
napi_get_prototype#
napi_status napi_get_prototype(napi_env env,
napi_value object,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] object
:napi_value
representing JavaScript Object whose prototype to return. This returns the equivalent ofObject.getPrototypeOf
(which is not the same as the function'sprototype
property).[out] result
:napi_value
representing prototype of the given object.
Returns napi_ok
if the API succeeded.
napi_get_typedarray_info#
napi_status napi_get_typedarray_info(napi_env env,
napi_value typedarray,
napi_typedarray_type* type,
size_t* length,
void** data,
napi_value* arraybuffer,
size_t* byte_offset)
[in] env
: The environment that the API is invoked under.[in] typedarray
:napi_value
representing the TypedArray whose properties to query.[out] type
: Scalar datatype of the elements within the TypedArray.[out] length
: Number of elements in the TypedArray.[out] data
: The data buffer underlying the typed array.[out] byte_offset
: The byte offset within the data buffer from which to start projecting the TypedArray.
Returns napi_ok
if the API succeeded.
This API returns various properties of a typed array.
Warning: Use caution while using this API since the underlying data buffer is managed by the VM
napi_get_dataview_info#
napi_status napi_get_dataview_info(napi_env env,
napi_value dataview,
size_t* byte_length,
void** data,
napi_value* arraybuffer,
size_t* byte_offset)
[in] env
: The environment that the API is invoked under.[in] dataview
:napi_value
representing the DataView whose properties to query.[out] byte_length
: Number of bytes in the DataView.[out] data
: The data buffer underlying the DataView.[out] arraybuffer
: ArrayBuffer underlying the DataView.[out] byte_offset
: The byte offset within the data buffer from which to start projecting the DataView.
Returns napi_ok
if the API succeeded.
This API returns various properties of a DataView.
napi_get_value_bool#
napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript Boolean.[out] result
: C boolean primitive equivalent of the given JavaScript Boolean.
Returns napi_ok
if the API succeeded. If a non-boolean napi_value
is
passed in it returns napi_boolean_expected
.
This API returns the C boolean primitive equivalent of the given JavaScript Boolean.
napi_get_value_double#
napi_status napi_get_value_double(napi_env env,
napi_value value,
double* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript Number.[out] result
: C double primitive equivalent of the given JavaScript Number.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed
in it returns napi_number_expected
.
This API returns the C double primitive equivalent of the given JavaScript Number.
napi_get_value_external#
napi_status napi_get_value_external(napi_env env,
napi_value value,
void** result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript external value.[out] result
: Pointer to the data wrapped by the JavaScript external value.
Returns napi_ok
if the API succeeded. If a non-external napi_value
is
passed in it returns napi_invalid_arg
.
This API retrieves the external data pointer that was previously passed to
napi_create_external()
.
napi_get_value_int32#
napi_status napi_get_value_int32(napi_env env,
napi_value value,
int32_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript Number.[out] result
: C int32 primitive equivalent of the given JavaScript Number.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed in `napi_number_expected .
This API returns the C int32 primitive equivalent of the given JavaScript Number. If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the the value is > 2^31 -1.
napi_get_value_int64#
napi_status napi_get_value_int64(napi_env env,
napi_value value,
int64_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript Number.[out] result
: C int64 primitive equivalent of the given JavaScript Number.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed in it returns napi_number_expected
.
This API returns the C int64 primitive equivalent of the given JavaScript Number
napi_get_value_string_utf8#
napi_status napi_get_value_string_utf8(napi_env env,
napi_value value,
char* buf,
size_t bufsize,
size_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript string.[in] buf
: Buffer to write the UTF8-encoded string into. If NULL is passed in, the length of the string (in bytes) is returned.[in] bufsize
: Size of the destination buffer.[out] result
: Number of bytes copied into the buffer including the null. terminator. If the buffer size is insufficient, the string will be truncated including a null terminator.
Returns napi_ok
if the API succeeded. Ifa non-String napi_value
x is passed in it returns napi_string_expected
.
This API returns the UTF8-encoded string corresponding the value passed in.
napi_get_value_string_utf16#
napi_status napi_get_value_string_utf16(napi_env env,
napi_value value,
char16_t* buf,
size_t bufsize,
size_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript string.[in] buf
: Buffer to write the UTF16-LE-encoded string into. If NULL is passed in, the length of the string (in 2-byte code units) is returned.[in] bufsize
: Size of the destination buffer.[out] result
: Number of 2-byte code units copied into the buffer including the null terminateor. If the buffer size is insufficient, the string will be truncated including a null terminator.
Returns napi_ok
if the API succeeded. If a non-String napi_value
is passed in it returns napi_string_expected
.
This API returns the UTF16-encoded string corresponding the value passed in.
napi_get_value_uint32#
napi_status napi_get_value_uint32(napi_env env,
napi_value value,
uint32_t* result)
[in] env
: The environment that the API is invoked under.[in] value
:napi_value
representing JavaScript Number.[out] result
: C primitive equivalent of the givennapi_value
as auint32_t
.
Returns napi_ok
if the API succeeded. If a non-number napi_value
is passed in it returns napi_number_expected
.
This API returns the C primitive equivalent of the given napi_value
as a
uint32_t
.
Functions to get global instances#
napi_get_boolean#
napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The value of the boolean to retrieve.[out] result
:napi_value
representing JavaScript Boolean singleton to retrieve.
Returns napi_ok
if the API succeeded.
This API is used to return the JavaScript singleton object that is used to represent the given boolean value
napi_get_global#
napi_status napi_get_global(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing JavaScript Global Object.
Returns napi_ok
if the API succeeded.
This API returns the global Object.
napi_get_null#
napi_status napi_get_null(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing JavaScript Null Object.
Returns napi_ok
if the API succeeded.
This API returns the null Object.
napi_get_undefined#
napi_status napi_get_undefined(napi_env env, napi_value* result)
[in] env
: The environment that the API is invoked under.[out] result
:napi_value
representing JavaScript Undefined value.
Returns napi_ok
if the API succeeded.
This API returns the Undefined object.
Working with JavaScript Values - Abstract Operations#
N-API exposes a set of APIs to perform some abstract operations on JavaScript values. Some of these operations are documented under Section 7 of the ECMAScript Language Specification.
These APIs support doing one of the following:
- Coerce JavaScript values to specific JavaScript types (such as Number or String)
- Check the type of a JavaScript value
- Check for equality between two JavaScript values
napi_coerce_to_bool#
napi_status napi_coerce_to_bool(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScript Boolean.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToBoolean as defined in Section 7.1.2 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_coerce_to_number#
napi_status napi_coerce_to_number(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScript Number.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToNumber as defined in Section 7.1.3 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_coerce_to_object#
napi_status napi_coerce_to_object(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScript Object.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToObject as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_coerce_to_string#
napi_status napi_coerce_to_string(napi_env env,
napi_value value,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to coerce.[out] result
:napi_value
representing the coerced JavaScript String.
Returns napi_ok
if the API succeeded.
This API implements the abstract operation ToString as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_typeof#
napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value whose type to query.[out] result
: The type of the JavaScript value.
Returns napi_ok
if the API succeeded.
napi_invalid_arg
if the type ofvalue
is not a known ECMAScript type andvalue
is not an External value.
This API represents behavior similar to invoking the typeof
Operator on
the object as defined in Section 12.5.5 of the ECMAScript Language
Specification. However, it has support for detecting an External value.
If value
has a type that is invalid, an error is returned.
napi_instanceof#
napi_status napi_instanceof(napi_env env,
napi_value object,
napi_value constructor,
bool* result)
[in] env
: The environment that the API is invoked under.[in] object
: The JavaScript value to check.[in] constructor
: The JavaScript function object of the constructor function to check against.[out] result
: Boolean that is set to true ifobject instanceof constructor
is true.
Returns napi_ok
if the API succeeded.
This API represents invoking the instanceof
Operator on the object as
defined in
Section 12.10.4
of the ECMAScript Language Specification.
napi_is_array#
napi_status napi_is_array(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the given object is an array.
Returns napi_ok
if the API succeeded.
This API represents invoking the IsArray
operation on the object
as defined in Section 7.2.2
of the ECMAScript Language Specification.
napi_is_arraybuffer#
napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the given object is an ArrayBuffer.
Returns napi_ok
if the API succeeded.
This API checks if the Object passsed in is an array buffer.
napi_is_buffer#
napi_status napi_is_buffer(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents anode::Buffer
object.
Returns napi_ok
if the API succeeded.
This API checks if the Object passsed in is a buffer.
napi_is_error#
napi_status napi_is_error(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents an Error object.
Returns napi_ok
if the API succeeded.
This API checks if the Object passsed in is an Error.
napi_is_typedarray#
napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents a TypedArray.
Returns napi_ok
if the API succeeded.
This API checks if the Object passsed in is a typed array.
napi_is_dataview#
napi_status napi_is_dataview(napi_env env, napi_value value, bool* result)
[in] env
: The environment that the API is invoked under.[in] value
: The JavaScript value to check.[out] result
: Whether the givennapi_value
represents a DataView.
Returns napi_ok
if the API succeeded.
This API checks if the Object passed in is a DataView.
napi_strict_equals#
napi_status napi_strict_equals(napi_env env,
napi_value lhs,
napi_value rhs,
bool* result)
[in] env
: The environment that the API is invoked under.[in] lhs
: The JavaScript value to check.[in] rhs
: The JavaScript value to check against.[out] result
: Whether the twonapi_value
objects are equal.
Returns napi_ok
if the API succeeded.
This API represents the invocation of the Strict Equality algorithm as defined in Section 7.2.14 of the ECMAScript Language Specification.
Working with JavaScript Properties#
N-API exposes a set of APIs to get and set properties on JavaScript objects. Some of these types are documented under Section 7 of the ECMAScript Language Specification.
Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in N-API can be represented in one of the following forms:
- Named: a simple UTF8-encoded string
- Integer-Indexed: an index value represented by
uint32_t
- JavaScript value: these are represented in N-API by
napi_value
. This can be anapi_value
representing a String, Number or Symbol.
N-API values are represented by the type napi_value
.
Any N-API call that requires a JavaScript value takes in a napi_value
.
However, it's the caller's responsibility to make sure that the
napi_value
in question is of the JavaScript type expected by the API.
The APIs documented in this section provide a simple interface to
get and set properties on arbitrary JavaScript objects represented by
napi_value
.
For instance, consider the following JavaScript code snippet:
const obj = {};
obj.myProp = 123;
The equivalent can be done using N-API values with the following snippet:
napi_status status = napi_generic_failure;
// const obj = {}
napi_value obj, value;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;
// Create a napi_value for 123
status = napi_create_number(env, 123, &value);
if (status != napi_ok) return status;
// obj.myProp = 123
status = napi_set_named_property(env, obj, "myProp", value);
if (status != napi_ok) return status;
Indexed properties can be set in a similar manner. Consider the following JavaScript snippet:
const arr = [];
arr[123] = 'hello';
The equivalent can be done using N-API values with the following snippet:
napi_status status = napi_generic_failure;
// const arr = [];
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;
// Create a napi_value for 'hello'
status = napi_create_string_utf8(env, "hello", -1, &value);
if (status != napi_ok) return status;
// arr[123] = 'hello';
status = napi_set_element(env, arr, 123, value);
if (status != napi_ok) return status;
Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet:
const arr = [];
const value = arr[123];
The following is the approximate equivalent of the N-API counterpart:
napi_status status = napi_generic_failure;
// const arr = []
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;
// const value = arr[123]
status = napi_get_element(env, arr, 123, &value);
if (status != napi_ok) return status;
Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript:
const obj = {};
Object.defineProperties(obj, {
'foo': { value: 123, writable: true, configurable: true, enumerable: true },
'bar': { value: 456, writable: true, configurable: true, enumerable: true }
});
The following is the approximate equivalent of the N-API counterpart:
napi_status status = napi_status_generic_failure;
// const obj = {};
napi_value obj;
status = napi_create_obj(env, &obj);
if (status != napi_ok) return status;
// Create napi_values for 123 and 456
napi_value fooValue, barValue;
status = napi_create_number(env, 123, &fooValue);
if (status != napi_ok) return status;
status = napi_create_number(env, 456, &barValue);
if (status != napi_ok) return status;
// Set the properties
napi_property_descriptors descriptors[] = {
{ "foo", fooValue, 0, 0, 0, napi_default, 0 },
{ "bar", barValue, 0, 0, 0, napi_default, 0 }
}
status = napi_define_properties(env,
obj,
sizeof(descriptors) / sizeof(descriptors[0]),
descriptors);
if (status != napi_ok) return status;
Structures#
napi_property_attributes#
typedef enum {
napi_default = 0,
napi_writable = 1 << 0,
napi_enumerable = 1 << 1,
napi_configurable = 1 << 2,
// Used with napi_define_class to distinguish static properties
// from instance properties. Ignored by napi_define_properties.
napi_static = 1 << 10,
} napi_property_attributes;
napi_property_attributes
are flags used to control the behavior of properties
set on a JavaScript object. Other than napi_static
they correspond to the
attributes listed in Section 6.1.7.1
of the ECMAScript Language Specification.
They can be one or more of the following bitflags:
napi_default
- Used to indicate that no explicit attributes are set on the given property. By default, a property is read only, not enumerable and not configurable.napi_writable
- Used to indicate that a given property is writable.napi_enumerable
- Used to indicate that a given property is enumerable.napi_configurable
- Used to indicate that a given property is configurable, as defined in Section 6.1.7.1 of the ECMAScript Language Specification.napi_static
- Used to indicate that the property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only bynapi_define_class
. It is ignored bynapi_define_properties
.
napi_property_descriptor#
typedef struct {
// One of utf8name or name should be NULL.
const char* utf8name;
napi_value name;
napi_callback method;
napi_callback getter;
napi_callback setter;
napi_value value;
napi_property_attributes attributes;
void* data;
} napi_property_descriptor;
utf8name
: Optional String describing the key for the property, encoded as UTF8. One ofutf8name
orname
must be provided for the property.name
: Optional napi_value that points to a JavaScript string or symbol to be used as the key for the property. One ofutf8name
orname
must be provided for the property.value
: The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, setgetter
,setter
,method
anddata
toNULL
(since these members won't be used).getter
: A function to call when a get access of the property is performed. If this is passed in, setvalue
andmethod
toNULL
(since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a N-API call).setter
: A function to call when a set access of the property is performed. If this is passed in, setvalue
andmethod
toNULL
(since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a N-API call).method
: Set this to make the property descriptor object'svalue
property to be a JavaScript function represented bymethod
. If this is passed in, setvalue
,getter
andsetter
toNULL
(since these members won't be used).data
: The callback data passed intomethod
,getter
andsetter
if this function is invoked.attributes
: The attributes associated with the particular property. Seenapi_property_attributes
.
Functions#
napi_get_property_names#
napi_status napi_get_property_names(napi_env env,
napi_value object,
napi_value* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object from which to retrieve the properties.[out] result
: Anapi_value
representing an array of JavaScript values that represent the property names of the object. The API can be used to iterate overresult
usingnapi_get_array_length
andnapi_get_element
.
Returns napi_ok
if the API succeeded.
This API returns the array of propertys for the Object passed in
napi_set_property#
napi_status napi_set_property(napi_env env,
napi_value object,
napi_value key,
napi_value value);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object on which to set the property.[in] key
: The name of the property to set.[in] value
: The property value.
Returns napi_ok
if the API succeeded.
This API set a property on the Object passed in.
napi_get_property#
napi_status napi_get_property(napi_env env,
napi_value object,
napi_value key,
napi_value* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object from which to retrieve the property.[in] key
: The name of the property to retrieve.[out] result
: The value of the property.
Returns napi_ok
if the API succeeded.
This API gets the requested property from the Object passed in.
napi_has_property#
napi_status napi_has_property(napi_env env,
napi_value object,
napi_value key,
bool* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object to query.[in] key
: The name of the property whose existence to check.[out] result
: Whether the property exists on the object or not.
Returns napi_ok
if the API succeeded.
This API checks if the Object passed in has the named property.
napi_delete_property#
napi_status napi_delete_property(napi_env env,
napi_value object,
napi_value key,
bool* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object to query.[in] key
: The name of the property to delete.[out] result
: Whether the property deletion succeeded or not.result
can optionally be ignored by passingNULL
.
Returns napi_ok
if the API succeeded.
This API attempts to delete the key
own property from object
.
napi_has_own_property#
napi_status napi_has_own_property(napi_env env,
napi_value object,
napi_value key,
bool* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object to query.[in] key
: The name of the own property whose existence to check.[out] result
: Whether the own property exists on the object or not.
Returns napi_ok
if the API succeeded.
This API checks if the Object passed in has the named own property. key
must
be a string or a Symbol, or an error will be thrown. N-API will not perform any
conversion between data types.
napi_set_named_property#
napi_status napi_set_named_property(napi_env env,
napi_value object,
const char* utf8Name,
napi_value value);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object on which to set the property.[in] utf8Name
: The name of the property to set.[in] value
: The property value.
Returns napi_ok
if the API succeeded.
This method is equivalent to calling napi_set_property
with a napi_value
created from the string passed in as utf8Name
napi_get_named_property#
napi_status napi_get_named_property(napi_env env,
napi_value object,
const char* utf8Name,
napi_value* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object from which to retrieve the property.[in] utf8Name
: The name of the property to get.[out] result
: The value of the property.
Returns napi_ok
if the API succeeded.
This method is equivalent to calling napi_get_property
with a napi_value
created from the string passed in as utf8Name
napi_has_named_property#
napi_status napi_has_named_property(napi_env env,
napi_value object,
const char* utf8Name,
bool* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object to query.[in] utf8Name
: The name of the property whose existence to check.[out] result
: Whether the property exists on the object or not.
Returns napi_ok
if the API succeeded.
This method is equivalent to calling napi_has_property
with a napi_value
created from the string passed in as utf8Name
napi_set_element#
napi_status napi_set_element(napi_env env,
napi_value object,
uint32_t index,
napi_value value);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object from which to set the properties.[in] index
: The index of the property to set.[in] value
: The property value.
Returns napi_ok
if the API succeeded.
This API sets and element on the Object passed in.
napi_get_element#
napi_status napi_get_element(napi_env env,
napi_value object,
uint32_t index,
napi_value* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object from which to retrieve the property.[in] index
: The index of the property to get.[out] result
: The value of the property.
Returns napi_ok
if the API succeeded.
This API gets the element at the requested index.
napi_has_element#
napi_status napi_has_element(napi_env env,
napi_value object,
uint32_t index,
bool* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object to query.[in] index
: The index of the property whose existence to check.[out] result
: Whether the property exists on the object or not.
Returns napi_ok
if the API succeeded.
This API returns if the Object passed in has an element at the requested index.
napi_delete_element#
napi_status napi_delete_element(napi_env env,
napi_value object,
uint32_t index,
bool* result);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object to query.[in] index
: The index of the property to delete.[out] result
: Whether the element deletion succeeded or not.result
can optionally be ignored by passingNULL
.
Returns napi_ok
if the API succeeded.
This API attempts to delete the specified index
from object
.
napi_define_properties#
napi_status napi_define_properties(napi_env env,
napi_value object,
size_t property_count,
const napi_property_descriptor* properties);
[in] env
: The environment that the N-API call is invoked under.[in] object
: The object from which to retrieve the properties.[in] property_count
: The number of elements in theproperties
array.[in] properties
: The array of property descriptors.
Returns napi_ok
if the API succeeded.
This method allows the efficient definition of multiple properties on a given
object. The properties are defined using property descriptors (See
napi_property_descriptor
). Given an array of such property descriptors, this
API will set the properties on the object one at a time, as defined by
DefineOwnProperty (described in Section 9.1.6 of the ECMA262 specification).
Working with JavaScript Functions#
N-API provides a set of APIs that allow JavaScript code to
call back into native code. N-API APIs that support calling back
into native code take in a callback functions represented by
the napi_callback
type. When the JavaScript VM calls back to
native code, the napi_callback
function provided is invoked. The APIs
documented in this section allow the callback function to do the
following:
- Get information about the context in which the callback was invoked.
- Get the arguments passed into the callback.
- Return a
napi_value
back from the callback.
Additionally, N-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function.
napi_call_function#
napi_status napi_call_function(napi_env env,
napi_value recv,
napi_value func,
int argc,
const napi_value* argv,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] recv
: Thethis
object passed to the called function.[in] func
:napi_value
representing the JavaScript function to be invoked.[in] argc
: The count of elements in theargv
array.[in] argv
: Array ofnapi_values
representing JavaScript values passed in as arguments to the function.[out] result
:napi_value
representing the JavaScript object returned.
Returns napi_ok
if the API succeeded.
This method allows a JavaScript function object to be called from a native
add-on. This is an primary mechanism of calling back from the add-on's
native code into JavaScript. For special cases like calling into JavaScript
after an async operation, see napi_make_callback
.
A sample use case might look as follows. Consider the following JavaScript snippet:
function AddTwo(num) {
return num + 2;
}
Then, the above function can be invoked from a native add-on using the following code:
// Get the function named "AddTwo" on the global object
napi_value global, add_two, arg;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;
status = napi_get_named_property(env, global, "AddTwo", &add_two);
if (status != napi_ok) return;
// const arg = 1337
status = napi_create_number(env, 1337, &arg);
if (status != napi_ok) return;
napi_value* argv = &arg;
size_t argc = 1;
// AddTwo(arg);
napi_value return_val;
status = napi_call_function(env, global, add_two, argc, argv, &return_val);
if (status != napi_ok) return;
// Convert the result back to a native type
int32_t result;
status = napi_get_value_int32(env, return_val, &result);
if (status != napi_ok) return;
napi_create_function#
napi_status napi_create_function(napi_env env,
const char* utf8name,
napi_callback cb,
void* data,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] utf8Name
: The name of the function encoded as UTF8. This is visible within JavaScript as the new function object'sname
property.[in] cb
: The native function which should be called when this function object is invoked.[in] data
: User-provided data context. This will be passed back into the function when invoked later.[out] result
:napi_value
representing the JavaScript function object for the newly created function.
Returns napi_ok
if the API succeeded.
This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from Javascript.
Note: The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script.
In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows:
napi_value SayHello(napi_env env, napi_callback_info info) {
printf("Hello\n");
return nullptr;
}
void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
napi_status status;
napi_value fn;
status = napi_create_function(env, NULL, SayHello, NULL, &fn);
if (status != napi_ok) return;
status = napi_set_named_property(env, exports, "sayHello", fn);
if (status != napi_ok) return;
}
NAPI_MODULE(addon, Init)
Given the above code, the add-on can be used from JavaScript as follows:
const myaddon = require('./addon');
myaddon.sayHello();
Note: The string passed to require is not necessarily the name passed into
NAPI_MODULE
in the earlier snippet but the name of the target in binding.gyp
responsible for creating the .node
file.
napi_get_cb_info#
napi_status napi_get_cb_info(napi_env env,
napi_callback_info cbinfo,
size_t* argc,
napi_value* argv,
napi_value* thisArg,
void** data)
[in] env
: The environment that the API is invoked under.[in] cbinfo
: The callback info passed into the callback function.[in-out] argc
: Specifies the size of the providedargv
array and receives the actual count of arguments.[out] argv
: Buffer to which thenapi_value
representing the arguments are copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest ofargv
is filled withnapi_value
values that representundefined
.[out] this
: Receives the JavaScriptthis
argument for the call.[out] data
: Receives the data pointer for the callback.
Returns napi_ok
if the API succeeded.
This method is used within a callback function to retrieve details about the
call like the arguments and the this
pointer from a given callback info.
napi_is_construct_call#
napi_status napi_is_construct_call(napi_env env,
napi_callback_info cbinfo,
bool* result)
[in] env
: The environment that the API is invoked under.[in] cbinfo
: The callback info passed into the callback function.[out] result
: Whether the native function is being invoked as a constructor call.
Returns napi_ok
if the API succeeded.
This API checks if the the current callback was due to a consructor call.
napi_new_instance#
napi_status napi_new_instance(napi_env env,
napi_value cons,
size_t argc,
napi_value* argv,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] cons
:napi_value
representing the JavaScript function to be invoked as a constructor.[in] argc
: The count of elements in theargv
array.[in] argv
: Array of JavaScript values asnapi_value
representing the arguments to the constructor.[out] result
:napi_value
representing the JavaScript object returned, which in this case is the constructed object.
This method is used to instantiate a new JavaScript value using a given
napi_value
that represents the constructor for the object. For example,
consider the following snippet:
function MyObject(param) {
this.param = param;
}
const arg = 'hello';
const value = new MyObject(arg);
The following can be approximated in N-API using the following snippet:
// Get the constructor function MyObject
napi_value global, constructor, arg, value;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;
status = napi_get_named_property(env, global, "MyObject", &constructor);
if (status != napi_ok) return;
// const arg = "hello"
status = napi_create_string_utf8(env, "hello", -1, &arg);
if (status != napi_ok) return;
napi_value* argv = &arg;
size_t argc = 1;
// const value = new MyObject(arg)
status = napi_new_instance(env, constructor, argc, argv, &value);
Returns napi_ok
if the API succeeded.
napi_make_callback#
napi_status napi_make_callback(napi_env env,
napi_value recv,
napi_value func,
int argc,
const napi_value* argv,
napi_value* result)
[in] env
: The environment that the API is invoked under.[in] recv
: Thethis
object passed to the called function.[in] func
:napi_value
representing the JavaScript function to be invoked.[in] argc
: The count of elements in theargv
array.[in] argv
: Array of JavaScript values asnapi_value
representing the arguments to the function.[out] result
:napi_value
representing the JavaScript object returned.
Returns napi_ok
if the API succeeded.
This method allows a JavaScript function object to be called from a native
add-on. This API is similar to napi_call_function
. However, it is used to call
from native code back into JavaScript after returning from an async
operation (when there is no other script on the stack). It is a fairly simple
wrapper around node::MakeCallback
.
For an example on how to use napi_make_callback
, see the section on
Asynchronous Operations.
Object Wrap#
N-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript.
- The
napi_define_class
API defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the The C++ class. - When JavaScript code invokes the constructor, the constructor callback
uses
napi_wrap
to wrap a new C++ instance in a JavaScript object, then returns the wrapper object. - When JavaScript code invokes a method or property accessor on the class,
the corresponding
napi_callback
C++ function is invoked. For an instance callback,napi_unwrap
obtains the C++ instance that is the target of the call.
napi_define_class#
napi_status napi_define_class(napi_env env,
const char* utf8name,
napi_callback constructor,
void* data,
size_t property_count,
const napi_property_descriptor* properties,
napi_value* result);
[in] env
: The environment that the API is invoked under.[in] utf8name
: Name of the JavaScript constructor function; this is not required to be the same as the C++ class name, though it is recommended for clarity.[in] constructor
: Callback function that handles constructing instances of the class. (This should be a static method on the class, not an actual C++ constructor function.)[in] data
: Optional data to be passed to the constructor callback as thedata
property of the callback info.[in] property_count
: Number of items in theproperties
array argument.[in] properties
: Array of property descriptors describing static and instance data properties, accessors, and methods on the class Seenapi_property_descriptor
.[out] result
: Anapi_value
representing the constructor function for the class.
Returns napi_ok
if the API succeeded.
Defines a JavaScript class that corresponds to a C++ class, including:
- A JavaScript constructor function that has the class name and invokes the provided C++ constructor callback.
- Properties on the constructor function corresponding to static data
properties, accessors, and methods of the C++ class (defined by
property descriptors with the
napi_static
attribute). - Properties on the constructor function's
prototype
object corresponding to non-static data properties, accessors, and methods of the C++ class (defined by property descriptors without thenapi_static
attribute).
The C++ constructor callback should be a static method on the class that calls
the actual class constructor, then wraps the new C++ instance in a JavaScript
object, and returns the wrapper object. See napi_wrap()
for details.
The JavaScript constructor function returned from napi_define_class
is
often saved and used later, to construct new instances of the class from native
code, and/or check whether provided values are instances of the class. In that
case, to prevent the function value from being garbage-collected, create a
persistent reference to it using napi_create_reference
and ensure the
reference count is kept >= 1.
napi_wrap#
napi_status napi_wrap(napi_env env,
napi_value js_object,
void* native_object,
napi_finalize finalize_cb,
void* finalize_hint,
napi_ref* result);
[in] env
: The environment that the API is invoked under.[in] js_object
: The JavaScript object that will be the wrapper for the native object. This object must have been created from theprototype
of a constructor that was created usingnapi_define_class()
.[in] native_object
: The native instance that will be wrapped in the JavaScript object.[in] finalize_cb
: Optional native callback that can be used to free the native instance when the JavaScript object is ready for garbage-collection.[in] finalize_hint
: Optional contextual hint that is passed to the finalize callback.[out] result
: Optional reference to the wrapped object.
Returns napi_ok
if the API succeeded.
Wraps a native instance in a JavaScript object. The native instance can be
retrieved later using napi_unwrap()
.
When JavaScript code invokes a constructor for a class that was defined using
napi_define_class()
, the napi_callback
for the constructor is invoked.
After constructing an instance of the native class, the callback must then call
napi_wrap()
to wrap the newly constructed instance in the already-created
JavaScript object that is the this
argument to the constructor callback.
(That this
object was created from the constructor function's prototype
,
so it already has definitions of all the instance properties and methods.)
Typically when wrapping a class instance, a finalize callback should be
provided that simply deletes the native instance that is received as the data
argument to the finalize callback.
The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid.
Caution: The optional returned reference (if obtained) should be deleted via
napi_delete_reference
ONLY in response to the finalize callback
invocation. (If it is deleted before then, then the finalize callback may never
be invoked.) Therefore, when obtaining a reference a finalize callback is also
required in order to enable correct proper of the reference.
Note: This API may modify the prototype chain of the wrapper object.
Afterward, additional manipulation of the wrapper's prototype chain may cause
napi_unwrap()
to fail.
Note: Calling napi_wrap()
a second time on an object that already has a
native instance associated with it by virtue of a previous call to
napi_wrap()
will cause an error to be returned.
napi_unwrap#
napi_status napi_unwrap(napi_env env,
napi_value js_object,
void** result);
[in] env
: The environment that the API is invoked under.[in] js_object
: The object associated with the C++ class instance.[out] result
: Pointer to the wrapped C++ class instance.
Returns napi_ok
if the API succeeded.
Retrieves a native instance that was previously wrapped in a JavaScript
object using napi_wrap()
.
When JavaScript code invokes a method or property accessor on the class, the
corresponding napi_callback
is invoked. If the callback is for an instance
method or accessor, then the this
argument to the callback is the wrapper
object; the wrapped C++ instance that is the target of the call can be obtained
then by calling napi_unwrap()
on the wrapper object.
Asynchronous Operations#
Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This is important in order to allow them to avoid blocking overall execution of the Node.js application.
N-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.
N-API defines the napi_work
structure which is used to manage
asynchronous workers. Instances are created/deleted with
napi_create_async_work
and napi_delete_async_work
.
The execute
and complete
callbacks are functions that will be
invoked when the executor is ready to execute and when it completes its
task respectively. These functions implement the following interfaces:
typedef void (*napi_async_execute_callback)(napi_env env,
void* data);
typedef void (*napi_async_complete_callback)(napi_env env,
napi_status status,
void* data);
When these methods are invoked, the data
parameter passed will be the
addon-provided void* data that was passed into the
napi_create_async_work
call.
Once created the async worker can be queued
for execution using the napi_queue_async_work
function:
NAPI_EXTERN napi_status napi_queue_async_work(napi_env env,
napi_async_work work);
napi_cancel_async_work
can be used if the work needs
to be cancelled before the work has started execution.
After calling napi_cancel_async_work
, the complete
callback
will be invoked with a status value of napi_cancelled
.
The work should not be deleted before the complete
callback invocation, even when it was cancelled.
napi_create_async_work#
NAPI_EXTERN
napi_status napi_create_async_work(napi_env env,
napi_async_execute_callback execute,
napi_async_complete_callback complete,
void* data,
napi_async_work* result);
[in] env
: The environment that the API is invoked under.[in] execute
: The native function which should be called to excute the logic asynchronously.[in] complete
: The native function which will be called when the asynchronous logic is comple or is cancelled.[in] data
: User-provided data context. This will be passed back into the execute and complete functions.[out] result
:napi_async_work*
which is the handle to the newly created async work.
Returns napi_ok
if the API succeeded.
This API allocates a work object that is used to execute logic asynchronously.
It should be freed using napi_delete_async_work
once the work is no longer
required.
napi_delete_async_work#
NAPI_EXTERN napi_status napi_delete_async_work(napi_env env,
napi_async_work work);
[in] env
: The environment that the API is invoked under.[in] work
: The handle returned by the call tonapi_create_async_work
.
Returns napi_ok
if the API succeeded.
This API frees a previously allocated work object.
napi_queue_async_work#
NAPI_EXTERN napi_status napi_queue_async_work(napi_env env,
napi_async_work work);
[in] env
: The environment that the API is invoked under.[in] work
: The handle returned by the call tonapi_create_async_work
.
Returns napi_ok
if the API succeeded.
This API requests that the previously allocated work be scheduled for execution.
napi_cancel_async_work#
NAPI_EXTERN napi_status napi_cancel_async_work(napi_env env,
napi_async_work work);
[in] env
: The environment that the API is invoked under.[in] work
: The handle returned by the call tonapi_create_async_work
.
Returns napi_ok
if the API succeeded.
This API cancels queued work if it has not yet
been started. If it has already started executing, it cannot be
cancelled and napi_generic_failure
will be returned. If successful,
the complete
callback will be invoked with a status value of
napi_cancelled
. The work should not be deleted before the complete
callback invocation, even if it has been successfully cancelled.
Version Management#
napi_get_version#
NAPI_EXTERN napi_status napi_get_version(napi_env env,
uint32_t* result);
[in] env
: The environment that the API is invoked under.[out] result
: The highest version of N-API supported.
Returns napi_ok
if the API succeeded.
This API returns the highest N-API version supported by the Node.js runtime. N-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it:
- Call
napi_get_version()
to determine if the API is available. - If available, dynamically load a pointer to the function using
uv_dlsym()
. - Use the dynamically loaded pointer to invoke the function.
- If the function is not available, provide an alternate implementation that does not use the function.
Child Process#
Stability: 2 - Stable
The child_process
module provides the ability to spawn child processes in
a manner that is similar, but not identical, to popen(3). This capability
is primarily provided by the child_process.spawn()
function:
const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
console.log(`stderr: ${data}`);
});
ls.on('close', (code) => {
console.log(`child process exited with code ${code}`);
});
By default, pipes for stdin
, stdout
and stderr
are established between
the parent Node.js process and the spawned child. It is possible to stream data
through these pipes in a non-blocking way. Note, however, that some programs
use line-buffered I/O internally. While that does not affect Node.js, it can
mean that data sent to the child process may not be immediately consumed.
The child_process.spawn()
method spawns the child process asynchronously,
without blocking the Node.js event loop. The child_process.spawnSync()
function provides equivalent functionality in a synchronous manner that blocks
the event loop until the spawned process either exits or is terminated.
For convenience, the child_process
module provides a handful of synchronous
and asynchronous alternatives to child_process.spawn()
and
child_process.spawnSync()
. Note that each of these alternatives are
implemented on top of child_process.spawn()
or child_process.spawnSync()
.
child_process.exec()
: spawns a shell and runs a command within that shell, passing thestdout
andstderr
to a callback function when complete.child_process.execFile()
: similar tochild_process.exec()
except that it spawns the command directly without first spawning a shell.child_process.fork()
: spawns a new Node.js process and invokes a specified module with an IPC communication channel established that allows sending messages between parent and child.child_process.execSync()
: a synchronous version ofchild_process.exec()
that will block the Node.js event loop.child_process.execFileSync()
: a synchronous version ofchild_process.execFile()
that will block the Node.js event loop.
For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.
Asynchronous Process Creation#
The child_process.spawn()
, child_process.fork()
, child_process.exec()
,
and child_process.execFile()
methods all follow the idiomatic asynchronous
programming pattern typical of other Node.js APIs.
Each of the methods returns a ChildProcess
instance. These objects
implement the Node.js EventEmitter
API, allowing the parent process to
register listener functions that are called when certain events occur during
the life cycle of the child process.
The child_process.exec()
and child_process.execFile()
methods additionally
allow for an optional callback
function to be specified that is invoked
when the child process terminates.
Spawning .bat
and .cmd
files on Windows#
The importance of the distinction between child_process.exec()
and
child_process.execFile()
can vary based on platform. On Unix-type operating
systems (Unix, Linux, macOS) child_process.execFile()
can be more efficient
because it does not spawn a shell. On Windows, however, .bat
and .cmd
files are not executable on their own without a terminal, and therefore cannot
be launched using child_process.execFile()
. When running on Windows, .bat
and .cmd
files can be invoked using child_process.spawn()
with the shell
option set, with child_process.exec()
, or by spawning cmd.exe
and passing
the .bat
or .cmd
file as an argument (which is what the shell
option and
child_process.exec()
do). In any case, if the script filename contains
spaces it needs to be quoted.
// On Windows Only ...
const { spawn } = require('child_process');
const bat = spawn('cmd.exe', ['/c', 'my.bat']);
bat.stdout.on('data', (data) => {
console.log(data.toString());
});
bat.stderr.on('data', (data) => {
console.log(data.toString());
});
bat.on('exit', (code) => {
console.log(`Child exited with code ${code}`);
});
// OR...
const { exec } = require('child_process');
exec('my.bat', (err, stdout, stderr) => {
if (err) {
console.error(err);
return;
}
console.log(stdout);
});
// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
// ...
});
child_process.exec(command[, options][, callback])#
command
<string> The command to run, with space-separated argumentsoptions
<Object>cwd
<string> Current working directory of the child processenv
<Object> Environment key-value pairsencoding
<string> (Default:'utf8'
)shell
<string> Shell to execute the command with (Default:'/bin/sh'
on UNIX,process.env.ComSpec
on Windows. See Shell Requirements and Default Windows Shell.)timeout
<number> (Default:0
)maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. (Default:200*1024
) If exceeded, the child process is terminated. See caveat atmaxBuffer
and Unicode.killSignal
<string> | <integer> (Default:'SIGTERM'
)uid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)
callback
<Function> called with the output when process terminates- Returns: <ChildProcess>
Spawns a shell then executes the command
within that shell, buffering any
generated output. The command
string passed to the exec function is processed
directly by the shell and special characters (vary based on
shell)
need to be dealt with accordingly:
exec('"/path/to/test file/test.sh" arg1 arg2');
//Double quotes are used so that the space in the path is not interpreted as
//multiple arguments
exec('echo "The \\$HOME variable is $HOME"');
//The $HOME variable is escaped in the first instance, but not in the second
Note: Never pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.
const { exec } = require('child_process');
exec('cat *.js bad_file | wc -l', (error, stdout, stderr) => {
if (error) {
console.error(`exec error: ${error}`);
return;
}
console.log(`stdout: ${stdout}`);
console.log(`stderr: ${stderr}`);
});
If a callback
function is provided, it is called with the arguments
(error, stdout, stderr)
. On success, error
will be null
. On error,
error
will be an instance of Error
. The error.code
property will be
the exit code of the child process while error.signal
will be set to the
signal that terminated the process. Any exit code other than 0
is considered
to be an error.
The stdout
and stderr
arguments passed to the callback will contain the
stdout and stderr output of the child process. By default, Node.js will decode
the output as UTF-8 and pass strings to the callback. The encoding
option
can be used to specify the character encoding used to decode the stdout and
stderr output. If encoding
is 'buffer'
, or an unrecognized character
encoding, Buffer
objects will be passed to the callback instead.
The options
argument may be passed as the second argument to customize how
the process is spawned. The default options are:
const defaults = {
encoding: 'utf8',
timeout: 0,
maxBuffer: 200 * 1024,
killSignal: 'SIGTERM',
cwd: null,
env: null
};
If timeout
is greater than 0
, the parent will send the signal
identified by the killSignal
property (the default is 'SIGTERM'
) if the
child runs longer than timeout
milliseconds.
Note: Unlike the exec(3) POSIX system call, child_process.exec()
does not
replace the existing process and uses a shell to execute the command.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise for an object with stdout
and stderr
properties. In case of an
error, a rejected promise is returned, with the same error
object given in the
callback, but with an additional two properties stdout
and stderr
.
For example:
const util = require('util');
const exec = util.promisify(require('child_process').exec);
async function lsExample() {
const { stdout, stderr } = await exec('ls');
console.log('stdout:', stdout);
console.log('stderr:', stderr);
}
lsExample();
child_process.execFile(file[, args][, options][, callback])#
file
<string> The name or path of the executable file to runargs
<Array> List of string argumentsoptions
<Object>cwd
<string> Current working directory of the child processenv
<Object> Environment key-value pairsencoding
<string> (Default:'utf8'
)timeout
<number> (Default:0
)maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. (Default:200*1024
) If exceeded, the child process is terminated. See caveat atmaxBuffer
and Unicode.killSignal
<string> | <integer> (Default:'SIGTERM'
)uid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)
callback
<Function> called with the output when process terminates- Returns: <ChildProcess>
The child_process.execFile()
function is similar to child_process.exec()
except that it does not spawn a shell. Rather, the specified executable file
is spawned directly as a new process making it slightly more efficient than
child_process.exec()
.
The same options as child_process.exec()
are supported. Since a shell is not
spawned, behaviors such as I/O redirection and file globbing are not supported.
const { execFile } = require('child_process');
const child = execFile('node', ['--version'], (error, stdout, stderr) => {
if (error) {
throw error;
}
console.log(stdout);
});
The stdout
and stderr
arguments passed to the callback will contain the
stdout and stderr output of the child process. By default, Node.js will decode
the output as UTF-8 and pass strings to the callback. The encoding
option
can be used to specify the character encoding used to decode the stdout and
stderr output. If encoding
is 'buffer'
, or an unrecognized character
encoding, Buffer
objects will be passed to the callback instead.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise for an object with stdout
and stderr
properties. In case of an
error, a rejected promise is returned, with the same error
object given in the
callback, but with an additional two properties stdout
and stderr
.
const util = require('util');
const execFile = util.promisify(require('child_process').execFile);
async function getVersion() {
const { stdout } = await execFile('node', ['--version']);
console.log(stdout);
}
getVersion();
child_process.fork(modulePath[, args][, options])#
modulePath
<string> The module to run in the childargs
<Array> List of string argumentsoptions
<Object>cwd
<string> Current working directory of the child processenv
<Object> Environment key-value pairsexecPath
<string> Executable used to create the child processexecArgv
<Array> List of string arguments passed to the executable (Default:process.execArgv
)silent
<boolean> Iftrue
, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the'pipe'
and'inherit'
options forchild_process.spawn()
'sstdio
for more details (Default:false
)stdio
<Array> | <string> Seechild_process.spawn()
'sstdio
. When this option is provided, it overridessilent
. If the array variant is used, it must contain exactly one item with value'ipc'
or an error will be thrown. For instance[0, 1, 2, 'ipc']
.uid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)
- Returns: <ChildProcess>
The child_process.fork()
method is a special case of
child_process.spawn()
used specifically to spawn new Node.js processes.
Like child_process.spawn()
, a ChildProcess
object is returned. The returned
ChildProcess
will have an additional communication channel built-in that
allows messages to be passed back and forth between the parent and child. See
child.send()
for details.
It is important to keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.
By default, child_process.fork()
will spawn new Node.js instances using the
process.execPath
of the parent process. The execPath
property in the
options
object allows for an alternative execution path to be used.
Node.js processes launched with a custom execPath
will communicate with the
parent process using the file descriptor (fd) identified using the
environment variable NODE_CHANNEL_FD
on the child process. The input and
output on this fd is expected to be line delimited JSON objects.
Note: Unlike the fork(2) POSIX system call, child_process.fork()
does
not clone the current process.
child_process.spawn(command[, args][, options])#
command
<string> The command to runargs
<Array> List of string argumentsoptions
<Object>cwd
<string> Current working directory of the child processenv
<Object> Environment key-value pairsargv0
<string> Explicitly set the value ofargv[0]
sent to the child process. This will be set tocommand
if not specified.stdio
<Array> | <string> Child's stdio configuration. (Seeoptions.stdio
)detached
<boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, seeoptions.detached
)uid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)shell
<boolean> | <string> Iftrue
, runscommand
inside of a shell. Uses'/bin/sh'
on UNIX, andprocess.env.ComSpec
on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell. Defaults tofalse
(no shell).
- Returns: <ChildProcess>
The child_process.spawn()
method spawns a new process using the given
command
, with command line arguments in args
. If omitted, args
defaults
to an empty array.
Note: If the shell
option is enabled, do not pass unsanitised user input to
this function. Any input containing shell metacharacters may be used to
trigger arbitrary command execution.
A third argument may be used to specify additional options, with these defaults:
const defaults = {
cwd: undefined,
env: process.env
};
Use cwd
to specify the working directory from which the process is spawned.
If not given, the default is to inherit the current working directory.
Use env
to specify environment variables that will be visible to the new
process, the default is process.env
.
Example of running ls -lh /usr
, capturing stdout
, stderr
, and the
exit code:
const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
console.log(`stderr: ${data}`);
});
ls.on('close', (code) => {
console.log(`child process exited with code ${code}`);
});
Example: A very elaborate way to run ps ax | grep ssh
const { spawn } = require('child_process');
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);
ps.stdout.on('data', (data) => {
grep.stdin.write(data);
});
ps.stderr.on('data', (data) => {
console.log(`ps stderr: ${data}`);
});
ps.on('close', (code) => {
if (code !== 0) {
console.log(`ps process exited with code ${code}`);
}
grep.stdin.end();
});
grep.stdout.on('data', (data) => {
console.log(data.toString());
});
grep.stderr.on('data', (data) => {
console.log(`grep stderr: ${data}`);
});
grep.on('close', (code) => {
if (code !== 0) {
console.log(`grep process exited with code ${code}`);
}
});
Example of checking for failed spawn
:
const { spawn } = require('child_process');
const child = spawn('bad_command');
child.on('error', (err) => {
console.log('Failed to start child process.');
});
Note: Certain platforms (macOS, Linux) will use the value of argv[0]
for
the process title while others (Windows, SunOS) will use command
.
Note: Node.js currently overwrites argv[0]
with process.execPath
on
startup, so process.argv[0]
in a Node.js child process will not match the
argv0
parameter passed to spawn
from the parent, retrieve it with the
process.argv0
property instead.
options.detached#
On Windows, setting options.detached
to true
makes it possible for the
child process to continue running after the parent exits. The child will have
its own console window. Once enabled for a child process, it cannot be
disabled.
On non-Windows platforms, if options.detached
is set to true
, the child
process will be made the leader of a new process group and session. Note that
child processes may continue running after the parent exits regardless of
whether they are detached or not. See setsid(2) for more information.
By default, the parent will wait for the detached child to exit. To prevent
the parent from waiting for a given child
, use the child.unref()
method.
Doing so will cause the parent's event loop to not include the child in its
reference count, allowing the parent to exit independently of the child, unless
there is an established IPC channel between the child and parent.
When using the detached
option to start a long-running process, the process
will not stay running in the background after the parent exits unless it is
provided with a stdio
configuration that is not connected to the parent.
If the parent's stdio
is inherited, the child will remain attached to the
controlling terminal.
Example of a long-running process, by detaching and also ignoring its parent
stdio
file descriptors, in order to ignore the parent's termination:
const { spawn } = require('child_process');
const child = spawn(process.argv[0], ['child_program.js'], {
detached: true,
stdio: 'ignore'
});
child.unref();
Alternatively one can redirect the child process' output into files:
const fs = require('fs');
const { spawn } = require('child_process');
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');
const child = spawn('prg', [], {
detached: true,
stdio: [ 'ignore', out, err ]
});
child.unref();
options.stdio#
The options.stdio
option is used to configure the pipes that are established
between the parent and child process. By default, the child's stdin, stdout,
and stderr are redirected to corresponding child.stdin
, child.stdout
, and
child.stderr
streams on the ChildProcess
object. This is equivalent to
setting the options.stdio
equal to ['pipe', 'pipe', 'pipe']
.
For convenience, options.stdio
may be one of the following strings:
'pipe'
- equivalent to['pipe', 'pipe', 'pipe']
(the default)'ignore'
- equivalent to['ignore', 'ignore', 'ignore']
'inherit'
- equivalent to[process.stdin, process.stdout, process.stderr]
or[0,1,2]
Otherwise, the value of options.stdio
is an array where each index corresponds
to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout,
and stderr, respectively. Additional fds can be specified to create additional
pipes between the parent and child. The value is one of the following:
'pipe'
- Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on thechild_process
object aschild.stdio[fd]
. Pipes created for fds 0 - 2 are also available aschild.stdin
,child.stdout
andchild.stderr
, respectively.'ipc'
- Create an IPC channel for passing messages/file descriptors between parent and child. AChildProcess
may have at most one IPC stdio file descriptor. Setting this option enables thechild.send()
method. If the child writes JSON messages to this file descriptor, thechild.on('message')
event handler will be triggered in the parent. If the child is a Node.js process, the presence of an IPC channel will enableprocess.send()
,process.disconnect()
,process.on('disconnect')
, andprocess.on('message')
within the child.'ignore'
- Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0 - 2 for the processes it spawns, setting the fd to'ignore'
will cause Node.js to open/dev/null
and attach it to the child's fd.- <Stream> object - Share a readable or writable stream that refers to a tty,
file, socket, or a pipe with the child process. The stream's underlying
file descriptor is duplicated in the child process to the fd that
corresponds to the index in the
stdio
array. Note that the stream must have an underlying descriptor (file streams do not until the'open'
event has occurred). - Positive integer - The integer value is interpreted as a file descriptor that is is currently open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared.
null
,undefined
- Use default value. For stdio fds 0, 1 and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is'ignore'
.
Example:
const { spawn } = require('child_process');
// Child will use parent's stdios
spawn('prg', [], { stdio: 'inherit' });
// Spawn child sharing only stderr
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
// Open an extra fd=4, to interact with programs presenting a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
It is worth noting that when an IPC channel is established between the
parent and child processes, and the child is a Node.js process, the child
is launched with the IPC channel unreferenced (using unref()
) until the
child registers an event handler for the process.on('disconnect')
event
or the process.on('message')
event.This allows the child to exit normally
without the process being held open by the open IPC channel.
See also: child_process.exec()
and child_process.fork()
Synchronous Process Creation#
The child_process.spawnSync()
, child_process.execSync()
, and
child_process.execFileSync()
methods are synchronous and WILL block
the Node.js event loop, pausing execution of any additional code until the
spawned process exits.
Blocking calls like these are mostly useful for simplifying general purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.
child_process.execFileSync(file[, args][, options])#
file
<string> The name or path of the executable file to runargs
<Array> List of string argumentsoptions
<Object>cwd
<string> Current working directory of the child processinput
<string> | <Buffer> | <Uint8Array> The value which will be passed as stdin to the spawned process- supplying this value will override
stdio[0]
- supplying this value will override
stdio
<string> | <Array> Child's stdio configuration. (Default:'pipe'
)stderr
by default will be output to the parent process' stderr unlessstdio
is specified
env
<Object> Environment key-value pairsuid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. (Default:undefined
)killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed. (Default:'SIGTERM'
)maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. (Default:200*1024
) If exceeded, the child process is terminated. See caveat atmaxBuffer
and Unicode.encoding
<string> The encoding used for all stdio inputs and outputs. (Default:'buffer'
)
- Returns: <Buffer> | <string> The stdout from the command
The child_process.execFileSync()
method is generally identical to
child_process.execFile()
with the exception that the method will not return
until the child process has fully closed. When a timeout has been encountered
and killSignal
is sent, the method won't return until the process has
completely exited.
Note: If the child process intercepts and handles the SIGTERM
signal and
does not exit, the parent process will still wait until the child process has
exited.
If the process times out, or has a non-zero exit code, this method will
throw. The Error
object will contain the entire result from
child_process.spawnSync()
child_process.execSync(command[, options])#
command
<string> The command to runoptions
<Object>cwd
<string> Current working directory of the child processinput
<string> | <Buffer> | <Uint8Array> The value which will be passed as stdin to the spawned process- supplying this value will override
stdio[0]
- supplying this value will override
stdio
<string> | <Array> Child's stdio configuration. (Default:'pipe'
)stderr
by default will be output to the parent process' stderr unlessstdio
is specified
env
<Object> Environment key-value pairsshell
<string> Shell to execute the command with (Default:'/bin/sh'
on UNIX,process.env.ComSpec
on Windows. See Shell Requirements and Default Windows Shell.)uid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. (Default:undefined
)killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed. (Default:'SIGTERM'
)maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. (Default:200*1024
) If exceeded, the child process is terminated. See caveat atmaxBuffer
and Unicode.encoding
<string> The encoding used for all stdio inputs and outputs. (Default:'buffer'
)
- Returns: <Buffer> | <string> The stdout from the command
The child_process.execSync()
method is generally identical to
child_process.exec()
with the exception that the method will not return until
the child process has fully closed. When a timeout has been encountered and
killSignal
is sent, the method won't return until the process has completely
exited. Note that if the child process intercepts and handles the SIGTERM
signal and doesn't exit, the parent process will wait until the child
process has exited.
If the process times out, or has a non-zero exit code, this method will
throw. The Error
object will contain the entire result from
child_process.spawnSync()
Note: Never pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.
child_process.spawnSync(command[, args][, options])#
command
<string> The command to runargs
<Array> List of string argumentsoptions
<Object>cwd
<string> Current working directory of the child processinput
<string> | <Buffer> | <Uint8Array> The value which will be passed as stdin to the spawned process- supplying this value will override
stdio[0]
- supplying this value will override
stdio
<string> | <Array> Child's stdio configuration.env
<Object> Environment key-value pairsuid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)timeout
<number> In milliseconds the maximum amount of time the process is allowed to run. (Default:undefined
)killSignal
<string> | <integer> The signal value to be used when the spawned process will be killed. (Default:'SIGTERM'
)maxBuffer
<number> Largest amount of data in bytes allowed on stdout or stderr. (Default:200*1024
) If exceeded, the child process is terminated. See caveat atmaxBuffer
and Unicode.encoding
<string> The encoding used for all stdio inputs and outputs. (Default:'buffer'
)shell
<boolean> | <string> Iftrue
, runscommand
inside of a shell. Uses'/bin/sh'
on UNIX, andprocess.env.ComSpec
on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell. Defaults tofalse
(no shell).
- Returns: <Object>
pid
<number> Pid of the child processoutput
<Array> Array of results from stdio outputstdout
<Buffer> | <string> The contents ofoutput[1]
stderr
<Buffer> | <string> The contents ofoutput[2]
status
<number> The exit code of the child processsignal
<string> The signal used to kill the child processerror
<Error> The error object if the child process failed or timed out
The child_process.spawnSync()
method is generally identical to
child_process.spawn()
with the exception that the function will not return
until the child process has fully closed. When a timeout has been encountered
and killSignal
is sent, the method won't return until the process has
completely exited. Note that if the process intercepts and handles the
SIGTERM
signal and doesn't exit, the parent process will wait until the child
process has exited.
Note: If the shell
option is enabled, do not pass unsanitised user input
to this function. Any input containing shell metacharacters may be used to
trigger arbitrary command execution.
Class: ChildProcess#
Instances of the ChildProcess
class are EventEmitters
that represent
spawned child processes.
Instances of ChildProcess
are not intended to be created directly. Rather,
use the child_process.spawn()
, child_process.exec()
,
child_process.execFile()
, or child_process.fork()
methods to create
instances of ChildProcess
.
Event: 'close'#
code
<number> the exit code if the child exited on its own.signal
<string> the signal by which the child process was terminated.
The 'close'
event is emitted when the stdio streams of a child process have
been closed. This is distinct from the 'exit'
event, since multiple
processes might share the same stdio streams.
Event: 'disconnect'#
The 'disconnect'
event is emitted after calling the
child.disconnect()
method in parent process or process.disconnect()
in child process. After
disconnecting it is no longer possible to send or receive messages, and the
child.connected
property is false
.
Event: 'error'#
err
<Error> the error.
The 'error'
event is emitted whenever:
- The process could not be spawned, or
- The process could not be killed, or
- Sending a message to the child process failed.
Note: The 'exit'
event may or may not fire after an error has occurred.
When listening to both the 'exit'
and 'error'
events, it is important
to guard against accidentally invoking handler functions multiple times.
See also child.kill()
and child.send()
.
Event: 'exit'#
code
<number> the exit code if the child exited on its own.signal
<string> the signal by which the child process was terminated.
The 'exit'
event is emitted after the child process ends. If the process
exited, code
is the final exit code of the process, otherwise null
. If the
process terminated due to receipt of a signal, signal
is the string name of
the signal, otherwise null
. One of the two will always be non-null.
Note that when the 'exit'
event is triggered, child process stdio streams
might still be open.
Also, note that Node.js establishes signal handlers for SIGINT
and
SIGTERM
and Node.js processes will not terminate immediately due to receipt
of those signals. Rather, Node.js will perform a sequence of cleanup actions
and then will re-raise the handled signal.
See waitpid(2).
Event: 'message'#
message
<Object> a parsed JSON object or primitive value.sendHandle
<Handle> anet.Socket
ornet.Server
object, or undefined.
The 'message'
event is triggered when a child process uses process.send()
to send messages.
child.channel#
- <Object> A pipe representing the IPC channel to the child process.
The child.channel
property is a reference to the child's IPC channel. If no
IPC channel currently exists, this property is undefined
.
child.connected#
- <boolean> Set to
false
afterchild.disconnect()
is called
The child.connected
property indicates whether it is still possible to send
and receive messages from a child process. When child.connected
is false
, it
is no longer possible to send or receive messages.
child.disconnect()#
Closes the IPC channel between parent and child, allowing the child to exit
gracefully once there are no other connections keeping it alive. After calling
this method the child.connected
and process.connected
properties in both
the parent and child (respectively) will be set to false
, and it will be no
longer possible to pass messages between the processes.
The 'disconnect'
event will be emitted when there are no messages in the
process of being received. This will most often be triggered immediately after
calling child.disconnect()
.
Note that when the child process is a Node.js instance (e.g. spawned using
child_process.fork()
), the process.disconnect()
method can be invoked
within the child process to close the IPC channel as well.
child.kill([signal])#
signal
<string>
The child.kill()
methods sends a signal to the child process. If no argument
is given, the process will be sent the 'SIGTERM'
signal. See signal(7) for
a list of available signals.
const { spawn } = require('child_process');
const grep = spawn('grep', ['ssh']);
grep.on('close', (code, signal) => {
console.log(
`child process terminated due to receipt of signal ${signal}`);
});
// Send SIGHUP to process
grep.kill('SIGHUP');
The ChildProcess
object may emit an 'error'
event if the signal cannot be
delivered. Sending a signal to a child process that has already exited is not
an error but may have unforeseen consequences. Specifically, if the process
identifier (PID) has been reassigned to another process, the signal will be
delivered to that process instead which can have unexpected results.
Note that while the function is called kill
, the signal delivered to the
child process may not actually terminate the process.
See kill(2) for reference.
Also note: on Linux, child processes of child processes will not be terminated
when attempting to kill their parent. This is likely to happen when running a
new process in a shell or with use of the shell
option of ChildProcess
, such
as in this example:
'use strict';
const { spawn } = require('child_process');
const child = spawn(
'sh',
[
'-c',
`node -e "setInterval(() => {
console.log(process.pid, 'is alive')
}, 500);"`
], {
stdio: ['inherit', 'inherit', 'inherit']
}
);
setTimeout(() => {
child.kill(); // does not terminate the node process in the shell
}, 2000);
child.pid#
- <number> Integer
Returns the process identifier (PID) of the child process.
Example:
const { spawn } = require('child_process');
const grep = spawn('grep', ['ssh']);
console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();
child.send(message[, sendHandle[, options]][, callback])#
message
<Object>sendHandle
<Handle>options
<Object>callback
<Function>- Returns: <boolean>
When an IPC channel has been established between the parent and child (
i.e. when using child_process.fork()
), the child.send()
method can be
used to send messages to the child process. When the child process is a Node.js
instance, these messages can be received via the process.on('message')
event.
For example, in the parent script:
const cp = require('child_process');
const n = cp.fork(`${__dirname}/sub.js`);
n.on('message', (m) => {
console.log('PARENT got message:', m);
});
n.send({ hello: 'world' });
And then the child script, 'sub.js'
might look like this:
process.on('message', (m) => {
console.log('CHILD got message:', m);
});
process.send({ foo: 'bar' });
Child Node.js processes will have a process.send()
method of their own that
allows the child to send messages back to the parent.
There is a special case when sending a {cmd: 'NODE_foo'}
message. All messages
containing a NODE_
prefix in its cmd
property are considered to be reserved
for use within Node.js core and will not be emitted in the child's
process.on('message')
event. Rather, such messages are emitted using the
process.on('internalMessage')
event and are consumed internally by Node.js.
Applications should avoid using such messages or listening for
'internalMessage'
events as it is subject to change without notice.
The optional sendHandle
argument that may be passed to child.send()
is for
passing a TCP server or socket object to the child process. The child will
receive the object as the second argument passed to the callback function
registered on the process.on('message')
event. Any data that is received
and buffered in the socket will not be sent to the child.
The options
argument, if present, is an object used to parameterize the
sending of certain types of handles. options
supports the following
properties:
keepOpen
- A Boolean value that can be used when passing instances ofnet.Socket
. Whentrue
, the socket is kept open in the sending process. Defaults tofalse
.
The optional callback
is a function that is invoked after the message is
sent but before the child may have received it. The function is called with a
single argument: null
on success, or an Error
object on failure.
If no callback
function is provided and the message cannot be sent, an
'error'
event will be emitted by the ChildProcess
object. This can happen,
for instance, when the child process has already exited.
child.send()
will return false
if the channel has closed or when the
backlog of unsent messages exceeds a threshold that makes it unwise to send
more. Otherwise, the method returns true
. The callback
function can be
used to implement flow control.
Example: sending a server object#
The sendHandle
argument can be used, for instance, to pass the handle of
a TCP server object to the child process as illustrated in the example below:
const child = require('child_process').fork('child.js');
// Open up the server object and send the handle.
const server = require('net').createServer();
server.on('connection', (socket) => {
socket.end('handled by parent');
});
server.listen(1337, () => {
child.send('server', server);
});
The child would then receive the server object as:
process.on('message', (m, server) => {
if (m === 'server') {
server.on('connection', (socket) => {
socket.end('handled by child');
});
}
});
Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.
While the example above uses a server created using the net
module, dgram
module servers use exactly the same workflow with the exceptions of listening on
a 'message'
event instead of 'connection'
and using server.bind()
instead of
server.listen()
. This is, however, currently only supported on UNIX platforms.
Example: sending a socket object#
Similarly, the sendHandler
argument can be used to pass the handle of a
socket to the child process. The example below spawns two children that each
handle connections with "normal" or "special" priority:
const { fork } = require('child_process');
const normal = fork('child.js', ['normal']);
const special = fork('child.js', ['special']);
// Open up the server and send sockets to child. Use pauseOnConnect to prevent
// the sockets from being read before they are sent to the child process.
const server = require('net').createServer({ pauseOnConnect: true });
server.on('connection', (socket) => {
// If this is special priority
if (socket.remoteAddress === '74.125.127.100') {
special.send('socket', socket);
return;
}
// This is normal priority
normal.send('socket', socket);
});
server.listen(1337);
The child.js
would receive the socket handle as the second argument passed
to the event callback function:
process.on('message', (m, socket) => {
if (m === 'socket') {
if (socket) {
// Check that the client socket exists.
// It is possible for the socket to be closed between the time it is
// sent and the time it is received in the child process.
socket.end(`Request handled with ${process.argv[2]} priority`);
}
}
});
Once a socket has been passed to a child, the parent is no longer capable of
tracking when the socket is destroyed. To indicate this, the .connections
property becomes null
. It is recommended not to use .maxConnections
when
this occurs.
It is also recommended that any 'message'
handlers in the child process
verify that socket
exists, as the connection may have been closed during the
time it takes to send the connection to the child.
Note: This function uses JSON.stringify()
internally to serialize the
message
.
child.stderr#
A Readable Stream
that represents the child process's stderr
.
If the child was spawned with stdio[2]
set to anything other than 'pipe'
,
then this will be null
.
child.stderr
is an alias for child.stdio[2]
. Both properties will refer to
the same value.
child.stdin#
A Writable Stream
that represents the child process's stdin
.
Note that if a child process waits to read all of its input, the child will not
continue until this stream has been closed via end()
.
If the child was spawned with stdio[0]
set to anything other than 'pipe'
,
then this will be null
.
child.stdin
is an alias for child.stdio[0]
. Both properties will refer to
the same value.
child.stdio#
A sparse array of pipes to the child process, corresponding with positions in
the stdio
option passed to child_process.spawn()
that have been set
to the value 'pipe'
. Note that child.stdio[0]
, child.stdio[1]
, and
child.stdio[2]
are also available as child.stdin
, child.stdout
, and
child.stderr
, respectively.
In the following example, only the child's fd 1
(stdout) is configured as a
pipe, so only the parent's child.stdio[1]
is a stream, all other values in
the array are null
.
const assert = require('assert');
const fs = require('fs');
const child_process = require('child_process');
const child = child_process.spawn('ls', {
stdio: [
0, // Use parent's stdin for child
'pipe', // Pipe child's stdout to parent
fs.openSync('err.out', 'w') // Direct child's stderr to a file
]
});
assert.strictEqual(child.stdio[0], null);
assert.strictEqual(child.stdio[0], child.stdin);
assert(child.stdout);
assert.strictEqual(child.stdio[1], child.stdout);
assert.strictEqual(child.stdio[2], null);
assert.strictEqual(child.stdio[2], child.stderr);
child.stdout#
A Readable Stream
that represents the child process's stdout
.
If the child was spawned with stdio[1]
set to anything other than 'pipe'
,
then this will be null
.
child.stdout
is an alias for child.stdio[1]
. Both properties will refer
to the same value.
maxBuffer
and Unicode#
The maxBuffer
option specifies the largest number of bytes allowed on stdout
or stderr
. If this value is exceeded, then the child process is terminated.
This impacts output that includes multibyte character encodings such as UTF-8 or
UTF-16. For instance, console.log('中文测试')
will send 13 UTF-8 encoded bytes
to stdout
although there are only 4 characters.
Shell Requirements#
The shell should understand the -c
switch on UNIX or /d /s /c
on Windows.
On Windows, command line parsing should be compatible with 'cmd.exe'
.
Default Windows Shell#
Although Microsoft specifies %COMSPEC%
must contain the path to
'cmd.exe'
in the root environment, child processes are not always subject to
the same requirement. Thus, in child_process
functions where a shell can be
spawned, 'cmd.exe'
is used as a fallback if process.env.ComSpec
is
unavailable.
Cluster#
Stability: 2 - Stable
A single instance of Node.js runs in a single thread. To take advantage of multi-core systems the user will sometimes want to launch a cluster of Node.js processes to handle the load.
The cluster module allows easy creation of child processes that all share server ports.
const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;
if (cluster.isMaster) {
console.log(`Master ${process.pid} is running`);
// Fork workers.
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
cluster.on('exit', (worker, code, signal) => {
console.log(`worker ${worker.process.pid} died`);
});
} else {
// Workers can share any TCP connection
// In this case it is an HTTP server
http.createServer((req, res) => {
res.writeHead(200);
res.end('hello world\n');
}).listen(8000);
console.log(`Worker ${process.pid} started`);
}
Running Node.js will now share port 8000 between the workers:
$ node server.js
Master 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started
Please note that on Windows, it is not yet possible to set up a named pipe server in a worker.
How It Works#
The worker processes are spawned using the child_process.fork()
method,
so that they can communicate with the parent via IPC and pass server
handles back and forth.
The cluster module supports two methods of distributing incoming connections.
The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.
The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.
The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.
Because server.listen()
hands off most of the work to the master
process, there are three cases where the behavior between a normal
Node.js process and a cluster worker differs:
server.listen({fd: 7})
Because the message is passed to the master, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.server.listen(handle)
Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the master process.server.listen(0)
Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they dolisten(0)
. In essence, the port is random the first time, but predictable thereafter. To listen on a unique port, generate a port number based on the cluster worker ID.
Note: Node.js does not provide routing logic. It is, therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login.
Because workers are all separate processes, they can be killed or re-spawned depending on a program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers, however. It is the application's responsibility to manage the worker pool based on its own needs.
Class: Worker#
A Worker object contains all public information and method about a worker.
In the master it can be obtained using cluster.workers
. In a worker
it can be obtained using cluster.worker
.
Event: 'disconnect'#
Similar to the cluster.on('disconnect')
event, but specific to this worker.
cluster.fork().on('disconnect', () => {
// Worker has disconnected
});
Event: 'error'#
This event is the same as the one provided by child_process.fork()
.
Within a worker, process.on('error')
may also be used.
Event: 'exit'#
code
<number> the exit code, if it exited normally.signal
<string> the name of the signal (e.g.'SIGHUP'
) that caused the process to be killed.
Similar to the cluster.on('exit')
event, but specific to this worker.
const worker = cluster.fork();
worker.on('exit', (code, signal) => {
if (signal) {
console.log(`worker was killed by signal: ${signal}`);
} else if (code !== 0) {
console.log(`worker exited with error code: ${code}`);
} else {
console.log('worker success!');
}
});
Event: 'listening'#
address
<Object>
Similar to the cluster.on('listening')
event, but specific to this worker.
cluster.fork().on('listening', (address) => {
// Worker is listening
});
It is not emitted in the worker.
Event: 'message'#
message
<Object>handle
<undefined> | <Object>
Similar to the cluster.on('message')
event, but specific to this worker.
Within a worker, process.on('message')
may also be used.
As an example, here is a cluster that keeps count of the number of requests in the master process using the message system:
const cluster = require('cluster');
const http = require('http');
if (cluster.isMaster) {
// Keep track of http requests
let numReqs = 0;
setInterval(() => {
console.log(`numReqs = ${numReqs}`);
}, 1000);
// Count requests
function messageHandler(msg) {
if (msg.cmd && msg.cmd === 'notifyRequest') {
numReqs += 1;
}
}
// Start workers and listen for messages containing notifyRequest
const numCPUs = require('os').cpus().length;
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
for (const id in cluster.workers) {
cluster.workers[id].on('message', messageHandler);
}
} else {
// Worker processes have a http server.
http.Server((req, res) => {
res.writeHead(200);
res.end('hello world\n');
// notify master about the request
process.send({ cmd: 'notifyRequest' });
}).listen(8000);
}
Event: 'online'#
Similar to the cluster.on('online')
event, but specific to this worker.
cluster.fork().on('online', () => {
// Worker is online
});
It is not emitted in the worker.
worker.disconnect()#
- Returns: <Worker> A reference to
worker
.
In a worker, this function will close all servers, wait for the 'close'
event on
those servers, and then disconnect the IPC channel.
In the master, an internal message is sent to the worker causing it to call
.disconnect()
on itself.
Causes .exitedAfterDisconnect
to be set.
Note that after a server is closed, it will no longer accept new connections,
but connections may be accepted by any other listening worker. Existing
connections will be allowed to close as usual. When no more connections exist,
see server.close()
, the IPC channel to the worker will close allowing it to
die gracefully.
The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.
Note that in a worker, process.disconnect
exists, but it is not this function,
it is disconnect
.
Because long living server connections may block workers from disconnecting, it
may be useful to send a message, so application specific actions may be taken to
close them. It also may be useful to implement a timeout, killing a worker if
the 'disconnect'
event has not been emitted after some time.
if (cluster.isMaster) {
const worker = cluster.fork();
let timeout;
worker.on('listening', (address) => {
worker.send('shutdown');
worker.disconnect();
timeout = setTimeout(() => {
worker.kill();
}, 2000);
});
worker.on('disconnect', () => {
clearTimeout(timeout);
});
} else if (cluster.isWorker) {
const net = require('net');
const server = net.createServer((socket) => {
// connections never end
});
server.listen(8000);
process.on('message', (msg) => {
if (msg === 'shutdown') {
// initiate graceful close of any connections to server
}
});
}
worker.exitedAfterDisconnect#
Set by calling .kill()
or .disconnect()
. Until then, it is undefined
.
The boolean worker.exitedAfterDisconnect
allows distinguishing between
voluntary and accidental exit, the master may choose not to respawn a worker
based on this value.
cluster.on('exit', (worker, code, signal) => {
if (worker.exitedAfterDisconnect === true) {
console.log('Oh, it was just voluntary – no need to worry');
}
});
// kill worker
worker.kill();
worker.id#
Each new worker is given its own unique id, this id is stored in the
id
.
While a worker is alive, this is the key that indexes it in cluster.workers
worker.isConnected()#
This function returns true
if the worker is connected to its master via its
IPC channel, false
otherwise. A worker is connected to its master after it
has been created. It is disconnected after the 'disconnect'
event is emitted.
worker.isDead()#
This function returns true
if the worker's process has terminated (either
because of exiting or being signaled). Otherwise, it returns false
.
worker.kill([signal='SIGTERM'])#
signal
<string> Name of the kill signal to send to the worker process.
This function will kill the worker. In the master, it does this by disconnecting
the worker.process
, and once disconnected, killing with signal
. In the
worker, it does it by disconnecting the channel, and then exiting with code 0
.
Causes .exitedAfterDisconnect
to be set.
This method is aliased as worker.destroy()
for backwards compatibility.
Note that in a worker, process.kill()
exists, but it is not this function,
it is kill
.
worker.process#
All workers are created using child_process.fork()
, the returned object
from this function is stored as .process
. In a worker, the global process
is stored.
See: Child Process module
Note that workers will call process.exit(0)
if the 'disconnect'
event occurs
on process
and .exitedAfterDisconnect
is not true
. This protects against
accidental disconnection.
worker.send(message[, sendHandle][, callback])#
message
<Object>sendHandle
<Handle>callback
<Function>- Returns: Boolean
Send a message to a worker or master, optionally with a handle.
In the master this sends a message to a specific worker. It is identical to
ChildProcess.send()
.
In a worker this sends a message to the master. It is identical to
process.send()
.
This example will echo back all messages from the master:
if (cluster.isMaster) {
const worker = cluster.fork();
worker.send('hi there');
} else if (cluster.isWorker) {
process.on('message', (msg) => {
process.send(msg);
});
}
worker.suicide#
Stability: 0 - Deprecated: Use worker.exitedAfterDisconnect
instead.
An alias to worker.exitedAfterDisconnect
.
Set by calling .kill()
or .disconnect()
. Until then, it is undefined
.
The boolean worker.suicide
is used to distinguish between voluntary
and accidental exit, the master may choose not to respawn a worker based on
this value.
cluster.on('exit', (worker, code, signal) => {
if (worker.suicide === true) {
console.log('Oh, it was just voluntary – no need to worry');
}
});
// kill worker
worker.kill();
This API only exists for backwards compatibility and will be removed in the future.
Event: 'disconnect'#
worker
<cluster.Worker>
Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect()).
There may be a delay between the 'disconnect'
and 'exit'
events. These events
can be used to detect if the process is stuck in a cleanup or if there are
long-living connections.
cluster.on('disconnect', (worker) => {
console.log(`The worker #${worker.id} has disconnected`);
});
Event: 'exit'#
worker
<cluster.Worker>code
<number> the exit code, if it exited normally.signal
<string> the name of the signal (e.g.'SIGHUP'
) that caused the process to be killed.
When any of the workers die the cluster module will emit the 'exit'
event.
This can be used to restart the worker by calling .fork()
again.
cluster.on('exit', (worker, code, signal) => {
console.log('worker %d died (%s). restarting...',
worker.process.pid, signal || code);
cluster.fork();
});
See child_process event: 'exit'.
Event: 'fork'#
worker
<cluster.Worker>
When a new worker is forked the cluster module will emit a 'fork'
event.
This can be used to log worker activity, and create a custom timeout.
const timeouts = [];
function errorMsg() {
console.error('Something must be wrong with the connection ...');
}
cluster.on('fork', (worker) => {
timeouts[worker.id] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', (worker, address) => {
clearTimeout(timeouts[worker.id]);
});
cluster.on('exit', (worker, code, signal) => {
clearTimeout(timeouts[worker.id]);
errorMsg();
});
Event: 'listening'#
worker
<cluster.Worker>address
<Object>
After calling listen()
from a worker, when the 'listening'
event is emitted
on the server a 'listening'
event will also be emitted on cluster
in the
master.
The event handler is executed with two arguments, the worker
contains the
worker object and the address
object contains the following connection
properties: address
, port
and addressType
. This is very useful if the
worker is listening on more than one address.
cluster.on('listening', (worker, address) => {
console.log(
`A worker is now connected to ${address.address}:${address.port}`);
});
The addressType
is one of:
4
(TCPv4)6
(TCPv6)-1
(unix domain socket)"udp4"
or"udp6"
(UDP v4 or v6)
Event: 'message'#
worker
<cluster.Worker>message
<Object>handle
<undefined> | <Object>
Emitted when the cluster master receives a message from any worker.
See child_process event: 'message'.
Before Node.js v6.0, this event emitted only the message and the handle, but not the worker object, contrary to what the documentation stated.
If support for older versions is required but a worker object is not required, it is possible to work around the discrepancy by checking the number of arguments:
cluster.on('message', (worker, message, handle) => {
if (arguments.length === 2) {
handle = message;
message = worker;
worker = undefined;
}
// ...
});
Event: 'online'#
worker
<cluster.Worker>
After forking a new worker, the worker should respond with an online message.
When the master receives an online message it will emit this event.
The difference between 'fork'
and 'online'
is that fork is emitted when the
master forks a worker, and 'online' is emitted when the worker is running.
cluster.on('online', (worker) => {
console.log('Yay, the worker responded after it was forked');
});
Event: 'setup'#
settings
<Object>
Emitted every time .setupMaster()
is called.
The settings
object is the cluster.settings
object at the time
.setupMaster()
was called and is advisory only, since multiple calls to
.setupMaster()
can be made in a single tick.
If accuracy is important, use cluster.settings
.
cluster.disconnect([callback])#
callback
<Function> called when all workers are disconnected and handles are closed
Calls .disconnect()
on each worker in cluster.workers
.
When they are disconnected all internal handles will be closed, allowing the master process to die gracefully if no other event is waiting.
The method takes an optional callback argument which will be called when finished.
This can only be called from the master process.
cluster.fork([env])#
env
<Object> Key/value pairs to add to worker process environment.- return <cluster.Worker>
Spawn a new worker process.
This can only be called from the master process.
cluster.isMaster#
True if the process is a master. This is determined
by the process.env.NODE_UNIQUE_ID
. If process.env.NODE_UNIQUE_ID
is
undefined, then isMaster
is true
.
cluster.isWorker#
True if the process is not a master (it is the negation of cluster.isMaster
).
cluster.schedulingPolicy#
The scheduling policy, either cluster.SCHED_RR
for round-robin or
cluster.SCHED_NONE
to leave it to the operating system. This is a
global setting and effectively frozen once either the first worker is spawned,
or cluster.setupMaster()
is called, whichever comes first.
SCHED_RR
is the default on all operating systems except Windows.
Windows will change to SCHED_RR
once libuv is able to effectively
distribute IOCP handles without incurring a large performance hit.
cluster.schedulingPolicy
can also be set through the
NODE_CLUSTER_SCHED_POLICY
environment variable. Valid
values are "rr"
and "none"
.
cluster.settings#
- <Object>
execArgv
<Array> List of string arguments passed to the Node.js executable. (Default=process.execArgv
)exec
<string> File path to worker file. (Default=process.argv[1]
)args
<Array> String arguments passed to worker. (Default=process.argv.slice(2)
)silent
<boolean> Whether or not to send output to parent's stdio. (Default=false
)stdio
<Array> Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an'ipc'
entry. When this option is provided, it overridessilent
.uid
<number> Sets the user identity of the process. (See setuid(2).)gid
<number> Sets the group identity of the process. (See setgid(2).)inspectPort
<number> | <function> Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the master'sprocess.debugPort
.
After calling .setupMaster()
(or .fork()
) this settings object will contain
the settings, including the default values.
This object is not intended to be changed or set manually.
cluster.setupMaster([settings])#
settings
<Object> seecluster.settings
setupMaster
is used to change the default 'fork' behavior. Once called,
the settings will be present in cluster.settings
.
Note that:
- Any settings changes only affect future calls to
.fork()
and have no effect on workers that are already running. - The only attribute of a worker that cannot be set via
.setupMaster()
is theenv
passed to.fork()
. - The defaults above apply to the first call only, the defaults for later
calls is the current value at the time of
cluster.setupMaster()
is called.
Example:
const cluster = require('cluster');
cluster.setupMaster({
exec: 'worker.js',
args: ['--use', 'https'],
silent: true
});
cluster.fork(); // https worker
cluster.setupMaster({
exec: 'worker.js',
args: ['--use', 'http']
});
cluster.fork(); // http worker
This can only be called from the master process.
cluster.worker#
A reference to the current worker object. Not available in the master process.
const cluster = require('cluster');
if (cluster.isMaster) {
console.log('I am master');
cluster.fork();
cluster.fork();
} else if (cluster.isWorker) {
console.log(`I am worker #${cluster.worker.id}`);
}
cluster.workers#
A hash that stores the active worker objects, keyed by id
field. Makes it
easy to loop through all the workers. It is only available in the master
process.
A worker is removed from cluster.workers after the worker has disconnected and
exited. The order between these two events cannot be determined in advance.
However, it is guaranteed that the removal from the cluster.workers list happens
before last 'disconnect'
or 'exit'
event is emitted.
// Go through all workers
function eachWorker(callback) {
for (const id in cluster.workers) {
callback(cluster.workers[id]);
}
}
eachWorker((worker) => {
worker.send('big announcement to all workers');
});
Using the worker's unique id is the easiest way to locate the worker.
socket.on('data', (id) => {
const worker = cluster.workers[id];
});
Command Line Options#
Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options.
To view this documentation as a manual page in a terminal, run man node
.
Synopsis#
node [options] [v8 options] [script.js | -e "script" | -] [--] [arguments]
node debug [script.js | -e "script" | <host>:<port>] …
node --v8-options
Execute without arguments to start the REPL.
For more info about node debug
, please see the debugger documentation.
Options#
-v
, --version
#
Print node's version.
-h
, --help
#
Print node command line options. The output of this option is less detailed than this document.
-e
, --eval "script"
#
Evaluate the following argument as JavaScript. The modules which are
predefined in the REPL can also be used in script
.
-p
, --print "script"
#
Identical to -e
but prints the result.
-c
, --check
#
Syntax check the script without executing.
-i
, --interactive
#
Opens the REPL even if stdin does not appear to be a terminal.
-r
, --require module
#
Preload the specified module at startup.
Follows require()
's module resolution
rules. module
may be either a path to a file, or a node module name.
--inspect[=[host:]port]
#
Activate inspector on host:port. Default is 127.0.0.1:9229.
V8 inspector integration allows tools such as Chrome DevTools and IDEs to debug and profile Node.js instances. The tools attach to Node.js instances via a tcp port and communicate using the Chrome Debugging Protocol.
--inspect-brk[=[host:]port]
#
Activate inspector on host:port and break at start of user script. Default host:port is 127.0.0.1:9229.
--inspect-port=[host:]port
#
Set the host:port to be used when the inspector is activated.
Useful when activating the inspector by sending the SIGUSR1
signal.
Default host is 127.0.0.1.
--no-deprecation
#
Silence deprecation warnings.
--trace-deprecation
#
Print stack traces for deprecations.
--throw-deprecation
#
Throw errors for deprecations.
--pending-deprecation
#
Emit pending deprecation warnings.
Note: Pending deprecations are generally identical to a runtime deprecation
with the notable exception that they are turned off by default and will not
be emitted unless either the --pending-deprecation
command line flag, or the
NODE_PENDING_DEPRECATION=1
environment variable, is set. Pending deprecations
are used to provide a kind of selective "early warning" mechanism that
developers may leverage to detect deprecated API usage.
--no-warnings
#
Silence all process warnings (including deprecations).
--napi-modules
#
Enable loading native modules compiled with the ABI-stable Node.js API (N-API) (experimental).
--abort-on-uncaught-exception
#
Aborting instead of exiting causes a core file to be generated for post-mortem
analysis using a debugger (such as lldb
, gdb
, and mdb
).
--trace-warnings
#
Print stack traces for process warnings (including deprecations).
--redirect-warnings=file
#
Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead.
--trace-sync-io
#
Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.
--trace-events-enabled
#
Enables the collection of trace event tracing information.
--trace-event-categories
#
A comma separated list of categories that should be traced when trace event
tracing is enabled using --trace-events-enabled
.
--zero-fill-buffers
#
Automatically zero-fills all newly allocated Buffer and SlowBuffer instances.
--preserve-symlinks
#
Instructs the module loader to preserve symbolic links when resolving and caching modules.
By default, when Node.js loads a module from a path that is symbolically linked
to a different on-disk location, Node.js will dereference the link and use the
actual on-disk "real path" of the module as both an identifier and as a root
path to locate other dependency modules. In most cases, this default behavior
is acceptable. However, when using symbolically linked peer dependencies, as
illustrated in the example below, the default behavior causes an exception to
be thrown if moduleA
attempts to require moduleB
as a peer dependency:
{appDir}
├── app
│ ├── index.js
│ └── node_modules
│ ├── moduleA -> {appDir}/moduleA
│ └── moduleB
│ ├── index.js
│ └── package.json
└── moduleA
├── index.js
└── package.json
The --preserve-symlinks
command line flag instructs Node.js to use the
symlink path for modules as opposed to the real path, allowing symbolically
linked peer dependencies to be found.
Note, however, that using --preserve-symlinks
can have other side effects.
Specifically, symbolically linked native modules can fail to load if those
are linked from more than one location in the dependency tree (Node.js would
see those as two separate modules and would attempt to load the module multiple
times, causing an exception to be thrown).
--track-heap-objects
#
Track heap object allocations for heap snapshots.
--prof-process
#
Process v8 profiler output generated using the v8 option --prof
.
--v8-options
#
Print v8 command line options.
Note: V8 options allow words to be separated by both dashes (-
) or
underscores (_
).
For example, --stack-trace-limit
is equivalent to --stack_trace_limit
.
--tls-cipher-list=list
#
Specify an alternative default TLS cipher list. (Requires Node.js to be built with crypto support. (Default))
--enable-fips
#
Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with
./configure --openssl-fips
)
--force-fips
#
Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.)
(Same requirements as --enable-fips
)
--openssl-config=file
#
Load an OpenSSL configuration file on startup. Among other uses, this can be
used to enable FIPS-compliant crypto if Node.js is built with
./configure --openssl-fips
.
--use-openssl-ca
, --use-bundled-ca
#
Use OpenSSL's default CA store or use bundled Mozilla CA store as supplied by current NodeJS version. The default store is selectable at build-time.
Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environmental variables.
The bundled CA store, as supplied by NodeJS, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.
See SSL_CERT_DIR
and SSL_CERT_FILE
.
--icu-data-dir=file
#
Specify ICU data load path. (overrides NODE_ICU_DATA
)
-
#
Alias for stdin, analogous to the use of - in other command line utilities, meaning that the script will be read from stdin, and the rest of the options are passed to that script.
--
#
Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument will be used as a script filename.
Environment Variables#
NODE_DEBUG=module[,…]
#
','
-separated list of core modules that should print debug information.
NODE_PATH=path[:…]
#
':'
-separated list of directories prefixed to the module search path.
Note: On Windows, this is a ';'
-separated list instead.
NODE_DISABLE_COLORS=1
#
When set to 1
colors will not be used in the REPL.
NODE_ICU_DATA=file
#
Data path for ICU (Intl object) data. Will extend linked-in data when compiled with small-icu support.
NODE_NO_WARNINGS=1
#
When set to 1
, process warnings are silenced.
NODE_OPTIONS=options...
#
options...
are interpreted as if they had been specified on the command line
before the actual command line (so they can be overridden). Node will exit with
an error if an option that is not allowed in the environment is used, such as
-p
or a script file.
Node options that are allowed are:
--enable-fips
--force-fips
--icu-data-dir
--inspect-brk
--inspect-port
--inspect
--napi-modules
--no-deprecation
--no-warnings
--openssl-config
--redirect-warnings
--require
,-r
--throw-deprecation
--tls-cipher-list
--trace-deprecation
--trace-events-categories
--trace-events-enabled
--trace-sync-io
--trace-warnings
--track-heap-objects
--use-bundled-ca
--use-openssl-ca
--v8-pool-size
--zero-fill-buffers
V8 options that are allowed are:
--abort-on-uncaught-exception
--max-old-space-size
NODE_PENDING_DEPRECATION=1
#
When set to 1
, emit pending deprecation warnings.
Note: Pending deprecations are generally identical to a runtime deprecation
with the notable exception that they are turned off by default and will not
be emitted unless either the --pending-deprecation
command line flag, or the
NODE_PENDING_DEPRECATION=1
environment variable, is set. Pending deprecations
are used to provide a kind of selective "early warning" mechanism that
developers may leverage to detect deprecated API usage.
NODE_PRESERVE_SYMLINKS=1
#
When set to 1
, instructs the module loader to preserve symbolic links when
resolving and caching modules.
NODE_REPL_HISTORY=file
#
Path to the file used to store the persistent REPL history. The default path is
~/.node_repl_history
, which is overridden by this variable. Setting the value
to an empty string (""
or " "
) disables persistent REPL history.
NODE_EXTRA_CA_CERTS=file
#
When set, the well known "root" CAs (like VeriSign) will be extended with the
extra certificates in file
. The file should consist of one or more trusted
certificates in PEM format. A message will be emitted (once) with
process.emitWarning()
if the file is missing or
malformed, but any errors are otherwise ignored.
Note that neither the well known nor extra certificates are used when the ca
options property is explicitly specified for a TLS or HTTPS client or server.
OPENSSL_CONF=file
#
Load an OpenSSL configuration file on startup. Among other uses, this can be
used to enable FIPS-compliant crypto if Node.js is built with ./configure
--openssl-fips
.
If the --openssl-config
command line option is used, the environment
variable is ignored.
SSL_CERT_DIR=dir
#
If --use-openssl-ca
is enabled, this overrides and sets OpenSSL's directory
containing trusted certificates.
Note: Be aware that unless the child environment is explicitly set, this evironment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.
SSL_CERT_FILE=file
#
If --use-openssl-ca
is enabled, this overrides and sets OpenSSL's file
containing trusted certificates.
Note: Be aware that unless the child environment is explicitly set, this evironment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.
NODE_REDIRECT_WARNINGS=file
#
When set, process warnings will be emitted to the given file instead of
printing to stderr. The file will be created if it does not exist, and will be
appended to if it does. If an error occurs while attempting to write the
warning to the file, the warning will be written to stderr instead. This is
equivalent to using the --redirect-warnings=file
command-line flag.
Console#
Stability: 2 - Stable
The console
module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
- A
Console
class with methods such asconsole.log()
,console.error()
andconsole.warn()
that can be used to write to any Node.js stream. - A global
console
instance configured to write toprocess.stdout
andprocess.stderr
. The globalconsole
can be used without callingrequire('console')
.
Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.
Example using the global console
:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
Example using the Console
class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
Class: Console#
The Console
class can be used to create a simple logger with configurable
output streams and can be accessed using either require('console').Console
or console.Console
(or their destructured counterparts):
const { Console } = require('console');
const { Console } = console;
new Console(stdout[, stderr])#
stdout
<Writable>stderr
<Writable>
Creates a new Console
by passing one or two writable stream instances.
stdout
is a writable stream to print log or info output. stderr
is used for warning or error output. If stderr
is not passed, warning and error
output will be sent to stdout
.
const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
const logger = new Console(output, errorOutput);
// use it like console
const count = 5;
logger.log('count: %d', count);
// in stdout.log: count 5
The global console
is a special Console
whose output is sent to
process.stdout
and process.stderr
. It is equivalent to calling:
new Console(process.stdout, process.stderr);
console.assert(value[, message][, ...args])#
value
<any>message
<any>...args
<any>
A simple assertion test that verifies whether value
is truthy. If it is not,
an AssertionError
is thrown. If provided, the error message
is formatted
using util.format()
and used as the error message.
console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work
Note: The console.assert()
method is implemented differently in Node.js
than the console.assert()
method available in browsers.
Specifically, in browsers, calling console.assert()
with a falsy
assertion will cause the message
to be printed to the console without
interrupting execution of subsequent code. In Node.js, however, a falsy
assertion will cause an AssertionError
to be thrown.
Functionality approximating that implemented by browsers can be implemented
by extending Node.js' console
and overriding the console.assert()
method.
In the following example, a simple module is created that extends and overrides
the default behavior of console
in Node.js.
'use strict';
// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.create(console, {
assert: {
value: function assert(assertion, message, ...args) {
try {
console.assert(assertion, message, ...args);
} catch (err) {
console.error(err.stack);
}
},
configurable: true,
enumerable: true,
writable: true,
},
});
module.exports = myConsole;
This can then be used as a direct replacement for the built in console:
const console = require('./myConsole');
console.assert(false, 'this message will print, but no error thrown');
console.log('this will also print');
console.dir(obj[, options])#
Uses util.inspect()
on obj
and prints the resulting string to stdout
.
This function bypasses any custom inspect()
function defined on obj
. An
optional options
object may be passed to alter certain aspects of the
formatted string:
showHidden
- iftrue
then the object's non-enumerable and symbol properties will be shown too. Defaults tofalse
.depth
- tellsutil.inspect()
how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to2
. To make it recurse indefinitely, passnull
.colors
- iftrue
, then the output will be styled with ANSI color codes. Defaults tofalse
. Colors are customizable; see customizingutil.inspect()
colors.
console.error([data][, ...args])#
data
<any>...args
<any>
Prints to stderr
with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3) (the arguments are all passed to
util.format()
).
const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr
If formatting elements (e.g. %d
) are not found in the first string then
util.inspect()
is called on each argument and the resulting string
values are concatenated. See util.format()
for more information.
console.info([data][, ...args])#
data
<any>...args
<any>
The console.info()
function is an alias for console.log()
.
console.log([data][, ...args])#
data
<any>...args
<any>
Prints to stdout
with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3) (the arguments are all passed to
util.format()
).
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
See util.format()
for more information.
console.time(label)#
label
<string>
Starts a timer that can be used to compute the duration of an operation. Timers
are identified by a unique label
. Use the same label
when calling
console.timeEnd()
to stop the timer and output the elapsed time in
milliseconds to stdout
. Timer durations are accurate to the sub-millisecond.
console.timeEnd(label)#
label
<string>
Stops a timer that was previously started by calling console.time()
and
prints the result to stdout
:
console.time('100-elements');
for (let i = 0; i < 100; i++) {}
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms
Note: As of Node.js v6.0.0, console.timeEnd()
deletes the timer to avoid
leaking it. On older versions, the timer persisted. This allowed
console.timeEnd()
to be called multiple times for the same label. This
functionality was unintended and is no longer supported.
console.trace([message][, ...args])#
message
<any>...args
<any>
Prints to stderr
the string 'Trace :'
, followed by the util.format()
formatted message and stack trace to the current position in the code.
console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
// Trace: Show me
// at repl:2:9
// at REPLServer.defaultEval (repl.js:248:27)
// at bound (domain.js:287:14)
// at REPLServer.runBound [as eval] (domain.js:300:12)
// at REPLServer.<anonymous> (repl.js:412:12)
// at emitOne (events.js:82:20)
// at REPLServer.emit (events.js:169:7)
// at REPLServer.Interface._onLine (readline.js:210:10)
// at REPLServer.Interface._line (readline.js:549:8)
// at REPLServer.Interface._ttyWrite (readline.js:826:14)
console.warn([data][, ...args])#
data
<any>...args
<any>
The console.warn()
function is an alias for console.error()
.
Crypto#
Stability: 2 - Stable
The crypto
module provides cryptographic functionality that includes a set of
wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign and verify functions.
Use require('crypto')
to access this module.
const crypto = require('crypto');
const secret = 'abcdefg';
const hash = crypto.createHmac('sha256', secret)
.update('I love cupcakes')
.digest('hex');
console.log(hash);
// Prints:
// c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e
Determining if crypto support is unavailable#
It is possible for Node.js to be built without including support for the
crypto
module. In such cases, calling require('crypto')
will result in an
error being thrown.
let crypto;
try {
crypto = require('crypto');
} catch (err) {
console.log('crypto support is disabled!');
}
Class: Certificate#
SPKAC is a Certificate Signing Request mechanism originally implemented by
Netscape and now specified formally as part of HTML5's keygen
element.
The crypto
module provides the Certificate
class for working with SPKAC
data. The most common usage is handling output generated by the HTML5
<keygen>
element. Node.js uses OpenSSL's SPKAC implementation internally.
new crypto.Certificate()#
Instances of the Certificate
class can be created using the new
keyword
or by calling crypto.Certificate()
as a function:
const crypto = require('crypto');
const cert1 = new crypto.Certificate();
const cert2 = crypto.Certificate();
certificate.exportChallenge(spkac)#
spkac
<string> | <Buffer> | <TypedArray> | <DataView>- Returns <Buffer> The challenge component of the
spkac
data structure, which includes a public key and a challenge.
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string
certificate.exportPublicKey(spkac)#
spkac
<string> | <Buffer> | <TypedArray> | <DataView>- Returns <Buffer> The public key component of the
spkac
data structure, which includes a public key and a challenge.
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>
certificate.verifySpkac(spkac)#
spkac
<Buffer> | <TypedArray> | <DataView>- Returns <boolean>
true
if the givenspkac
data structure is valid,false
otherwise.
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or false
Class: Cipher#
Instances of the Cipher
class are used to encrypt data. The class can be
used in one of two ways:
- As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or
- Using the
cipher.update()
andcipher.final()
methods to produce the encrypted data.
The crypto.createCipher()
or crypto.createCipheriv()
methods are
used to create Cipher
instances. Cipher
objects are not to be created
directly using the new
keyword.
Example: Using Cipher
objects as streams:
const crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');
let encrypted = '';
cipher.on('readable', () => {
const data = cipher.read();
if (data)
encrypted += data.toString('hex');
});
cipher.on('end', () => {
console.log(encrypted);
// Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504
});
cipher.write('some clear text data');
cipher.end();
Example: Using Cipher
and piped streams:
const crypto = require('crypto');
const fs = require('fs');
const cipher = crypto.createCipher('aes192', 'a password');
const input = fs.createReadStream('test.js');
const output = fs.createWriteStream('test.enc');
input.pipe(cipher).pipe(output);
Example: Using the cipher.update()
and cipher.final()
methods:
const crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');
let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
encrypted += cipher.final('hex');
console.log(encrypted);
// Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504
cipher.final([outputEncoding])#
outputEncoding
<string>
Returns any remaining enciphered contents. If outputEncoding
parameter is one of 'latin1'
, 'base64'
or 'hex'
, a string is returned.
If an outputEncoding
is not provided, a Buffer
is returned.
Once the cipher.final()
method has been called, the Cipher
object can no
longer be used to encrypt data. Attempts to call cipher.final()
more than
once will result in an error being thrown.
cipher.setAAD(buffer)#
buffer
<Buffer>- Returns the <Cipher> for method chaining.
When using an authenticated encryption mode (only GCM
is currently
supported), the cipher.setAAD()
method sets the value used for the
additional authenticated data (AAD) input parameter.
The cipher.setAAD()
method must be called before cipher.update()
.
cipher.getAuthTag()#
When using an authenticated encryption mode (only GCM
is currently
supported), the cipher.getAuthTag()
method returns a Buffer
containing
the authentication tag that has been computed from the given data.
The cipher.getAuthTag()
method should only be called after encryption has
been completed using the cipher.final()
method.
cipher.setAutoPadding([autoPadding])#
autoPadding
<boolean> Defaults totrue
.- Returns the <Cipher> for method chaining.
When using block encryption algorithms, the Cipher
class will automatically
add padding to the input data to the appropriate block size. To disable the
default padding call cipher.setAutoPadding(false)
.
When autoPadding
is false
, the length of the entire input data must be a
multiple of the cipher's block size or cipher.final()
will throw an Error.
Disabling automatic padding is useful for non-standard padding, for instance
using 0x0
instead of PKCS padding.
The cipher.setAutoPadding()
method must be called before
cipher.final()
.
cipher.update(data[, inputEncoding][, outputEncoding])#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>outputEncoding
<string>
Updates the cipher with data
. If the inputEncoding
argument is given,
its value must be one of 'utf8'
, 'ascii'
, or 'latin1'
and the data
argument is a string using the specified encoding. If the inputEncoding
argument is not given, data
must be a Buffer
, TypedArray
, or
DataView
. If data
is a Buffer
, TypedArray
, or DataView
, then
inputEncoding
is ignored.
The outputEncoding
specifies the output format of the enciphered
data, and can be 'latin1'
, 'base64'
or 'hex'
. If the outputEncoding
is specified, a string using the specified encoding is returned. If no
outputEncoding
is provided, a Buffer
is returned.
The cipher.update()
method can be called multiple times with new data until
cipher.final()
is called. Calling cipher.update()
after
cipher.final()
will result in an error being thrown.
Class: Decipher#
Instances of the Decipher
class are used to decrypt data. The class can be
used in one of two ways:
- As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or
- Using the
decipher.update()
anddecipher.final()
methods to produce the unencrypted data.
The crypto.createDecipher()
or crypto.createDecipheriv()
methods are
used to create Decipher
instances. Decipher
objects are not to be created
directly using the new
keyword.
Example: Using Decipher
objects as streams:
const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');
let decrypted = '';
decipher.on('readable', () => {
const data = decipher.read();
if (data)
decrypted += data.toString('utf8');
});
decipher.on('end', () => {
console.log(decrypted);
// Prints: some clear text data
});
const encrypted =
'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
decipher.write(encrypted, 'hex');
decipher.end();
Example: Using Decipher
and piped streams:
const crypto = require('crypto');
const fs = require('fs');
const decipher = crypto.createDecipher('aes192', 'a password');
const input = fs.createReadStream('test.enc');
const output = fs.createWriteStream('test.js');
input.pipe(decipher).pipe(output);
Example: Using the decipher.update()
and decipher.final()
methods:
const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');
const encrypted =
'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text data
decipher.final([outputEncoding])#
outputEncoding
<string>
Returns any remaining deciphered contents. If outputEncoding
parameter is one of 'latin1'
, 'ascii'
or 'utf8'
, a string is returned.
If an outputEncoding
is not provided, a Buffer
is returned.
Once the decipher.final()
method has been called, the Decipher
object can
no longer be used to decrypt data. Attempts to call decipher.final()
more
than once will result in an error being thrown.
decipher.setAAD(buffer)#
buffer
<Buffer> | <TypedArray> | <DataView>- Returns the <Cipher> for method chaining.
When using an authenticated encryption mode (only GCM
is currently
supported), the decipher.setAAD()
method sets the value used for the
additional authenticated data (AAD) input parameter.
The decipher.setAAD()
method must be called before decipher.update()
.
decipher.setAuthTag(buffer)#
buffer
<Buffer> | <TypedArray> | <DataView>- Returns the <Cipher> for method chaining.
When using an authenticated encryption mode (only GCM
is currently
supported), the decipher.setAuthTag()
method is used to pass in the
received authentication tag. If no tag is provided, or if the cipher text
has been tampered with, decipher.final()
with throw, indicating that the
cipher text should be discarded due to failed authentication.
The decipher.setAuthTag()
method must be called before
decipher.final()
.
decipher.setAutoPadding([autoPadding])#
autoPadding
<boolean> Defaults totrue
.- Returns the <Cipher> for method chaining.
When data has been encrypted without standard block padding, calling
decipher.setAutoPadding(false)
will disable automatic padding to prevent
decipher.final()
from checking for and removing padding.
Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size.
The decipher.setAutoPadding()
method must be called before
decipher.final()
.
decipher.update(data[, inputEncoding][, outputEncoding])#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>outputEncoding
<string>
Updates the decipher with data
. If the inputEncoding
argument is given,
its value must be one of 'latin1'
, 'base64'
, or 'hex'
and the data
argument is a string using the specified encoding. If the inputEncoding
argument is not given, data
must be a Buffer
. If data
is a
Buffer
then inputEncoding
is ignored.
The outputEncoding
specifies the output format of the enciphered
data, and can be 'latin1'
, 'ascii'
or 'utf8'
. If the outputEncoding
is specified, a string using the specified encoding is returned. If no
outputEncoding
is provided, a Buffer
is returned.
The decipher.update()
method can be called multiple times with new data until
decipher.final()
is called. Calling decipher.update()
after
decipher.final()
will result in an error being thrown.
Class: DiffieHellman#
The DiffieHellman
class is a utility for creating Diffie-Hellman key
exchanges.
Instances of the DiffieHellman
class can be created using the
crypto.createDiffieHellman()
function.
const crypto = require('crypto');
const assert = require('assert');
// Generate Alice's keys...
const alice = crypto.createDiffieHellman(2048);
const aliceKey = alice.generateKeys();
// Generate Bob's keys...
const bob = crypto.createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();
// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);
// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])#
otherPublicKey
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>outputEncoding
<string>
Computes the shared secret using otherPublicKey
as the other
party's public key and returns the computed shared secret. The supplied
key is interpreted using the specified inputEncoding
, and secret is
encoded using specified outputEncoding
. Encodings can be
'latin1'
, 'hex'
, or 'base64'
. If the inputEncoding
is not
provided, otherPublicKey
is expected to be a Buffer
,
TypedArray
, or DataView
.
If outputEncoding
is given a string is returned; otherwise, a
Buffer
is returned.
diffieHellman.generateKeys([encoding])#
encoding
<string>
Generates private and public Diffie-Hellman key values, and returns
the public key in the specified encoding
. This key should be
transferred to the other party. Encoding can be 'latin1'
, 'hex'
,
or 'base64'
. If encoding
is provided a string is returned; otherwise a
Buffer
is returned.
diffieHellman.getGenerator([encoding])#
encoding
<string>
Returns the Diffie-Hellman generator in the specified encoding
, which can
be 'latin1'
, 'hex'
, or 'base64'
. If encoding
is provided a string is
returned; otherwise a Buffer
is returned.
diffieHellman.getPrime([encoding])#
encoding
<string>
Returns the Diffie-Hellman prime in the specified encoding
, which can
be 'latin1'
, 'hex'
, or 'base64'
. If encoding
is provided a string is
returned; otherwise a Buffer
is returned.
diffieHellman.getPrivateKey([encoding])#
encoding
<string>
Returns the Diffie-Hellman private key in the specified encoding
,
which can be 'latin1'
, 'hex'
, or 'base64'
. If encoding
is provided a
string is returned; otherwise a Buffer
is returned.
diffieHellman.getPublicKey([encoding])#
encoding
<string>
Returns the Diffie-Hellman public key in the specified encoding
, which
can be 'latin1'
, 'hex'
, or 'base64'
. If encoding
is provided a
string is returned; otherwise a Buffer
is returned.
diffieHellman.setPrivateKey(privateKey[, encoding])#
privateKey
<string> | <Buffer> | <TypedArray> | <DataView>encoding
<string>
Sets the Diffie-Hellman private key. If the encoding
argument is provided
and is either 'latin1'
, 'hex'
, or 'base64'
, privateKey
is expected
to be a string. If no encoding
is provided, privateKey
is expected
to be a Buffer
, TypedArray
, or DataView
.
diffieHellman.setPublicKey(publicKey[, encoding])#
publicKey
<string> | <Buffer> | <TypedArray> | <DataView>encoding
<string>
Sets the Diffie-Hellman public key. If the encoding
argument is provided
and is either 'latin1'
, 'hex'
or 'base64'
, publicKey
is expected
to be a string. If no encoding
is provided, publicKey
is expected
to be a Buffer
, TypedArray
, or DataView
.
diffieHellman.verifyError#
A bit field containing any warnings and/or errors resulting from a check
performed during initialization of the DiffieHellman
object.
The following values are valid for this property (as defined in constants
module):
DH_CHECK_P_NOT_SAFE_PRIME
DH_CHECK_P_NOT_PRIME
DH_UNABLE_TO_CHECK_GENERATOR
DH_NOT_SUITABLE_GENERATOR
Class: ECDH#
The ECDH
class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH)
key exchanges.
Instances of the ECDH
class can be created using the
crypto.createECDH()
function.
const crypto = require('crypto');
const assert = require('assert');
// Generate Alice's keys...
const alice = crypto.createECDH('secp521r1');
const aliceKey = alice.generateKeys();
// Generate Bob's keys...
const bob = crypto.createECDH('secp521r1');
const bobKey = bob.generateKeys();
// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
// OK
ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])#
otherPublicKey
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>outputEncoding
<string>
Computes the shared secret using otherPublicKey
as the other
party's public key and returns the computed shared secret. The supplied
key is interpreted using specified inputEncoding
, and the returned secret
is encoded using the specified outputEncoding
. Encodings can be
'latin1'
, 'hex'
, or 'base64'
. If the inputEncoding
is not
provided, otherPublicKey
is expected to be a Buffer
, TypedArray
, or
DataView
.
If outputEncoding
is given a string will be returned; otherwise a
Buffer
is returned.
ecdh.generateKeys([encoding[, format]])#
Generates private and public EC Diffie-Hellman key values, and returns
the public key in the specified format
and encoding
. This key should be
transferred to the other party.
The format
argument specifies point encoding and can be 'compressed'
or
'uncompressed'
. If format
is not specified, the point will be returned in
'uncompressed'
format.
The encoding
argument can be 'latin1'
, 'hex'
, or 'base64'
. If
encoding
is provided a string is returned; otherwise a Buffer
is returned.
ecdh.getPrivateKey([encoding])#
encoding
<string>
Returns the EC Diffie-Hellman private key in the specified encoding
,
which can be 'latin1'
, 'hex'
, or 'base64'
. If encoding
is provided
a string is returned; otherwise a Buffer
is returned.
ecdh.getPublicKey([encoding][, format])#
Returns the EC Diffie-Hellman public key in the specified encoding
and
format
.
The format
argument specifies point encoding and can be 'compressed'
or
'uncompressed'
. If format
is not specified the point will be returned in
'uncompressed'
format.
The encoding
argument can be 'latin1'
, 'hex'
, or 'base64'
. If
encoding
is specified, a string is returned; otherwise a Buffer
is
returned.
ecdh.setPrivateKey(privateKey[, encoding])#
privateKey
<string> | <Buffer> | <TypedArray> | <DataView>encoding
<string>
Sets the EC Diffie-Hellman private key. The encoding
can be 'latin1'
,
'hex'
or 'base64'
. If encoding
is provided, privateKey
is expected
to be a string; otherwise privateKey
is expected to be a Buffer
,
TypedArray
, or DataView
.
If privateKey
is not valid for the curve specified when the ECDH
object was
created, an error is thrown. Upon setting the private key, the associated
public point (key) is also generated and set in the ECDH object.
ecdh.setPublicKey(publicKey[, encoding])#
Stability: 0 - Deprecated
publicKey
<string> | <Buffer> | <TypedArray> | <DataView>encoding
<string>
Sets the EC Diffie-Hellman public key. Key encoding can be 'latin1'
,
'hex'
or 'base64'
. If encoding
is provided publicKey
is expected to
be a string; otherwise a Buffer
, TypedArray
, or DataView
is expected.
Note that there is not normally a reason to call this method because ECDH
only requires a private key and the other party's public key to compute the
shared secret. Typically either ecdh.generateKeys()
or
ecdh.setPrivateKey()
will be called. The ecdh.setPrivateKey()
method
attempts to generate the public point/key associated with the private key being
set.
Example (obtaining a shared secret):
const crypto = require('crypto');
const alice = crypto.createECDH('secp256k1');
const bob = crypto.createECDH('secp256k1');
// Note: This is a shortcut way to specify one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
crypto.createHash('sha256').update('alice', 'utf8').digest()
);
// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();
const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');
// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);
Class: Hash#
The Hash
class is a utility for creating hash digests of data. It can be
used in one of two ways:
- As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or
- Using the
hash.update()
andhash.digest()
methods to produce the computed hash.
The crypto.createHash()
method is used to create Hash
instances. Hash
objects are not to be created directly using the new
keyword.
Example: Using Hash
objects as streams:
const crypto = require('crypto');
const hash = crypto.createHash('sha256');
hash.on('readable', () => {
const data = hash.read();
if (data) {
console.log(data.toString('hex'));
// Prints:
// 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
}
});
hash.write('some data to hash');
hash.end();
Example: Using Hash
and piped streams:
const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');
const input = fs.createReadStream('test.js');
input.pipe(hash).pipe(process.stdout);
Example: Using the hash.update()
and hash.digest()
methods:
const crypto = require('crypto');
const hash = crypto.createHash('sha256');
hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
// 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
hash.digest([encoding])#
encoding
<string>
Calculates the digest of all of the data passed to be hashed (using the
hash.update()
method). The encoding
can be 'hex'
, 'latin1'
or
'base64'
. If encoding
is provided a string will be returned; otherwise
a Buffer
is returned.
The Hash
object can not be used again after hash.digest()
method has been
called. Multiple calls will cause an error to be thrown.
hash.update(data[, inputEncoding])#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>
Updates the hash content with the given data
, the encoding of which
is given in inputEncoding
and can be 'utf8'
, 'ascii'
or
'latin1'
. If encoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
Class: Hmac#
The Hmac
Class is a utility for creating cryptographic HMAC digests. It can
be used in one of two ways:
- As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or
- Using the
hmac.update()
andhmac.digest()
methods to produce the computed HMAC digest.
The crypto.createHmac()
method is used to create Hmac
instances. Hmac
objects are not to be created directly using the new
keyword.
Example: Using Hmac
objects as streams:
const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');
hmac.on('readable', () => {
const data = hmac.read();
if (data) {
console.log(data.toString('hex'));
// Prints:
// 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
}
});
hmac.write('some data to hash');
hmac.end();
Example: Using Hmac
and piped streams:
const crypto = require('crypto');
const fs = require('fs');
const hmac = crypto.createHmac('sha256', 'a secret');
const input = fs.createReadStream('test.js');
input.pipe(hmac).pipe(process.stdout);
Example: Using the hmac.update()
and hmac.digest()
methods:
const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');
hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
// 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
hmac.digest([encoding])#
encoding
<string>
Calculates the HMAC digest of all of the data passed using hmac.update()
.
The encoding
can be 'hex'
, 'latin1'
or 'base64'
. If encoding
is
provided a string is returned; otherwise a Buffer
is returned;
The Hmac
object can not be used again after hmac.digest()
has been
called. Multiple calls to hmac.digest()
will result in an error being thrown.
hmac.update(data[, inputEncoding])#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>
Updates the Hmac
content with the given data
, the encoding of which
is given in inputEncoding
and can be 'utf8'
, 'ascii'
or
'latin1'
. If encoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
Class: Sign#
The Sign
Class is a utility for generating signatures. It can be used in one
of two ways:
- As a writable stream, where data to be signed is written and the
sign.sign()
method is used to generate and return the signature, or - Using the
sign.update()
andsign.sign()
methods to produce the signature.
The crypto.createSign()
method is used to create Sign
instances. Sign
objects are not to be created directly using the new
keyword.
Example: Using Sign
objects as streams:
const crypto = require('crypto');
const sign = crypto.createSign('RSA-SHA256');
sign.write('some data to sign');
sign.end();
const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature
Example: Using the sign.update()
and sign.sign()
methods:
const crypto = require('crypto');
const sign = crypto.createSign('RSA-SHA256');
sign.update('some data to sign');
const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature
A Sign
instance can also be created by just passing in the digest
algorithm name, in which case OpenSSL will infer the full signature algorithm
from the type of the PEM-formatted private key, including algorithms that
do not have directly exposed name constants, e.g. 'ecdsa-with-SHA256'.
Example: signing using ECDSA with SHA256
const crypto = require('crypto');
const sign = crypto.createSign('sha256');
sign.update('some data to sign');
const privateKey =
`-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIF+jnWY1D5kbVYDNvxxo/Y+ku2uJPDwS0r/VuPZQrjjVoAoGCCqGSM49
AwEHoUQDQgAEurOxfSxmqIRYzJVagdZfMMSjRNNhB8i3mXyIMq704m2m52FdfKZ2
pQhByd5eyj3lgZ7m7jbchtdgyOF8Io/1ng==
-----END EC PRIVATE KEY-----`;
console.log(sign.sign(privateKey).toString('hex'));
sign.sign(privateKey[, outputFormat])#
Calculates the signature on all the data passed through using either
sign.update()
or sign.write()
.
The privateKey
argument can be an object or a string. If privateKey
is a
string, it is treated as a raw key with no passphrase. If privateKey
is an
object, it must contain one or more of the following properties:
key
: <string> - PEM encoded private key (required)passphrase
: <string> - passphrase for the private keypadding
: <integer> - Optional padding value for RSA, one of the following:crypto.constants.RSA_PKCS1_PADDING
(default)crypto.constants.RSA_PKCS1_PSS_PADDING
Note that
RSA_PKCS1_PSS_PADDING
will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.saltLength
: <integer> - salt length for when padding isRSA_PKCS1_PSS_PADDING
. The special valuecrypto.constants.RSA_PSS_SALTLEN_DIGEST
sets the salt length to the digest size,crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN
(default) sets it to the maximum permissible value.
The outputFormat
can specify one of 'latin1'
, 'hex'
or 'base64'
. If
outputFormat
is provided a string is returned; otherwise a Buffer
is
returned.
The Sign
object can not be again used after sign.sign()
method has been
called. Multiple calls to sign.sign()
will result in an error being thrown.
sign.update(data[, inputEncoding])#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>
Updates the Sign
content with the given data
, the encoding of which
is given in inputEncoding
and can be 'utf8'
, 'ascii'
or
'latin1'
. If encoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
Class: Verify#
The Verify
class is a utility for verifying signatures. It can be used in one
of two ways:
- As a writable stream where written data is used to validate against the supplied signature, or
- Using the
verify.update()
andverify.verify()
methods to verify the signature.
The crypto.createVerify()
method is used to create Verify
instances.
Verify
objects are not to be created directly using the new
keyword.
Example: Using Verify
objects as streams:
const crypto = require('crypto');
const verify = crypto.createVerify('RSA-SHA256');
verify.write('some data to sign');
verify.end();
const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false
Example: Using the verify.update()
and verify.verify()
methods:
const crypto = require('crypto');
const verify = crypto.createVerify('RSA-SHA256');
verify.update('some data to sign');
const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false
verify.update(data[, inputEncoding])#
data
<string> | <Buffer> | <TypedArray> | <DataView>inputEncoding
<string>
Updates the Verify
content with the given data
, the encoding of which
is given in inputEncoding
and can be 'utf8'
, 'ascii'
or
'latin1'
. If encoding
is not provided, and the data
is a string, an
encoding of 'utf8'
is enforced. If data
is a Buffer
, TypedArray
, or
DataView
, then inputEncoding
is ignored.
This can be called many times with new data as it is streamed.
verify.verify(object, signature[, signatureFormat])#
object
<string> | <Object>signature
<string> | <Buffer> | <TypedArray> | <DataView>signatureFormat
<string>
Verifies the provided data using the given object
and signature
.
The object
argument can be either a string containing a PEM encoded object,
which can be an RSA public key, a DSA public key, or an X.509 certificate,
or an object with one or more of the following properties:
key
: <string> - PEM encoded public key (required)padding
: <integer> - Optional padding value for RSA, one of the following:crypto.constants.RSA_PKCS1_PADDING
(default)crypto.constants.RSA_PKCS1_PSS_PADDING
Note that
RSA_PKCS1_PSS_PADDING
will use MGF1 with the same hash function used to verify the message as specified in section 3.1 of RFC 4055.saltLength
: <integer> - salt length for when padding isRSA_PKCS1_PSS_PADDING
. The special valuecrypto.constants.RSA_PSS_SALTLEN_DIGEST
sets the salt length to the digest size,crypto.constants.RSA_PSS_SALTLEN_AUTO
(default) causes it to be determined automatically.
The signature
argument is the previously calculated signature for the data, in
the signatureFormat
which can be 'latin1'
, 'hex'
or 'base64'
.
If a signatureFormat
is specified, the signature
is expected to be a
string; otherwise signature
is expected to be a Buffer
,
TypedArray
, or DataView
.
Returns true
or false
depending on the validity of the signature for
the data and public key.
The verify
object can not be used again after verify.verify()
has been
called. Multiple calls to verify.verify()
will result in an error being
thrown.
crypto
module methods and properties#
crypto.constants#
Returns an object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto Constants.
crypto.DEFAULT_ENCODING#
The default encoding to use for functions that can take either strings
or buffers. The default value is 'buffer'
, which makes methods
default to Buffer
objects.
The crypto.DEFAULT_ENCODING
mechanism is provided for backwards compatibility
with legacy programs that expect 'latin1'
to be the default encoding.
New applications should expect the default to be 'buffer'
. This property may
become deprecated in a future Node.js release.
crypto.fips#
Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js.
crypto.createCipher(algorithm, password)#
algorithm
<string>password
<string> | <Buffer> | <TypedArray> | <DataView>
Creates and returns a Cipher
object that uses the given algorithm
and
password
.
The algorithm
is dependent on OpenSSL, examples are 'aes192'
, etc. On
recent OpenSSL releases, openssl list-cipher-algorithms
will display the
available cipher algorithms.
The password
is used to derive the cipher key and initialization vector (IV).
The value must be either a 'latin1'
encoded string, a Buffer
, a
TypedArray
, or a DataView
.
The implementation of crypto.createCipher()
derives keys using the OpenSSL
function EVP_BytesToKey
with the digest algorithm set to MD5, one
iteration, and no salt. The lack of salt allows dictionary attacks as the same
password always creates the same key. The low iteration count and
non-cryptographically secure hash algorithm allow passwords to be tested very
rapidly.
In line with OpenSSL's recommendation to use pbkdf2 instead of
EVP_BytesToKey
it is recommended that developers derive a key and IV on
their own using crypto.pbkdf2()
and to use crypto.createCipheriv()
to create the Cipher
object.
crypto.createCipheriv(algorithm, key, iv)#
algorithm
<string>key
<string> | <Buffer> | <TypedArray> | <DataView>iv
<string> | <Buffer> | <TypedArray> | <DataView>
Creates and returns a Cipher
object, with the given algorithm
, key
and
initialization vector (iv
).
The algorithm
is dependent on OpenSSL, examples are 'aes192'
, etc. On
recent OpenSSL releases, openssl list-cipher-algorithms
will display the
available cipher algorithms.
The key
is the raw key used by the algorithm
and iv
is an
initialization vector. Both arguments must be 'utf8'
encoded strings,
Buffers, TypedArray
, or DataView
s.
crypto.createCredentials(details)#
Stability: 0 - Deprecated: Use tls.createSecureContext()
instead.
details
<Object> Identical totls.createSecureContext()
.
The crypto.createCredentials()
method is a deprecated function for creating
and returning a tls.SecureContext
. It should not be used. Replace it with
tls.createSecureContext()
which has the exact same arguments and return
value.
Returns a tls.SecureContext
, as-if tls.createSecureContext()
had been
called.
crypto.createDecipher(algorithm, password)#
algorithm
<string>password
<string> | <Buffer> | <TypedArray> | <DataView>
Creates and returns a Decipher
object that uses the given algorithm
and
password
(key).
The implementation of crypto.createDecipher()
derives keys using the OpenSSL
function EVP_BytesToKey
with the digest algorithm set to MD5, one
iteration, and no salt. The lack of salt allows dictionary attacks as the same
password always creates the same key. The low iteration count and
non-cryptographically secure hash algorithm allow passwords to be tested very
rapidly.
In line with OpenSSL's recommendation to use pbkdf2 instead of
EVP_BytesToKey
it is recommended that developers derive a key and IV on
their own using crypto.pbkdf2()
and to use crypto.createDecipheriv()
to create the Decipher
object.
crypto.createDecipheriv(algorithm, key, iv)#
algorithm
<string>key
<string> | <Buffer> | <TypedArray> | <DataView>iv
<string> | <Buffer> | <TypedArray> | <DataView>
Creates and returns a Decipher
object that uses the given algorithm
, key
and initialization vector (iv
).
The algorithm
is dependent on OpenSSL, examples are 'aes192'
, etc. On
recent OpenSSL releases, openssl list-cipher-algorithms
will display the
available cipher algorithms.
The key
is the raw key used by the algorithm
and iv
is an
initialization vector. Both arguments must be 'utf8'
encoded strings or
buffers.
crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])#
prime
<string> | <Buffer> | <TypedArray> | <DataView>primeEncoding
<string>generator
<number> | <string> | <Buffer> | <TypedArray> | <DataView> Defaults to2
.generatorEncoding
<string>
Creates a DiffieHellman
key exchange object using the supplied prime
and an
optional specific generator
.
The generator
argument can be a number, string, or Buffer
. If
generator
is not specified, the value 2
is used.
The primeEncoding
and generatorEncoding
arguments can be 'latin1'
,
'hex'
, or 'base64'
.
If primeEncoding
is specified, prime
is expected to be a string; otherwise
a Buffer
, TypedArray
, or DataView
is expected.
If generatorEncoding
is specified, generator
is expected to be a string;
otherwise a number, Buffer
, TypedArray
, or DataView
is expected.
crypto.createDiffieHellman(primeLength[, generator])#
primeLength
<number>generator
<number> | <string> | <Buffer> | <TypedArray> | <DataView> Defaults to2
.
Creates a DiffieHellman
key exchange object and generates a prime of
primeLength
bits using an optional specific numeric generator
.
If generator
is not specified, the value 2
is used.
crypto.createECDH(curveName)#
curveName
<string>
Creates an Elliptic Curve Diffie-Hellman (ECDH
) key exchange object using a
predefined curve specified by the curveName
string. Use
crypto.getCurves()
to obtain a list of available curve names. On recent
OpenSSL releases, openssl ecparam -list_curves
will also display the name
and description of each available elliptic curve.
crypto.createHash(algorithm)#
algorithm
<string>
Creates and returns a Hash
object that can be used to generate hash digests
using the given algorithm
.
The algorithm
is dependent on the available algorithms supported by the
version of OpenSSL on the platform. Examples are 'sha256'
, 'sha512'
, etc.
On recent releases of OpenSSL, openssl list-message-digest-algorithms
will
display the available digest algorithms.
Example: generating the sha256 sum of a file
const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');
const input = fs.createReadStream(filename);
input.on('readable', () => {
const data = input.read();
if (data)
hash.update(data);
else {
console.log(`${hash.digest('hex')} ${filename}`);
}
});
crypto.createHmac(algorithm, key)#
algorithm
<string>key
<string> | <Buffer> | <TypedArray> | <DataView>
Creates and returns an Hmac
object that uses the given algorithm
and key
.
The algorithm
is dependent on the available algorithms supported by the
version of OpenSSL on the platform. Examples are 'sha256'
, 'sha512'
, etc.
On recent releases of OpenSSL, openssl list-message-digest-algorithms
will
display the available digest algorithms.
The key
is the HMAC key used to generate the cryptographic HMAC hash.
Example: generating the sha256 HMAC of a file
const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');
const hmac = crypto.createHmac('sha256', 'a secret');
const input = fs.createReadStream(filename);
input.on('readable', () => {
const data = input.read();
if (data)
hmac.update(data);
else {
console.log(`${hmac.digest('hex')} ${filename}`);
}
});
crypto.createSign(algorithm)#
algorithm
<string>
Creates and returns a Sign
object that uses the given algorithm
.
Use crypto.getHashes()
to obtain an array of names of the available
signing algorithms.
crypto.createVerify(algorithm)#
algorithm
<string>
Creates and returns a Verify
object that uses the given algorithm.
Use crypto.getHashes()
to obtain an array of names of the available
signing algorithms.
crypto.getCiphers()#
Returns an array with the names of the supported cipher algorithms.
Example:
const ciphers = crypto.getCiphers();
console.log(ciphers); // ['aes-128-cbc', 'aes-128-ccm', ...]
crypto.getCurves()#
Returns an array with the names of the supported elliptic curves.
Example:
const curves = crypto.getCurves();
console.log(curves); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]
crypto.getDiffieHellman(groupName)#
groupName
<string>
Creates a predefined DiffieHellman
key exchange object. The
supported groups are: 'modp1'
, 'modp2'
, 'modp5'
(defined in
RFC 2412, but see Caveats) and 'modp14'
, 'modp15'
,
'modp16'
, 'modp17'
, 'modp18'
(defined in RFC 3526). The
returned object mimics the interface of objects created by
crypto.createDiffieHellman()
, but will not allow changing
the keys (with diffieHellman.setPublicKey()
for example). The
advantage of using this method is that the parties do not have to
generate nor exchange a group modulus beforehand, saving both processor
and communication time.
Example (obtaining a shared secret):
const crypto = require('crypto');
const alice = crypto.getDiffieHellman('modp14');
const bob = crypto.getDiffieHellman('modp14');
alice.generateKeys();
bob.generateKeys();
const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');
/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);
crypto.getHashes()#
Returns an array of the names of the supported hash algorithms,
such as RSA-SHA256
.
Example:
const hashes = crypto.getHashes();
console.log(hashes); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]
crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)#
password
<string>salt
<string>iterations
<number>keylen
<number>digest
<string>callback
<Function>
Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2)
implementation. A selected HMAC digest algorithm specified by digest
is
applied to derive a key of the requested byte length (keylen
) from the
password
, salt
and iterations
.
The supplied callback
function is called with two arguments: err
and
derivedKey
. If an error occurs, err
will be set; otherwise err
will be
null. The successfully generated derivedKey
will be passed as a Buffer
.
The iterations
argument must be a number set as high as possible. The
higher the number of iterations, the more secure the derived key will be,
but will take a longer amount of time to complete.
The salt
should also be as unique as possible. It is recommended that the
salts are random and their lengths are greater than 16 bytes. See
NIST SP 800-132 for details.
Example:
const crypto = require('crypto');
crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, derivedKey) => {
if (err) throw err;
console.log(derivedKey.toString('hex')); // '3745e48...aa39b34'
});
An array of supported digest functions can be retrieved using
crypto.getHashes()
.
crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)#
Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2)
implementation. A selected HMAC digest algorithm specified by digest
is
applied to derive a key of the requested byte length (keylen
) from the
password
, salt
and iterations
.
If an error occurs an Error will be thrown, otherwise the derived key will be
returned as a Buffer
.
The iterations
argument must be a number set as high as possible. The
higher the number of iterations, the more secure the derived key will be,
but will take a longer amount of time to complete.
The salt
should also be as unique as possible. It is recommended that the
salts are random and their lengths are greater than 16 bytes. See
NIST SP 800-132 for details.
Example:
const crypto = require('crypto');
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 512, 'sha512');
console.log(key.toString('hex')); // '3745e48...aa39b34'
An array of supported digest functions can be retrieved using
crypto.getHashes()
.
crypto.privateDecrypt(privateKey, buffer)#
privateKey
<Object> | <string>buffer
<Buffer> | <TypedArray> | <DataView>
Decrypts buffer
with privateKey
.
privateKey
can be an object or a string. If privateKey
is a string, it is
treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING
.
crypto.privateEncrypt(privateKey, buffer)#
privateKey
<Object> | <string>buffer
<Buffer> | <TypedArray> | <DataView>
Encrypts buffer
with privateKey
.
privateKey
can be an object or a string. If privateKey
is a string, it is
treated as the key with no passphrase and will use RSA_PKCS1_PADDING
.
crypto.publicDecrypt(publicKey, buffer)#
publicKey
<Object> | <string>buffer
<Buffer> | <TypedArray> | <DataView>
Decrypts buffer
with publicKey
.
publicKey
can be an object or a string. If publicKey
is a string, it is
treated as the key with no passphrase and will use RSA_PKCS1_PADDING
.
Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.
crypto.publicEncrypt(publicKey, buffer)#
publicKey
<Object> | <string>buffer
<Buffer> | <TypedArray> | <DataView>
Encrypts the content of buffer
with publicKey
and returns a new
Buffer
with encrypted content.
publicKey
can be an object or a string. If publicKey
is a string, it is
treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING
.
Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.
crypto.randomBytes(size[, callback])#
size
<number>callback
<Function>
Generates cryptographically strong pseudo-random data. The size
argument
is a number indicating the number of bytes to generate.
If a callback
function is provided, the bytes are generated asynchronously
and the callback
function is invoked with two arguments: err
and buf
.
If an error occurs, err
will be an Error object; otherwise it is null. The
buf
argument is a Buffer
containing the generated bytes.
// Asynchronous
const crypto = require('crypto');
crypto.randomBytes(256, (err, buf) => {
if (err) throw err;
console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});
If the callback
function is not provided, the random bytes are generated
synchronously and returned as a Buffer
. An error will be thrown if
there is a problem generating the bytes.
// Synchronous
const buf = crypto.randomBytes(256);
console.log(
`${buf.length} bytes of random data: ${buf.toString('hex')}`);
The crypto.randomBytes()
method will block until there is sufficient entropy.
This should normally never take longer than a few milliseconds. The only time
when generating the random bytes may conceivably block for a longer period of
time is right after boot, when the whole system is still low on entropy.
crypto.randomFillSync(buffer[, offset][, size])#
buffer
<Buffer> | <Uint8Array> Must be supplied.offset
<number> Defaults to0
.size
<number> Defaults tobuffer.length - offset
.
Synchronous version of crypto.randomFill()
.
Returns buffer
const buf = Buffer.alloc(10);
console.log(crypto.randomFillSync(buf).toString('hex'));
crypto.randomFillSync(buf, 5);
console.log(buf.toString('hex'));
// The above is equivalent to the following:
crypto.randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));
crypto.randomFill(buffer[, offset][, size], callback)#
buffer
<Buffer> | <Uint8Array> Must be supplied.offset
<number> Defaults to0
.size
<number> Defaults tobuffer.length - offset
.callback
<Function>function(err, buf) {}
.
This function is similar to crypto.randomBytes()
but requires the first
argument to be a Buffer
that will be filled. It also
requires that a callback is passed in.
If the callback
function is not provided, an error will be thrown.
const buf = Buffer.alloc(10);
crypto.randomFill(buf, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
crypto.randomFill(buf, 5, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
// The above is equivalent to the following:
crypto.randomFill(buf, 5, 5, (err, buf) => {
if (err) throw err;
console.log(buf.toString('hex'));
});
crypto.setEngine(engine[, flags])#
engine
<string>flags
<crypto.constants> Defaults tocrypto.constants.ENGINE_METHOD_ALL
.
Load and set the engine
for some or all OpenSSL functions (selected by flags).
engine
could be either an id or a path to the engine's shared library.
The optional flags
argument uses ENGINE_METHOD_ALL
by default. The flags
is a bit field taking one of or a mix of the following flags (defined in
crypto.constants
):
crypto.constants.ENGINE_METHOD_RSA
crypto.constants.ENGINE_METHOD_DSA
crypto.constants.ENGINE_METHOD_DH
crypto.constants.ENGINE_METHOD_RAND
crypto.constants.ENGINE_METHOD_ECDH
crypto.constants.ENGINE_METHOD_ECDSA
crypto.constants.ENGINE_METHOD_CIPHERS
crypto.constants.ENGINE_METHOD_DIGESTS
crypto.constants.ENGINE_METHOD_STORE
crypto.constants.ENGINE_METHOD_PKEY_METHS
crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS
crypto.constants.ENGINE_METHOD_ALL
crypto.constants.ENGINE_METHOD_NONE
crypto.timingSafeEqual(a, b)#
a
<Buffer> | <TypedArray> | <DataView>b
<Buffer> | <TypedArray> | <DataView>
Returns true if a
is equal to b
, without leaking timing information that
would allow an attacker to guess one of the values. This is suitable for
comparing HMAC digests or secret values like authentication cookies or
capability urls.
a
and b
must both be Buffer
s, TypedArray
s, or DataView
s, and they
must have the same length.
Note: Use of crypto.timingSafeEqual
does not guarantee that the
surrounding code is timing-safe. Care should be taken to ensure that the
surrounding code does not introduce timing vulnerabilities.
Notes#
Legacy Streams API (pre Node.js v0.10)#
The Crypto module was added to Node.js before there was the concept of a
unified Stream API, and before there were Buffer
objects for handling
binary data. As such, the many of the crypto
defined classes have methods not
typically found on other Node.js classes that implement the streams
API (e.g. update()
, final()
, or digest()
). Also, many methods accepted
and returned 'latin1'
encoded strings by default rather than Buffers. This
default was changed after Node.js v0.8 to use Buffer
objects by default
instead.
Recent ECDH Changes#
Usage of ECDH
with non-dynamically generated key pairs has been simplified.
Now, ecdh.setPrivateKey()
can be called with a preselected private key
and the associated public point (key) will be computed and stored in the object.
This allows code to only store and provide the private part of the EC key pair.
ecdh.setPrivateKey()
now also validates that the private key is valid for
the selected curve.
The ecdh.setPublicKey()
method is now deprecated as its inclusion in the
API is not useful. Either a previously stored private key should be set, which
automatically generates the associated public key, or ecdh.generateKeys()
should be called. The main drawback of using ecdh.setPublicKey()
is that
it can be used to put the ECDH key pair into an inconsistent state.
Support for weak or compromised algorithms#
The crypto
module still supports some algorithms which are already
compromised and are not currently recommended for use. The API also allows
the use of ciphers and hashes with a small key size that are considered to be
too weak for safe use.
Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements.
Based on the recommendations of NIST SP 800-131A:
- MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures.
- The key used with RSA, DSA and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years.
- The DH groups of
modp1
,modp2
andmodp5
have a key size smaller than 2048 bits and are not recommended.
See the reference for other recommendations and details.
Crypto Constants#
The following constants exported by crypto.constants
apply to various uses of
the crypto
, tls
, and https
modules and are generally specific to OpenSSL.
OpenSSL Options#
Constant | Description |
---|---|
SSL_OP_ALL |
Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html for detail. |
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION |
Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html. |
SSL_OP_CIPHER_SERVER_PREFERENCE |
Attempts to use the server's preferences instead of the client's when selecting a cipher. Behavior depends on protocol version. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html. |
SSL_OP_CISCO_ANYCONNECT |
Instructs OpenSSL to use Cisco's "speshul" version of DTLS_BAD_VER. |
SSL_OP_COOKIE_EXCHANGE |
Instructs OpenSSL to turn on cookie exchange. |
SSL_OP_CRYPTOPRO_TLSEXT_BUG |
Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft. |
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS |
Instructs OpenSSL to disable a SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d. |
SSL_OP_EPHEMERAL_RSA |
Instructs OpenSSL to always use the tmp_rsa key when performing RSA operations. |
SSL_OP_LEGACY_SERVER_CONNECT |
Allows initial connection to servers that do not support RI. |
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER |
|
SSL_OP_MICROSOFT_SESS_ID_BUG |
|
SSL_OP_MSIE_SSLV2_RSA_PADDING |
Instructs OpenSSL to disable the workaround for a man-in-the-middle protocol-version vulnerability in the SSL 2.0 server implementation. |
SSL_OP_NETSCAPE_CA_DN_BUG |
|
SSL_OP_NETSCAPE_CHALLENGE_BUG |
|
SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG |
|
SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG |
|
SSL_OP_NO_COMPRESSION |
Instructs OpenSSL to disable support for SSL/TLS compression. |
SSL_OP_NO_QUERY_MTU |
|
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION |
Instructs OpenSSL to always start a new session when performing renegotiation. |
SSL_OP_NO_SSLv2 |
Instructs OpenSSL to turn off SSL v2 |
SSL_OP_NO_SSLv3 |
Instructs OpenSSL to turn off SSL v3 |
SSL_OP_NO_TICKET |
Instructs OpenSSL to disable use of RFC4507bis tickets. |
SSL_OP_NO_TLSv1 |
Instructs OpenSSL to turn off TLS v1 |
SSL_OP_NO_TLSv1_1 |
Instructs OpenSSL to turn off TLS v1.1 |
SSL_OP_NO_TLSv1_2 |
Instructs OpenSSL to turn off TLS v1.2 | SSL_OP_PKCS1_CHECK_1 |
SSL_OP_PKCS1_CHECK_2 |
|
SSL_OP_SINGLE_DH_USE |
Instructs OpenSSL to always create a new key when using temporary/ephemeral DH parameters. |
SSL_OP_SINGLE_ECDH_USE |
Instructs OpenSSL to always create a new key when using temporary/ephemeral ECDH parameters. | SSL_OP_SSLEAY_080_CLIENT_DH_BUG |
SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG |
|
SSL_OP_TLS_BLOCK_PADDING_BUG |
|
SSL_OP_TLS_D5_BUG |
|
SSL_OP_TLS_ROLLBACK_BUG |
Instructs OpenSSL to disable version rollback attack detection. |
OpenSSL Engine Constants#
Constant | Description |
---|---|
ENGINE_METHOD_RSA |
Limit engine usage to RSA |
ENGINE_METHOD_DSA |
Limit engine usage to DSA |
ENGINE_METHOD_DH |
Limit engine usage to DH |
ENGINE_METHOD_RAND |
Limit engine usage to RAND |
ENGINE_METHOD_ECDH |
Limit engine usage to ECDH |
ENGINE_METHOD_ECDSA |
Limit engine usage to ECDSA |
ENGINE_METHOD_CIPHERS |
Limit engine usage to CIPHERS |
ENGINE_METHOD_DIGESTS |
Limit engine usage to DIGESTS |
ENGINE_METHOD_STORE |
Limit engine usage to STORE |
ENGINE_METHOD_PKEY_METHS |
Limit engine usage to PKEY_METHDS |
ENGINE_METHOD_PKEY_ASN1_METHS |
Limit engine usage to PKEY_ASN1_METHS |
ENGINE_METHOD_ALL |
|
ENGINE_METHOD_NONE |
Other OpenSSL Constants#
Constant | Description |
---|---|
DH_CHECK_P_NOT_SAFE_PRIME |
|
DH_CHECK_P_NOT_PRIME |
|
DH_UNABLE_TO_CHECK_GENERATOR |
|
DH_NOT_SUITABLE_GENERATOR |
|
NPN_ENABLED |
|
ALPN_ENABLED |
|
RSA_PKCS1_PADDING |
|
RSA_SSLV23_PADDING |
|
RSA_NO_PADDING |
|
RSA_PKCS1_OAEP_PADDING |
|
RSA_X931_PADDING |
|
RSA_PKCS1_PSS_PADDING |
|
RSA_PSS_SALTLEN_DIGEST |
Sets the salt length for RSA_PKCS1_PSS_PADDING to the digest size
when signing or verifying. |
RSA_PSS_SALTLEN_MAX_SIGN |
Sets the salt length for RSA_PKCS1_PSS_PADDING to the maximum
permissible value when signing data. |
RSA_PSS_SALTLEN_AUTO |
Causes the salt length for RSA_PKCS1_PSS_PADDING to be determined
automatically when verifying a signature. |
POINT_CONVERSION_COMPRESSED |
|
POINT_CONVERSION_UNCOMPRESSED |
|
POINT_CONVERSION_HYBRID |
Node.js Crypto Constants#
Constant | Description |
---|---|
defaultCoreCipherList |
Specifies the built-in default cipher list used by Node.js. |
defaultCipherList |
Specifies the active default cipher list used by the current Node.js process. |
Debugger#
Stability: 2 - Stable
Node.js includes an out-of-process debugging utility accessible via a
TCP-based protocol and built-in debugging client. To use it, start Node.js
with the inspect
argument followed by the path to the script to debug; a prompt
will be displayed indicating successful launch of the debugger:
$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help see https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
2 setTimeout(() => {
3 console.log('world');
debug>
Node.js's debugger client is not a full-featured debugger, but simple step and inspection are possible.
Inserting the statement debugger;
into the source code of a script will
enable a breakpoint at that position in the code:
// myscript.js
global.x = 5;
setTimeout(() => {
debugger;
console.log('world');
}, 1000);
console.log('hello');
Once the debugger is run, a breakpoint will occur at line 3:
$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help see https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
2 setTimeout(() => {
3 debugger;
debug> cont
< hello
break in myscript.js:3
1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
2 setTimeout(() => {
> 3 debugger;
4 console.log('world');
5 }, 1000);
debug> next
break in myscript.js:4
2 setTimeout(() => {
3 debugger;
> 4 console.log('world');
5 }, 1000);
6 console.log('hello');
debug> repl
Press Ctrl + C to leave debug repl
> x
5
> 2+2
4
debug> next
< world
break in myscript.js:5
3 debugger;
4 console.log('world');
> 5 }, 1000);
6 console.log('hello');
7
debug> .exit
The repl
command allows code to be evaluated remotely. The next
command
steps to the next line. Type help
to see what other commands are available.
Pressing enter
without typing a command will repeat the previous debugger
command.
Watchers#
It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.
To begin watching an expression, type watch('my_expression')
. The command
watchers
will print the active watchers. To remove a watcher, type
unwatch('my_expression')
.
Command reference#
Stepping#
cont
,c
- Continue executionnext
,n
- Step nextstep
,s
- Step inout
,o
- Step outpause
- Pause running code (like pause button in Developer Tools)
Breakpoints#
setBreakpoint()
,sb()
- Set breakpoint on current linesetBreakpoint(line)
,sb(line)
- Set breakpoint on specific linesetBreakpoint('fn()')
,sb(...)
- Set breakpoint on a first statement in functions bodysetBreakpoint('script.js', 1)
,sb(...)
- Set breakpoint on first line of script.jsclearBreakpoint('script.js', 1)
,cb(...)
- Clear breakpoint in script.js on line 1
It is also possible to set a breakpoint in a file (module) that is not loaded yet:
$ node inspect test/fixtures/break-in-module/main.js
< Debugger listening on ws://127.0.0.1:9229/4e3db158-9791-4274-8909-914f7facf3bd
< For help see https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in test/fixtures/break-in-module/main.js:1
> 1 (function (exports, require, module, __filename, __dirname) { const mod = require('./mod.js');
2 mod.hello();
3 mod.hello();
debug> setBreakpoint('mod.js', 22)
Warning: script 'mod.js' was not loaded yet.
debug> c
break in test/fixtures/break-in-module/mod.js:22
20 // USE OR OTHER DEALINGS IN THE SOFTWARE.
21
>22 exports.hello = function() {
23 return 'hello from module';
24 };
debug>
Information#
backtrace
,bt
- Print backtrace of current execution framelist(5)
- List scripts source code with 5 line context (5 lines before and after)watch(expr)
- Add expression to watch listunwatch(expr)
- Remove expression from watch listwatchers
- List all watchers and their values (automatically listed on each breakpoint)repl
- Open debugger's repl for evaluation in debugging script's contextexec expr
- Execute an expression in debugging script's context
Execution control#
run
- Run script (automatically runs on debugger's start)restart
- Restart scriptkill
- Kill script
Various#
scripts
- List all loaded scriptsversion
- Display V8's version
Advanced Usage#
V8 Inspector Integration for Node.js#
V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome Debugging Protocol.
V8 Inspector can be enabled by passing the --inspect
flag when starting a
Node.js application. It is also possible to supply a custom port with that flag,
e.g. --inspect=9222
will accept DevTools connections on port 9222.
To break on the first line of the application code, pass the --inspect-brk
flag instead of --inspect
.
$ node --inspect index.js
Debugger listening on 127.0.0.1:9229.
To start debugging, open the following URL in Chrome:
chrome-devtools://devtools/bundled/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.)
Deprecated APIs#
Node.js may deprecate APIs when either: (a) use of the API is considered to be unsafe, (b) an improved alternative API has been made available, or (c) breaking changes to the API are expected in a future major release.
Node.js utilizes three kinds of Deprecations:
- Documentation-only
- Runtime
- End-of-Life
A Documentation-only deprecation is one that is expressed only within the Node.js API docs. These generate no side-effects while running Node.js.
A Runtime deprecation will, by default, generate a process warning that will
be printed to stderr
the first time the deprecated API is used. When the
--throw-deprecation
command-line flag is used, a Runtime deprecation will
cause an error to be thrown.
An End-of-Life deprecation is used to identify code that either has been removed or will soon be removed from Node.js.
Un-deprecation#
From time-to-time the deprecation of an API may be reversed. Such action may happen in either a semver-minor or semver-major release. In such situations, this document will be updated with information relevant to the decision. However, the deprecation identifier will not be modified.
List of Deprecated APIs#
DEP0001: http.OutgoingMessage.prototype.flush#
Type: Runtime
The OutgoingMessage.prototype.flush()
method is deprecated. Use
OutgoingMessage.prototype.flushHeaders()
instead.
DEP0002: require('_linklist')#
Type: Runtime
The _linklist
module is deprecated. Please use a userland alternative.
DEP0003: _writableState.buffer#
Type: Runtime
The _writableState.buffer
property is deprecated. Use the
_writableState.getBuffer()
method instead.
DEP0004: CryptoStream.prototype.readyState#
Type: Documentation-only
The CryptoStream.prototype.readyState
property is deprecated and should not
be used.
DEP0005: Buffer() constructor#
Type: Documentation-only
The Buffer()
function and new Buffer()
constructor are deprecated due to
API usability issues that can potentially lead to accidental security issues.
As an alternative, use of the following methods of constructing Buffer
objects
is strongly recommended:
Buffer.alloc(size[, fill[, encoding]])
- Create aBuffer
with initialized memory.Buffer.allocUnsafe(size)
- Create aBuffer
with uninitialized memory.Buffer.allocUnsafeSlow(size)
- Create aBuffer
with uninitialized memory.Buffer.from(array)
- Create aBuffer
with a copy ofarray
Buffer.from(arrayBuffer[, byteOffset[, length]])
- Create aBuffer
that wraps the givenarrayBuffer
.Buffer.from(buffer)
- Create aBuffer
that copiesbuffer
.Buffer.from(string[, encoding])
- Create aBuffer
that copiesstring
.
DEP0006: child_process options.customFds#
Type: Runtime
Within the child_process
module's spawn()
, fork()
, and exec()
methods, the options.customFds
option is deprecated. The options.stdio
option should be used instead.
DEP0007: cluster worker.suicide#
Type: Runtime
Within the cluster
module, the worker.suicide
property has been
deprecated. Please use worker.exitedAfterDisconnect
instead.
DEP0008: require('constants')#
Type: Documentation-only
The constants
module has been deprecated. When requiring access to constants
relevant to specific Node.js builtin modules, developers should instead refer
to the constants
property exposed by the relevant module. For instance,
require('fs').constants
and require('os').constants
.
DEP0009: crypto.pbkdf2 without digest#
Type: End-of-life
Use of the crypto.pbkdf2()
API without specifying a digest was deprecated
in Node.js 6.0 because the method defaulted to using the non-recommendend
'SHA1'
digest. Previously, a deprecation warning was printed. Starting in
Node.js 8.0.0, calling crypto.pbkdf2()
or crypto.pbkdf2Sync()
with an
undefined digest
will throw a TypeError
.
DEP0010: crypto.createCredentials#
Type: Runtime
The crypto.createCredentials()
API is deprecated. Please use
tls.createSecureContext()
instead.
DEP0011: crypto.Credentials#
Type: Runtime
The crypto.Credentials
class is deprecated. Please use tls.SecureContext
instead.
DEP0012: Domain.dispose#
Type: Runtime
Domain.dispose()
is deprecated. Recover from failed I/O actions
explicitly via error event handlers set on the domain instead.
DEP0013: fs async function without callback#
Type: Runtime
Calling an asynchronous function without a callback is deprecated.
DEP0014: fs.read legacy String interface#
Type: End-of-Life
The fs.read()
legacy String interface is deprecated. Use the Buffer API as
mentioned in the documentation instead.
DEP0015: fs.readSync legacy String interface#
Type: End-of-Life
The fs.readSync()
legacy String interface is deprecated. Use the Buffer
API as mentioned in the documentation instead.
DEP0016: GLOBAL/root#
Type: Runtime
The GLOBAL
and root
aliases for the global
property have been deprecated
and should no longer be used.
DEP0017: Intl.v8BreakIterator#
Type: Runtime
The Intl.v8BreakIterator
is deprecated and will be removed or replaced soon.
DEP0018: Unhandled promise rejections#
Type: Runtime
Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
DEP0019: require('.') resolved outside directory#
Type: Runtime
In certain cases, require('.')
may resolve outside the package directory.
This behavior is deprecated and will be removed in a future major Node.js
release.
DEP0020: Server.connections#
Type: Runtime
The Server.connections
property is deprecated. Please use the
Server.getConnections()
method instead.
DEP0021: Server.listenFD#
Type: Runtime
The Server.listenFD()
method is deprecated. Please use
Server.listen({fd: <number>})
instead.
DEP0022: os.tmpDir()#
Type: Runtime
The os.tmpDir()
API is deprecated. Please use os.tmpdir()
instead.
DEP0023: os.getNetworkInterfaces()#
Type: Runtime
The os.getNetworkInterfaces()
method is deprecated. Please use the
os.networkInterfaces
property instead.
DEP0024: REPLServer.prototype.convertToContext()#
Type: Runtime
The REPLServer.prototype.convertToContext()
API is deprecated and should
not be used.
DEP0025: require('sys')#
Type: Runtime
The sys
module is deprecated. Please use the util
module instead.
DEP0026: util.print()#
Type: Runtime
The util.print()
API is deprecated. Please use console.log()
instead.
DEP0027: util.puts()#
Type: Runtime
The util.puts()
API is deprecated. Please use console.log()
instead.
DEP0028: util.debug()#
Type: Runtime
The util.debug()
API is deprecated. Please use console.error()
instead.
DEP0029: util.error()#
Type: Runtime
The util.error()
API is deprecated. Please use console.error()
instead.
DEP0030: SlowBuffer#
Type: Documentation-only
The SlowBuffer
class has been deprecated. Please use
Buffer.allocUnsafeSlow(size)
instead.
DEP0031: ecdh.setPublicKey()#
Type: Documentation-only
The ecdh.setPublicKey()
method is now deprecated as its inclusion in the
API is not useful.
DEP0032: domain module#
Type: Documentation-only
The domain
module is deprecated and should not be used.
DEP0033: EventEmitter.listenerCount()#
Type: Documentation-only
The EventEmitter.listenerCount(emitter, eventName)
API has been
deprecated. Please use emitter.listenerCount(eventName)
instead.
DEP0034: fs.exists(path, callback)#
Type: Documentation-only
The fs.exists(path, callback)
API has been deprecated. Please use
fs.stat()
or fs.access()
instead.
DEP0035: fs.lchmod(path, mode, callback)#
Type: Documentation-only
The fs.lchmod(path, mode, callback)
API has been deprecated.
DEP0036: fs.lchmodSync(path, mode)#
Type: Documentation-only
The fs.lchmodSync(path, mode)
API has been deprecated.
DEP0037: fs.lchown(path, uid, gid, callback)#
Type: Documentation-only
The fs.lchown(path, uid, gid, callback)
API has been deprecated.
DEP0038: fs.lchownSync(path, uid, gid)#
Type: Documentation-only
The fs.lchownSync(path, uid, gid)
API has been deprecated.
DEP0039: require.extensions#
Type: Documentation-only
The require.extensions
property has been deprecated.
DEP0040: punycode module#
Type: Documentation-only
The punycode
module has been deprecated. Please use a userland alternative
instead.
DEP0041: NODE_REPL_HISTORY_FILE environment variable#
Type: Documentation-only
The NODE_REPL_HISTORY_FILE
environment variable has been deprecated.
DEP0042: tls.CryptoStream#
Type: Documentation-only
The tls.CryptoStream
class has been deprecated. Please use
tls.TLSSocket
instead.
DEP0043: tls.SecurePair#
Type: Documentation-only
The tls.SecurePair
class has been deprecated. Please use
tls.TLSSocket
instead.
DEP0044: util.isArray()#
Type: Documentation-only
The util.isArray()
API has been deprecated. Please use Array.isArray()
instead.
DEP0045: util.isBoolean()#
Type: Documentation-only
The util.isBoolean()
API has been deprecated.
DEP0046: util.isBuffer()#
Type: Documentation-only
The util.isBuffer()
API has been deprecated. Please use
Buffer.isBuffer()
instead.
DEP0047: util.isDate()#
Type: Documentation-only
The util.isDate()
API has been deprecated.
DEP0048: util.isError()#
Type: Documentation-only
The util.isError()
API has been deprecated.
DEP0049: util.isFunction()#
Type: Documentation-only
The util.isFunction()
API has been deprecated.
DEP0050: util.isNull()#
Type: Documentation-only
The util.isNull()
API has been deprecated.
DEP0051: util.isNullOrUndefined()#
Type: Documentation-only
The util.isNullOrUndefined()
API has been deprecated.
DEP0052: util.isNumber()#
Type: Documentation-only
The util.isNumber()
API has been deprecated.
DEP0053 util.isObject()#
Type: Documentation-only
The util.isObject()
API has been deprecated.
DEP0054: util.isPrimitive()#
Type: Documentation-only
The util.isPrimitive()
API has been deprecated.
DEP0055: util.isRegExp()#
Type: Documentation-only
The util.isRegExp()
API has been deprecated.
DEP0056: util.isString()#
Type: Documentation-only
The util.isString()
API has been deprecated.
DEP0057: util.isSymbol()#
Type: Documentation-only
The util.isSymbol()
API has been deprecated.
DEP0058: util.isUndefined()#
Type: Documentation-only
The util.isUndefined()
API has been deprecated.
DEP0059: util.log()#
Type: Documentation-only
The util.log()
API has been deprecated.
DEP0060: util._extend()#
Type: Documentation-only
The util._extend()
API has been deprecated.
DEP0061: fs.SyncWriteStream#
Type: Runtime
The fs.SyncWriteStream
class was never intended to be a publicly accessible
API. No alternative API is available. Please use a userland alternative.
DEP0062: node --debug#
Type: Runtime
--debug
activates the legacy V8 debugger interface, which has been removed as
of V8 5.8. It is replaced by Inspector which is activated with --inspect
instead.
DEP0063: ServerResponse.prototype.writeHeader()#
Type: Documentation-only
The http
module ServerResponse.prototype.writeHeader()
API has been
deprecated. Please use ServerResponse.prototype.writeHead()
instead.
Note: The ServerResponse.prototype.writeHeader()
method was never
documented as an officially supported API.
DEP0064: tls.createSecurePair()#
Type: Runtime
The tls.createSecurePair()
API was deprecated in documentation in Node.js
0.11.3. Users should use tls.Socket
instead.
DEP0065: repl.REPL_MODE_MAGIC and NODE_REPL_MODE=magic#
Type: Documentation-only
The repl
module's REPL_MODE_MAGIC
constant, used for replMode
option, has
been deprecated. Its behavior has been functionally identical to that of
REPL_MODE_SLOPPY
since Node.js v6.0.0, when V8 5.0 was imported. Please use
REPL_MODE_SLOPPY
instead.
The NODE_REPL_MODE
environment variable is used to set the underlying
replMode
of an interactive node
session. Its default value, magic
, is
similarly deprecated in favor of sloppy
.
DEP0066: outgoingMessage._headers, outgoingMessage._headerNames#
Type: Documentation-only
The http
module outgoingMessage._headers
and outgoingMessage._headerNames
properties have been deprecated. Please instead use one of the public methods
(e.g. outgoingMessage.getHeader()
, outgoingMessage.getHeaders()
,
outgoingMessage.getHeaderNames()
, outgoingMessage.hasHeader()
,
outgoingMessage.removeHeader()
, outgoingMessage.setHeader()
) for working
with outgoing headers.
Note: outgoingMessage._headers
and outgoingMessage._headerNames
were
never documented as officially supported properties.
DEP0067: OutgoingMessage.prototype._renderHeaders#
Type: Documentation-only
The http
module OutgoingMessage.prototype._renderHeaders()
API has been
deprecated.
Note: OutgoingMessage.prototype._renderHeaders
was never documented as
an officially supported API.
DEP0068: node debug#
Type: Runtime
node debug
corresponds to the legacy CLI debugger which has been replaced with
a V8-inspector based CLI debugger available through node inspect
.
DEP0069: vm.runInDebugContext(string)#
Type: Documentation-only
The DebugContext will be removed in V8 soon and will not be available in Node 10+.
Note: DebugContext was an experimental API.
DEP0070: async_hooks.currentId()#
Type: Runtime
async_hooks.currentId()
was renamed to async_hooks.executionAsyncId()
for
clarity.
Note: change was made while async_hooks
was an experimental API.
DEP0071: async_hooks.triggerId()#
Type: Runtime
async_hooks.triggerId()
was renamed to async_hooks.triggerAsyncId()
for
clarity.
Note: change was made while async_hooks
was an experimental API.
DEP0072: async_hooks.AsyncResource.triggerId()#
Type: Runtime
async_hooks.AsyncResource.triggerId()
was renamed to
async_hooks.AsyncResource.triggerAsyncId()
for clarity.
Note: change was made while async_hooks
was an experimental API.
DNS#
Stability: 2 - Stable
The dns
module contains functions belonging to two different categories:
1) Functions that use the underlying operating system facilities to perform
name resolution, and that do not necessarily perform any network communication.
This category contains only one function: dns.lookup()
. Developers
looking to perform name resolution in the same way that other applications on
the same operating system behave should use dns.lookup()
.
For example, looking up iana.org
.
const dns = require('dns');
dns.lookup('iana.org', (err, address, family) => {
console.log('address: %j family: IPv%s', address, family);
});
// address: "192.0.43.8" family: IPv4
2) Functions that connect to an actual DNS server to perform name resolution,
and that always use the network to perform DNS queries. This category
contains all functions in the dns
module except dns.lookup()
. These
functions do not use the same set of configuration files used by
dns.lookup()
(e.g. /etc/hosts
). These functions should be used by
developers who do not want to use the underlying operating system's facilities
for name resolution, and instead want to always perform DNS queries.
Below is an example that resolves 'archive.org'
then reverse resolves the IP
addresses that are returned.
const dns = require('dns');
dns.resolve4('archive.org', (err, addresses) => {
if (err) throw err;
console.log(`addresses: ${JSON.stringify(addresses)}`);
addresses.forEach((a) => {
dns.reverse(a, (err, hostnames) => {
if (err) {
throw err;
}
console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
});
});
});
There are subtle consequences in choosing one over the other, please consult the Implementation considerations section for more information.
Class dns.Resolver#
An independent resolver for DNS requests.
Note that creating a new resolver uses the default server settings. Setting
the servers used for a resolver using
resolver.setServers()
does not affect
other resolver:
const { Resolver } = require('dns');
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);
// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org', (err, addresses) => {
// ...
});
The following methods from the dns
module are available:
resolver.getServers()
resolver.setServers()
resolver.resolve()
resolver.resolve4()
resolver.resolve6()
resolver.resolveAny()
resolver.resolveCname()
resolver.resolveMx()
resolver.resolveNaptr()
resolver.resolveNs()
resolver.resolvePtr()
resolver.resolveSoa()
resolver.resolveSrv()
resolver.resolveTxt()
resolver.reverse()
resolver.cancel()#
Cancel all outstanding DNS queries made by this resolver. The corresponding
callbacks will be called with an error with code ECANCELLED
.
dns.getServers()#
Returns an array of IP address strings, formatted according to rfc5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.
For example:
[
'4.4.4.4',
'2001:4860:4860::8888',
'4.4.4.4:1053',
'[2001:4860:4860::8888]:1053'
]
dns.lookup(hostname[, options], callback)#
hostname
<string>options
<integer> | <Object>family
<integer> The record family. Must be4
or6
. IPv4 and IPv6 addresses are both returned by default.hints
<number> One or more supportedgetaddrinfo
flags. Multiple flags may be passed by bitwiseOR
ing their values.all
<boolean> Whentrue
, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Defaults tofalse
.
callback
<Function>
Resolves a hostname (e.g. 'nodejs.org'
) into the first found A (IPv4) or
AAAA (IPv6) record. All option
properties are optional. If options
is an
integer, then it must be 4
or 6
– if options
is not provided, then IPv4
and IPv6 addresses are both returned if found.
With the all
option set to true
, the arguments for callback
change to
(err, addresses)
, with addresses
being an array of objects with the
properties address
and family
.
On error, err
is an Error
object, where err.code
is the error code.
Keep in mind that err.code
will be set to 'ENOENT'
not only when
the hostname does not exist but also when the lookup fails in other ways
such as no available file descriptors.
dns.lookup()
does not necessarily have anything to do with the DNS protocol.
The implementation uses an operating system facility that can associate names
with addresses, and vice versa. This implementation can have subtle but
important consequences on the behavior of any Node.js program. Please take some
time to consult the Implementation considerations section before using
dns.lookup()
.
Example usage:
const dns = require('dns');
const options = {
family: 6,
hints: dns.ADDRCONFIG | dns.V4MAPPED,
};
dns.lookup('example.com', options, (err, address, family) =>
console.log('address: %j family: IPv%s', address, family));
// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6
// When options.all is true, the result will be an Array.
options.all = true;
dns.lookup('example.com', options, (err, addresses) =>
console.log('addresses: %j', addresses));
// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]
If this method is invoked as its util.promisify()
ed version, and all
is not set to true
, it returns a Promise for an object with address
and
family
properties.
Supported getaddrinfo flags#
The following flags can be passed as hints to dns.lookup()
.
dns.ADDRCONFIG
: Returned address types are determined by the types of addresses supported by the current system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured. Loopback addresses are not considered.dns.V4MAPPED
: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. Note that it is not supported on some operating systems (e.g FreeBSD 10.1).
dns.lookupService(address, port, callback)#
address
<string>port
<number>callback
<Function>
Resolves the given address
and port
into a hostname and service using
the operating system's underlying getnameinfo
implementation.
If address
is not a valid IP address, a TypeError
will be thrown.
The port
will be coerced to a number. If it is not a legal port, a TypeError
will be thrown.
On an error, err
is an Error
object, where err.code
is the error code.
const dns = require('dns');
dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
console.log(hostname, service);
// Prints: localhost ssh
});
If this method is invoked as its util.promisify()
ed version, it returns a
Promise for an object with hostname
and service
properties.
dns.resolve(hostname[, rrtype], callback)#
hostname
<string> Hostname to resolve.rrtype
<string> Resource record type. Default:'A'
.callback
<Function>err
<Error>records
<string[]> | <Object[]> | <Object>
Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org'
) into an array
of the resource records. The callback
function has arguments
(err, records)
. When successful, records
will be an array of resource
records. The type and structure of individual results varies based on rrtype
:
rrtype |
records contains |
Result type | Shorthand method |
---|---|---|---|
'A' |
IPv4 addresses (default) | <string> | dns.resolve4() |
'AAAA' |
IPv6 addresses | <string> | dns.resolve6() |
'CNAME' |
canonical name records | <string> | dns.resolveCname() |
'MX' |
mail exchange records | <Object> | dns.resolveMx() |
'NAPTR' |
name authority pointer records | <Object> | dns.resolveNaptr() |
'NS' |
name server records | <string> | dns.resolveNs() |
'PTR' |
pointer records | <string> | dns.resolvePtr() |
'SOA' |
start of authority records | <Object> | dns.resolveSoa() |
'SRV' |
service records | <Object> | dns.resolveSrv() |
'TXT' |
text records | <string> | dns.resolveTxt() |
'ANY' |
any records | <Object> | dns.resolveAny() |
On error, err
is an Error
object, where err.code
is one of the
DNS error codes.
dns.resolve4(hostname[, options], callback)#
hostname
<string> Hostname to resolve.options
<Object>ttl
<boolean> Retrieve the Time-To-Live value (TTL) of each record. Whentrue
, the callback receives an array of{ address: '1.2.3.4', ttl: 60 }
objects rather than an array of strings, with the TTL expressed in seconds.
callback
<Function>err
<Error>addresses
<string[]> | <Object[]>
Uses the DNS protocol to resolve a IPv4 addresses (A
records) for the
hostname
. The addresses
argument passed to the callback
function
will contain an array of IPv4 addresses (e.g.
['74.125.79.104', '74.125.79.105', '74.125.79.106']
).
dns.resolve6(hostname[, options], callback)#
hostname
<string> Hostname to resolve.options
<Object>ttl
<boolean> Retrieve the Time-To-Live value (TTL) of each record. Whentrue
, the callback receives an array of{ address: '0:1:2:3:4:5:6:7', ttl: 60 }
objects rather than an array of strings, with the TTL expressed in seconds.
callback
<Function>err
<Error>addresses
<string[]> | <Object[]>
Uses the DNS protocol to resolve a IPv6 addresses (AAAA
records) for the
hostname
. The addresses
argument passed to the callback
function
will contain an array of IPv6 addresses.
dns.resolveCname(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>addresses
<string[]>
Uses the DNS protocol to resolve CNAME
records for the hostname
. The
addresses
argument passed to the callback
function
will contain an array of canonical name records available for the hostname
(e.g. ['bar.example.com']
).
dns.resolveMx(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>addresses
<Object[]>
Uses the DNS protocol to resolve mail exchange records (MX
records) for the
hostname
. The addresses
argument passed to the callback
function will
contain an array of objects containing both a priority
and exchange
property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]
).
dns.resolveNaptr(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>addresses
<Object[]>
Uses the DNS protocol to resolve regular expression based records (NAPTR
records) for the hostname
. The addresses
argument passed to the callback
function will contain an array of objects with the following properties:
flags
service
regexp
replacement
order
preference
For example:
{
flags: 's',
service: 'SIP+D2U',
regexp: '',
replacement: '_sip._udp.example.com',
order: 30,
preference: 100
}
dns.resolveNs(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>addresses
<string[]>
Uses the DNS protocol to resolve name server records (NS
records) for the
hostname
. The addresses
argument passed to the callback
function will
contain an array of name server records available for hostname
(e.g. ['ns1.example.com', 'ns2.example.com']
).
dns.resolvePtr(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>addresses
<string[]>
Uses the DNS protocol to resolve pointer records (PTR
records) for the
hostname
. The addresses
argument passed to the callback
function will
be an array of strings containing the reply records.
dns.resolveSoa(hostname, callback)#
hostname
<string>callback
<Function>
Uses the DNS protocol to resolve a start of authority record (SOA
record) for
the hostname
. The address
argument passed to the callback
function will
be an object with the following properties:
nsname
hostmaster
serial
refresh
retry
expire
minttl
{
nsname: 'ns.example.com',
hostmaster: 'root.example.com',
serial: 2013101809,
refresh: 10000,
retry: 2400,
expire: 604800,
minttl: 3600
}
dns.resolveSrv(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>addresses
<Object[]>
Uses the DNS protocol to resolve service records (SRV
records) for the
hostname
. The addresses
argument passed to the callback
function will
be an array of objects with the following properties:
priority
weight
port
name
{
priority: 10,
weight: 5,
port: 21223,
name: 'service.example.com'
}
dns.resolveTxt(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>addresses
<string[]>
Uses the DNS protocol to resolve text queries (TXT
records) for the
hostname
. The addresses
argument passed to the callback
function is
is a two-dimensional array of the text records available for hostname
(e.g.,
[ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]
). Each sub-array contains TXT chunks of
one record. Depending on the use case, these could be either joined together or
treated separately.
dns.resolveAny(hostname, callback)#
hostname
<string>callback
<Function>err
<Error>ret
<Object[]>
Uses the DNS protocol to resolve all records (also known as ANY
or *
query).
The ret
argument passed to the callback
function will be an array containing
various types of records. Each object has a property type
that indicates the
type of the current record. And depending on the type
, additional properties
will be present on the object:
Type | Properties |
---|---|
"A" |
address / ttl |
"AAAA" |
address / ttl |
"CNAME" |
value |
"MX" |
Refer to dns.resolveMx() |
"NAPTR" |
Refer to dns.resolveNaptr() |
"NS" |
value |
"PTR" |
value |
"SOA" |
Refer to dns.resolveSoa() |
"SRV" |
Refer to dns.resolveSrv() |
"TXT" |
This type of record contains an array property called entries which refers to dns.resolveTxt() , eg. { entries: ['...'], type: 'TXT' } |
Here is a example of the ret
object passed to the callback:
[ { type: 'A', address: '127.0.0.1', ttl: 299 },
{ type: 'CNAME', value: 'example.com' },
{ type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
{ type: 'NS', value: 'ns1.example.com' },
{ type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
{ type: 'SOA',
nsname: 'ns1.example.com',
hostmaster: 'admin.example.com',
serial: 156696742,
refresh: 900,
retry: 900,
expire: 1800,
minttl: 60 } ]
dns.reverse(ip, callback)#
ip
<string>callback
<Function>err
<Error>hostnames
<string[]>
Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames.
On error, err
is an Error
object, where err.code
is
one of the DNS error codes.
dns.setServers(servers)#
servers
<string[]> array of rfc5952 formatted addresses
Sets the IP address and port of servers to be used when performing DNS
resolution. The servers
argument is an array of rfc5952 formatted
addresses. If the port is the IANA default DNS port (53) it can be omitted.
For example:
dns.setServers([
'4.4.4.4',
'[2001:4860:4860::8888]',
'4.4.4.4:1053',
'[2001:4860:4860::8888]:1053'
]);
An error will be thrown if an invalid address is provided.
The dns.setServers()
method must not be called while a DNS query is in
progress.
Error codes#
Each DNS query can return one of the following error codes:
dns.NODATA
: DNS server returned answer with no data.dns.FORMERR
: DNS server claims query was misformatted.dns.SERVFAIL
: DNS server returned general failure.dns.NOTFOUND
: Domain name not found.dns.NOTIMP
: DNS server does not implement requested operation.dns.REFUSED
: DNS server refused query.dns.BADQUERY
: Misformatted DNS query.dns.BADNAME
: Misformatted hostname.dns.BADFAMILY
: Unsupported address family.dns.BADRESP
: Misformatted DNS reply.dns.CONNREFUSED
: Could not contact DNS servers.dns.TIMEOUT
: Timeout while contacting DNS servers.dns.EOF
: End of file.dns.FILE
: Error reading file.dns.NOMEM
: Out of memory.dns.DESTRUCTION
: Channel is being destroyed.dns.BADSTR
: Misformatted string.dns.BADFLAGS
: Illegal flags specified.dns.NONAME
: Given hostname is not numeric.dns.BADHINTS
: Illegal hints flags specified.dns.NOTINITIALIZED
: c-ares library initialization not yet performed.dns.LOADIPHLPAPI
: Error loading iphlpapi.dll.dns.ADDRGETNETWORKPARAMS
: Could not find GetNetworkParams function.dns.CANCELLED
: DNS query cancelled.
Implementation considerations#
Although dns.lookup()
and the various dns.resolve*()/dns.reverse()
functions have the same goal of associating a network name with a network
address (or vice versa), their behavior is quite different. These differences
can have subtle but significant consequences on the behavior of Node.js
programs.
dns.lookup()
#
Under the hood, dns.lookup()
uses the same operating system facilities
as most other programs. For instance, dns.lookup()
will almost always
resolve a given name the same way as the ping
command. On most POSIX-like
operating systems, the behavior of the dns.lookup()
function can be
modified by changing settings in nsswitch.conf(5) and/or resolv.conf(5),
but note that changing these files will change the behavior of all other
programs running on the same operating system.
Though the call to dns.lookup()
will be asynchronous from JavaScript's
perspective, it is implemented as a synchronous call to getaddrinfo(3) that
runs on libuv's threadpool. Because libuv's threadpool has a fixed size, it
means that if for whatever reason the call to getaddrinfo(3) takes a long
time, other operations that could run on libuv's threadpool (such as filesystem
operations) will experience degraded performance. In order to mitigate this
issue, one potential solution is to increase the size of libuv's threadpool by
setting the 'UV_THREADPOOL_SIZE'
environment variable to a value greater than
4
(its current default value). For more information on libuv's threadpool, see
the official libuv documentation.
dns.resolve()
, dns.resolve*()
and dns.reverse()
#
These functions are implemented quite differently than dns.lookup()
. They
do not use getaddrinfo(3) and they always perform a DNS query on the
network. This network communication is always done asynchronously, and does not
use libuv's threadpool.
As a result, these functions cannot have the same negative impact on other
processing that happens on libuv's threadpool that dns.lookup()
can have.
They do not use the same set of configuration files than what dns.lookup()
uses. For instance, they do not use the configuration from /etc/hosts
.
Domain#
Stability: 0 - Deprecated
This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most end users should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future.
Domains provide a way to handle multiple different IO operations as a
single group. If any of the event emitters or callbacks registered to a
domain emit an 'error'
event, or throw an error, then the domain object
will be notified, rather than losing the context of the error in the
process.on('uncaughtException')
handler, or causing the program to
exit immediately with an error code.
Warning: Don't Ignore Errors!#
Domain error handlers are not a substitute for closing down a process when an error occurs.
By the very nature of how throw
works in JavaScript, there is almost
never any way to safely "pick up where you left off", without leaking
references, or creating some other sort of undefined brittle state.
The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, there may be many open connections, and it is not reasonable to abruptly shut those down because an error was triggered by someone else.
The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.
In this way, domain
usage goes hand-in-hand with the cluster module,
since the master process can fork a new worker when a worker
encounters an error. For Node.js programs that scale to multiple
machines, the terminating proxy or service registry can take note of
the failure, and react accordingly.
For example, this is not a good idea:
// XXX WARNING! BAD IDEA!
const d = require('domain').create();
d.on('error', (er) => {
// The error won't crash the process, but what it does is worse!
// Though we've prevented abrupt process restarting, we are leaking
// resources like crazy if this ever happens.
// This is no better than process.on('uncaughtException')!
console.log(`error, but oh well ${er.message}`);
});
d.run(() => {
require('http').createServer((req, res) => {
handleRequest(req, res);
}).listen(PORT);
});
By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety.
// Much better!
const cluster = require('cluster');
const PORT = +process.env.PORT || 1337;
if (cluster.isMaster) {
// A more realistic scenario would have more than 2 workers,
// and perhaps not put the master and worker in the same file.
//
// It is also possible to get a bit fancier about logging, and
// implement whatever custom logic is needed to prevent DoS
// attacks and other bad behavior.
//
// See the options in the cluster documentation.
//
// The important thing is that the master does very little,
// increasing our resilience to unexpected errors.
cluster.fork();
cluster.fork();
cluster.on('disconnect', (worker) => {
console.error('disconnect!');
cluster.fork();
});
} else {
// the worker
//
// This is where we put our bugs!
const domain = require('domain');
// See the cluster documentation for more details about using
// worker processes to serve requests. How it works, caveats, etc.
const server = require('http').createServer((req, res) => {
const d = domain.create();
d.on('error', (er) => {
console.error(`error ${er.stack}`);
// Note: We're in dangerous territory!
// By definition, something unexpected occurred,
// which we probably didn't want.
// Anything can happen now! Be very careful!
try {
// make sure we close down within 30 seconds
const killtimer = setTimeout(() => {
process.exit(1);
}, 30000);
// But don't keep the process open just for that!
killtimer.unref();
// stop taking new requests.
server.close();
// Let the master know we're dead. This will trigger a
// 'disconnect' in the cluster master, and then it will fork
// a new worker.
cluster.worker.disconnect();
// try to send an error to the request that triggered the problem
res.statusCode = 500;
res.setHeader('content-type', 'text/plain');
res.end('Oops, there was a problem!\n');
} catch (er2) {
// oh well, not much we can do at this point.
console.error(`Error sending 500! ${er2.stack}`);
}
});
// Because req and res were created before this domain existed,
// we need to explicitly add them.
// See the explanation of implicit vs explicit binding below.
d.add(req);
d.add(res);
// Now run the handler function in the domain.
d.run(() => {
handleRequest(req, res);
});
});
server.listen(PORT);
}
// This part is not important. Just an example routing thing.
// Put fancy application logic here.
function handleRequest(req, res) {
switch (req.url) {
case '/error':
// We do some async stuff, and then...
setTimeout(() => {
// Whoops!
flerb.bark();
}, timeout);
break;
default:
res.end('ok');
}
}
Additions to Error objects#
Any time an Error
object is routed through a domain, a few extra fields
are added to it.
error.domain
The domain that first handled the error.error.domainEmitter
The event emitter that emitted an'error'
event with the error object.error.domainBound
The callback function which was bound to the domain, and passed an error as its first argument.error.domainThrown
A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function.
Implicit Binding#
If domains are in use, then all new EventEmitter objects (including Stream objects, requests, responses, etc.) will be implicitly bound to the active domain at the time of their creation.
Additionally, callbacks passed to lowlevel event loop requests (such as to fs.open, or other callback-taking methods) will automatically be bound to the active domain. If they throw, then the domain will catch the error.
In order to prevent excessive memory usage, Domain objects themselves are not implicitly added as children of the active domain. If they were, then it would be too easy to prevent request and response objects from being properly garbage collected.
To nest Domain objects as children of a parent Domain they must be explicitly added.
Implicit binding routes thrown errors and 'error'
events to the
Domain's 'error'
event, but does not register the EventEmitter on the
Domain, so domain.dispose()
will not shut down the EventEmitter.
Implicit binding only takes care of thrown errors and 'error'
events.
Explicit Binding#
Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain.
For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request.
That is possible via explicit binding.
For example:
// create a top-level domain for the server
const domain = require('domain');
const http = require('http');
const serverDomain = domain.create();
serverDomain.run(() => {
// server is created in the scope of serverDomain
http.createServer((req, res) => {
// req and res are also created in the scope of serverDomain
// however, we'd prefer to have a separate domain for each request.
// create it first thing, and add req and res to it.
const reqd = domain.create();
reqd.add(req);
reqd.add(res);
reqd.on('error', (er) => {
console.error('Error', er, req.url);
try {
res.writeHead(500);
res.end('Error occurred, sorry.');
} catch (er2) {
console.error('Error sending 500', er2, req.url);
}
});
}).listen(1337);
});
domain.create()#
- Returns: <Domain>
Returns a new Domain object.
Class: Domain#
The Domain class encapsulates the functionality of routing errors and uncaught exceptions to the active Domain object.
Domain is a child class of EventEmitter
. To handle the errors that it
catches, listen to its 'error'
event.
domain.members#
An array of timers and event emitters that have been explicitly added to the domain.
domain.add(emitter)#
emitter
<EventEmitter> | <Timer> emitter or timer to be added to the domain
Explicitly adds an emitter to the domain. If any event handlers called by
the emitter throw an error, or if the emitter emits an 'error'
event, it
will be routed to the domain's 'error'
event, just like with implicit
binding.
This also works with timers that are returned from setInterval()
and
setTimeout()
. If their callback function throws, it will be caught by
the domain 'error' handler.
If the Timer or EventEmitter was already bound to a domain, it is removed from that one, and bound to this one instead.
domain.bind(callback)#
callback
<Function> The callback function- Returns: <Function> The bound function
The returned function will be a wrapper around the supplied callback
function. When the returned function is called, any errors that are
thrown will be routed to the domain's 'error'
event.
Example#
const d = domain.create();
function readSomeFile(filename, cb) {
fs.readFile(filename, 'utf8', d.bind((er, data) => {
// if this throws, it will also be passed to the domain
return cb(er, data ? JSON.parse(data) : null);
}));
}
d.on('error', (er) => {
// an error occurred somewhere.
// if we throw it now, it will crash the program
// with the normal line number and stack message.
});
domain.dispose()#
Stability: 0 - Deprecated. Please recover from failed IO actions explicitly via error event handlers set on the domain.
Once dispose
has been called, the domain will no longer be used by callbacks
bound into the domain via run
, bind
, or intercept
, and a 'dispose'
event
is emitted.
domain.enter()#
The enter
method is plumbing used by the run
, bind
, and intercept
methods to set the active domain. It sets domain.active
and process.domain
to the domain, and implicitly pushes the domain onto the domain stack managed
by the domain module (see domain.exit()
for details on the domain stack). The
call to enter
delimits the beginning of a chain of asynchronous calls and I/O
operations bound to a domain.
Calling enter
changes only the active domain, and does not alter the domain
itself. enter
and exit
can be called an arbitrary number of times on a
single domain.
If the domain on which enter
is called has been disposed, enter
will return
without setting the domain.
domain.exit()#
The exit
method exits the current domain, popping it off the domain stack.
Any time execution is going to switch to the context of a different chain of
asynchronous calls, it's important to ensure that the current domain is exited.
The call to exit
delimits either the end of or an interruption to the chain
of asynchronous calls and I/O operations bound to a domain.
If there are multiple, nested domains bound to the current execution context,
exit
will exit any domains nested within this domain.
Calling exit
changes only the active domain, and does not alter the domain
itself. enter
and exit
can be called an arbitrary number of times on a
single domain.
If the domain on which exit
is called has been disposed, exit
will return
without exiting the domain.
domain.intercept(callback)#
callback
<Function> The callback function- Returns: <Function> The intercepted function
This method is almost identical to domain.bind(callback)
. However, in
addition to catching thrown errors, it will also intercept Error
objects sent as the first argument to the function.
In this way, the common if (err) return callback(err);
pattern can be replaced
with a single error handler in a single place.
Example#
const d = domain.create();
function readSomeFile(filename, cb) {
fs.readFile(filename, 'utf8', d.intercept((data) => {
// note, the first argument is never passed to the
// callback since it is assumed to be the 'Error' argument
// and thus intercepted by the domain.
// if this throws, it will also be passed to the domain
// so the error-handling logic can be moved to the 'error'
// event on the domain instead of being repeated throughout
// the program.
return cb(null, JSON.parse(data));
}));
}
d.on('error', (er) => {
// an error occurred somewhere.
// if we throw it now, it will crash the program
// with the normal line number and stack message.
});
domain.remove(emitter)#
emitter
<EventEmitter> | <Timer> emitter or timer to be removed from the domain
The opposite of domain.add(emitter)
. Removes domain handling from the
specified emitter.
domain.run(fn[, ...args])#
fn
<Function>...args
<any>
Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and lowlevel requests that are created in that context. Optionally, arguments can be passed to the function.
This is the most basic way to use a domain.
Example:
const domain = require('domain');
const fs = require('fs');
const d = domain.create();
d.on('error', (er) => {
console.error('Caught error!', er);
});
d.run(() => {
process.nextTick(() => {
setTimeout(() => { // simulating some various async stuff
fs.open('non-existent file', 'r', (er, fd) => {
if (er) throw er;
// proceed...
});
}, 100);
});
});
In this example, the d.on('error')
handler will be triggered, rather
than crashing the program.
Domains and Promises#
As of Node 8.0.0, the handlers of Promises are run inside the domain in
which the call to .then
or .catch
itself was made:
const d1 = domain.create();
const d2 = domain.create();
let p;
d1.run(() => {
p = Promise.resolve(42);
});
d2.run(() => {
p.then((v) => {
// running in d2
});
});
A callback may be bound to a specific domain using domain.bind(callback)
:
const d1 = domain.create();
const d2 = domain.create();
let p;
d1.run(() => {
p = Promise.resolve(42);
});
d2.run(() => {
p.then(p.domain.bind((v) => {
// running in d1
}));
});
Note that domains will not interfere with the error handling mechanisms for
Promises, i.e. no error
event will be emitted for unhandled Promise
rejections.
Errors#
Applications running in Node.js will generally experience four categories of errors:
- Standard JavaScript errors such as:
- <EvalError> : thrown when a call to
eval()
fails. - <SyntaxError> : thrown in response to improper JavaScript language syntax.
- <RangeError> : thrown when a value is not within an expected range
- <ReferenceError> : thrown when using undefined variables
- <TypeError> : thrown when passing arguments of the wrong type
- <URIError> : thrown when a global URI handling function is misused.
- <EvalError> : thrown when a call to
- System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist, attempting to send data over a closed socket, etc;
- And User-specified errors triggered by application code.
- Assertion Errors are a special class of error that can be triggered whenever
Node.js detects an exceptional logic violation that should never occur. These
are raised typically by the
assert
module.
All JavaScript and System errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class.
Error Propagation and Interception#
Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of Error and the style of the API that is called.
All JavaScript errors are handled as exceptions that immediately generate
and throw an error using the standard JavaScript throw
mechanism. These
are handled using the try / catch
construct provided by the
JavaScript language.
// Throws with a ReferenceError because z is undefined
try {
const m = 1;
const n = m + z;
} catch (err) {
// Handle the error here.
}
Any use of the JavaScript throw
mechanism will raise an exception that
must be handled using try / catch
or the Node.js process will exit
immediately.
With few exceptions, Synchronous APIs (any blocking method that does not
accept a callback
function, such as fs.readFileSync
), will use throw
to report errors.
Errors that occur within Asynchronous APIs may be reported in multiple ways:
- Most asynchronous methods that accept a
callback
function will accept anError
object passed as the first argument to that function. If that first argument is notnull
and is an instance ofError
, then an error occurred that should be handled.
const fs = require('fs');
fs.readFile('a file that does not exist', (err, data) => {
if (err) {
console.error('There was an error reading the file!', err);
return;
}
// Otherwise handle the data
});
When an asynchronous method is called on an object that is an
EventEmitter
, errors can be routed to that object's'error'
event.const net = require('net'); const connection = net.connect('localhost'); // Adding an 'error' event handler to a stream: connection.on('error', (err) => { // If the connection is reset by the server, or if it can't // connect at all, or on any sort of error encountered by // the connection, the error will be sent here. console.error(err); }); connection.pipe(process.stdout);
A handful of typically asynchronous methods in the Node.js API may still use the
throw
mechanism to raise exceptions that must be handled usingtry / catch
. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.
The use of the 'error'
event mechanism is most common for stream-based
and event emitter-based APIs, which themselves represent a series of
asynchronous operations over time (as opposed to a single operation that may
pass or fail).
For all EventEmitter
objects, if an 'error'
event handler is not
provided, the error will be thrown, causing the Node.js process to report an
unhandled exception and crash unless either: The domain
module is
used appropriately or a handler has been registered for the
process.on('uncaughtException')
event.
const EventEmitter = require('events');
const ee = new EventEmitter();
setImmediate(() => {
// This will crash the process because no 'error' event
// handler has been added.
ee.emit('error', new Error('This will crash'));
});
Errors generated in this way cannot be intercepted using try / catch
as
they are thrown after the calling code has already exited.
Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.
Node.js style callbacks#
Most asynchronous methods exposed by the Node.js core API follow an idiomatic
pattern referred to as a "Node.js style callback". With this pattern, a
callback function is passed to the method as an argument. When the operation
either completes or an error is raised, the callback function is called with
the Error object (if any) passed as the first argument. If no error was raised,
the first argument will be passed as null
.
const fs = require('fs');
function nodeStyleCallback(err, data) {
if (err) {
console.error('There was an error', err);
return;
}
console.log(data);
}
fs.readFile('/some/file/that/does-not-exist', nodeStyleCallback);
fs.readFile('/some/file/that/does-exist', nodeStyleCallback);
The JavaScript try / catch
mechanism cannot be used to intercept errors
generated by asynchronous APIs. A common mistake for beginners is to try to
use throw
inside a Node.js style callback:
// THIS WILL NOT WORK:
const fs = require('fs');
try {
fs.readFile('/some/file/that/does-not-exist', (err, data) => {
// mistaken assumption: throwing here...
if (err) {
throw err;
}
});
} catch (err) {
// This will not catch the throw!
console.error(err);
}
This will not work because the callback function passed to fs.readFile()
is
called asynchronously. By the time the callback has been called, the
surrounding code (including the try { } catch (err) { }
block will have
already exited. Throwing an error inside the callback can crash the Node.js
process in most cases. If domains are enabled, or a handler has been
registered with process.on('uncaughtException')
, such errors can be
intercepted.
Class: Error#
A generic JavaScript Error
object that does not denote any specific
circumstance of why the error occurred. Error
objects capture a "stack trace"
detailing the point in the code at which the Error
was instantiated, and may
provide a text description of the error.
All errors generated by Node.js, including all System and JavaScript errors,
will either be instances of, or inherit from, the Error
class.
new Error(message)#
message
<string>
Creates a new Error
object and sets the error.message
property to the
provided text message. If an object is passed as message
, the text message
is generated by calling message.toString()
. The error.stack
property will
represent the point in the code at which new Error()
was called. Stack traces
are dependent on V8's stack trace API. Stack traces extend only to either
(a) the beginning of synchronous code execution, or (b) the number of frames
given by the property Error.stackTraceLimit
, whichever is smaller.
Error.captureStackTrace(targetObject[, constructorOpt])#
targetObject
<Object>constructorOpt
<Function>
Creates a .stack
property on targetObject
, which when accessed returns
a string representing the location in the code at which
Error.captureStackTrace()
was called.
const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack; // similar to `new Error().stack`
The first line of the trace will be prefixed with ${myObject.name}: ${myObject.message}
.
The optional constructorOpt
argument accepts a function. If given, all frames
above constructorOpt
, including constructorOpt
, will be omitted from the
generated stack trace.
The constructorOpt
argument is useful for hiding implementation
details of error generation from an end user. For instance:
function MyError() {
Error.captureStackTrace(this, MyError);
}
// Without passing MyError to captureStackTrace, the MyError
// frame would show up in the .stack property. By passing
// the constructor, we omit that frame, and retain all frames below it.
new MyError().stack;
Error.stackTraceLimit#
The Error.stackTraceLimit
property specifies the number of stack frames
collected by a stack trace (whether generated by new Error().stack
or
Error.captureStackTrace(obj)
).
The default value is 10
but may be set to any valid JavaScript number. Changes
will affect any stack trace captured after the value has been changed.
If set to a non-number value, or set to a negative number, stack traces will not capture any frames.
error.code#
The error.code
property is a string label that identifies the kind of error.
See Node.js Error Codes for details about specific codes.
error.message#
The error.message
property is the string description of the error as set by
calling new Error(message)
. The message
passed to the constructor will also
appear in the first line of the stack trace of the Error
, however changing
this property after the Error
object is created may not change the first
line of the stack trace (for example, when error.stack
is read before this
property is changed).
const err = new Error('The message');
console.error(err.message);
// Prints: The message
error.stack#
The error.stack
property is a string describing the point in the code at which
the Error
was instantiated.
For example:
Error: Things keep happening!
at /home/gbusey/file.js:525:2
at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
at increaseSynergy (/home/gbusey/actors.js:701:6)
The first line is formatted as <error class name>: <error message>
, and
is followed by a series of stack frames (each line beginning with "at ").
Each frame describes a call site within the code that lead to the error being
generated. V8 attempts to display a name for each function (by variable name,
function name, or object method name), but occasionally it will not be able to
find a suitable name. If V8 cannot determine a name for the function, only
location information will be displayed for that frame. Otherwise, the
determined function name will be displayed with location information appended
in parentheses.
It is important to note that frames are only generated for JavaScript
functions. If, for example, execution synchronously passes through a C++ addon
function called cheetahify
, which itself calls a JavaScript function, the
frame representing the cheetahify
call will not be present in the stack
traces:
const cheetahify = require('./native-binding.node');
function makeFaster() {
// cheetahify *synchronously* calls speedy.
cheetahify(function speedy() {
throw new Error('oh no!');
});
}
makeFaster();
// will throw:
// /home/gbusey/file.js:6
// throw new Error('oh no!');
// ^
// Error: oh no!
// at speedy (/home/gbusey/file.js:6:11)
// at makeFaster (/home/gbusey/file.js:5:3)
// at Object.<anonymous> (/home/gbusey/file.js:10:1)
// at Module._compile (module.js:456:26)
// at Object.Module._extensions..js (module.js:474:10)
// at Module.load (module.js:356:32)
// at Function.Module._load (module.js:312:12)
// at Function.Module.runMain (module.js:497:10)
// at startup (node.js:119:16)
// at node.js:906:3
The location information will be one of:
native
, if the frame represents a call internal to V8 (as in[].forEach
).plain-filename.js:line:column
, if the frame represents a call internal to Node.js./absolute/path/to/file.js:line:column
, if the frame represents a call in a user program, or its dependencies.
The string representing the stack trace is lazily generated when the
error.stack
property is accessed.
The number of frames captured by the stack trace is bounded by the smaller of
Error.stackTraceLimit
or the number of available frames on the current event
loop tick.
System-level errors are generated as augmented Error
instances, which are
detailed here.
Class: RangeError#
A subclass of Error
that indicates that a provided argument was not within the
set or range of acceptable values for a function; whether that is a numeric
range, or outside the set of options for a given function parameter.
For example:
require('net').connect(-1);
// throws "RangeError: "port" option should be >= 0 and < 65536: -1"
Node.js will generate and throw RangeError
instances immediately as a form
of argument validation.
Class: ReferenceError#
A subclass of Error
that indicates that an attempt is being made to access a
variable that is not defined. Such errors commonly indicate typos in code, or
an otherwise broken program.
While client code may generate and propagate these errors, in practice, only V8 will do so.
doesNotExist;
// throws ReferenceError, doesNotExist is not a variable in this program.
Unless an application is dynamically generating and running code,
ReferenceError
instances should always be considered a bug in the code
or its dependencies.
Class: SyntaxError#
A subclass of Error
that indicates that a program is not valid JavaScript.
These errors may only be generated and propagated as a result of code
evaluation. Code evaluation may happen as a result of eval
, Function
,
require
, or vm. These errors are almost always indicative of a broken
program.
try {
require('vm').runInThisContext('binary ! isNotOk');
} catch (err) {
// err will be a SyntaxError
}
SyntaxError
instances are unrecoverable in the context that created them –
they may only be caught by other contexts.
Class: TypeError#
A subclass of Error
that indicates that a provided argument is not an
allowable type. For example, passing a function to a parameter which expects a
string would be considered a TypeError.
require('url').parse(() => { });
// throws TypeError, since it expected a string
Node.js will generate and throw TypeError
instances immediately as a form
of argument validation.
Exceptions vs. Errors#
A JavaScript exception is a value that is thrown as a result of an invalid
operation or as the target of a throw
statement. While it is not required
that these values are instances of Error
or classes which inherit from
Error
, all exceptions thrown by Node.js or the JavaScript runtime will be
instances of Error.
Some exceptions are unrecoverable at the JavaScript layer. Such exceptions
will always cause the Node.js process to crash. Examples include assert()
checks or abort()
calls in the C++ layer.
System Errors#
System errors are generated when exceptions occur within the program's runtime environment. Typically, these are operational errors that occur when an application violates an operating system constraint such as attempting to read a file that does not exist or when the user does not have sufficient permissions.
System errors are typically generated at the syscall level: an exhaustive list
of error codes and their meanings is available by running man 2 intro
or
man 3 errno
on most Unices; or online.
In Node.js, system errors are represented as augmented Error
objects with
added properties.
Class: System Error#
error.code#
The error.code
property is a string representing the error code, which is
typically E
followed by a sequence of capital letters.
error.errno#
The error.errno
property is a number or a string.
The number is a negative value which corresponds to the error code defined
in libuv Error handling
. See uv-errno.h header file
(deps/uv/include/uv-errno.h
in the Node.js source tree) for details. In case
of a string, it is the same as error.code
.
error.syscall#
The error.syscall
property is a string describing the syscall that failed.
error.path#
When present (e.g. in fs
or child_process
), the error.path
property is a
string containing a relevant invalid pathname.
error.address#
When present (e.g. in net
or dgram
), the error.address
property is a
string describing the address to which the connection failed.
error.port#
When present (e.g. in net
or dgram
), the error.port
property is a number
representing the connection's port that is not available.
Common System Errors#
This list is not exhaustive, but enumerates many of the common system errors encountered when writing a Node.js program. An exhaustive list may be found here.
EACCES
(Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions.EADDRINUSE
(Address already in use): An attempt to bind a server (net
,http
, orhttps
) to a local address failed due to another server on the local system already occupying that address.ECONNREFUSED
(Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.ECONNRESET
(Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via thehttp
andnet
modules.EEXIST
(File exists): An existing file was the target of an operation that required that the target not exist.EISDIR
(Is a directory): An operation expected a file, but the given pathname was a directory.EMFILE
(Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, runulimit -n 2048
in the same shell that will run the Node.js process.ENOENT
(No such file or directory): Commonly raised byfs
operations to indicate that a component of the specified pathname does not exist -- no entity (file or directory) could be found by the given path.ENOTDIR
(Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised byfs.readdir
.ENOTEMPTY
(Directory not empty): A directory with entries was the target of an operation that requires an empty directory -- usuallyfs.unlink
.EPERM
(Operation not permitted): An attempt was made to perform an operation that requires elevated privileges.EPIPE
(Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at thenet
andhttp
layers, indicative that the remote side of the stream being written to has been closed.ETIMEDOUT
(Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered byhttp
ornet
-- often a sign that asocket.end()
was not properly called.
Node.js Error Codes#
ERR_ARG_NOT_ITERABLE#
Used generically to identify that an iterable argument (i.e. a value that works
with for...of
loops) is required, but not provided to a Node.js API.
ERR_FALSY_VALUE_REJECTION#
Used by the util.callbackify()
API when a callbackified Promise
is rejected
with a falsy value (e.g. null
).
ERR_INVALID_ARG_TYPE#
Used generically to identify that an argument of the wrong type has been passed to a Node.js API.
ERR_INVALID_CALLBACK#
Used generically to identify that a callback function is required and has not been provided to a Node.js API.
ERR_INVALID_FILE_URL_HOST#
Used when a Node.js API that consumes file:
URLs (such as certain functions in
the fs
module) encounters a file URL with an incompatible host. Currently,
this situation can only occur on Unix-like systems, where only localhost
or an
empty host is supported.
ERR_INVALID_FILE_URL_PATH#
Used when a Node.js API that consumes file:
URLs (such as certain
functions in the fs
module) encounters a file URL with an incompatible
path. The exact semantics for determining whether a path can be used is
platform-dependent.
ERR_INVALID_HANDLE_TYPE#
Used when an attempt is made to send an unsupported "handle" over an IPC
communication channel to a child process. See child.send()
and
process.send()
for more information.
ERR_INVALID_OPT_VALUE#
Used generically to identify when an invalid or unexpected value has been passed in an options object.
ERR_INVALID_SYNC_FORK_INPUT#
Used when a Buffer
, Uint8Array
or string
is provided as stdio input to a
synchronous fork. See the documentation for the
child_process
module for more information.
ERR_INVALID_THIS#
Used generically to identify that a Node.js API function is called with an
incompatible this
value.
Example:
const { URLSearchParams } = require('url');
const urlSearchParams = new URLSearchParams('foo=bar&baz=new');
const buf = Buffer.alloc(1);
urlSearchParams.has.call(buf, 'foo');
// Throws a TypeError with code 'ERR_INVALID_THIS'
ERR_INVALID_TUPLE#
Used when an element in the iterable
provided to the WHATWG URLSearchParams
constructor does not
represent a [name, value]
tuple – that is, if an element is not iterable, or
does not consist of exactly two elements.
ERR_INVALID_URL#
Used when an invalid URL is passed to the WHATWG
URL
constructor to be parsed. The thrown error object
typically has an additional property 'input'
that contains the URL that failed
to parse.
ERR_INVALID_URL_SCHEME#
Used generically to signify an attempt to use a URL of an incompatible scheme
(aka protocol) for a specific purpose. It is currently only used in the
WHATWG URL API support in the fs
module (which only accepts URLs with
'file'
scheme), but may be used in other Node.js APIs as well in the future.
ERR_IPC_CHANNEL_CLOSED#
Used when an attempt is made to use an IPC communication channel that has already been closed.
ERR_IPC_DISCONNECTED#
Used when an attempt is made to disconnect an already disconnected IPC
communication channel between two Node.js processes. See the documentation for
the child_process
module for more information.
ERR_IPC_ONE_PIPE#
Used when an attempt is made to create a child Node.js process using more than
one IPC communication channel. See the documentation for the
child_process
module for more information.
ERR_IPC_SYNC_FORK#
Used when an attempt is made to open an IPC communication channel with a
synchronous forked Node.js process. See the documentation for the
child_process
module for more information.
ERR_MISSING_ARGS#
Used when a required argument of a Node.js API is not passed. This is currently
only used in the WHATWG URL API for strict compliance with the specification
(which in some cases may accept func(undefined)
but not func()
). In most
native Node.js APIs, func(undefined)
and func()
are treated identically, and
the ERR_INVALID_ARG_TYPE
error code may be used instead.
ERR_SOCKET_ALREADY_BOUND#
Used when an attempt is made to bind a socket that has already been bound.
ERR_SOCKET_BAD_PORT#
Used when an API function expecting a port > 0 and < 65536 receives an invalid value.
ERR_SOCKET_BAD_TYPE#
Used when an API function expecting a socket type (udp4
or udp6
) receives an
invalid value.
ERR_SOCKET_CANNOT_SEND#
Used when data cannot be sent on a socket.
ERR_SOCKET_DGRAM_NOT_RUNNING#
Used when a call is made and the UDP subsystem is not running.
ERR_STDERR_CLOSE#
Used when an attempt is made to close the process.stderr
stream. By design,
Node.js does not allow stdout
or stderr
Streams to be closed by user code.
ERR_STDOUT_CLOSE#
Used when an attempt is made to close the process.stdout
stream. By design,
Node.js does not allow stdout
or stderr
Streams to be closed by user code.
ERR_UNKNOWN_BUILTIN_MODULE#
Used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself.
ERR_UNKNOWN_SIGNAL#
Used when an invalid or unknown process signal is passed to an API expecting a
valid signal (such as child.kill()
).
ERR_UNKNOWN_STDIN_TYPE#
Used when an attempt is made to launch a Node.js process with an unknown stdin
file type. Errors of this kind cannot typically be caused by errors in user
code, although it is not impossible. Occurrences of this error are most likely
an indication of a bug within Node.js itself.
ERR_UNKNOWN_STREAM_TYPE#
Used when an attempt is made to launch a Node.js process with an unknown
stdout
or stderr
file type. Errors of this kind cannot typically be caused
by errors in user code, although it is not impossible. Occurrences of this error
are most likely an indication of a bug within Node.js itself.
Events#
Stability: 2 - Stable
Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") periodically emit named events that cause Function objects ("listeners") to be called.
For instance: a net.Server
object emits an event each time a peer
connects to it; a fs.ReadStream
emits an event when the file is opened;
a stream emits an event whenever data is available to be read.
All objects that emit events are instances of the EventEmitter
class. These
objects expose an eventEmitter.on()
function that allows one or more
functions to be attached to named events emitted by the object. Typically,
event names are camel-cased strings but any valid JavaScript property key
can be used.
When the EventEmitter
object emits an event, all of the functions attached
to that specific event are called synchronously. Any values returned by the
called listeners are ignored and will be discarded.
The following example shows a simple EventEmitter
instance with a single
listener. The eventEmitter.on()
method is used to register listeners, while
the eventEmitter.emit()
method is used to trigger the event.
const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
console.log('an event occurred!');
});
myEmitter.emit('event');
Passing arguments and this
to listeners#
The eventEmitter.emit()
method allows an arbitrary set of arguments to be
passed to the listener functions. It is important to keep in mind that when an
ordinary listener function is called by the EventEmitter
, the standard this
keyword is intentionally set to reference the EventEmitter
to which the
listener is attached.
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
console.log(a, b, this);
// Prints:
// a b MyEmitter {
// domain: null,
// _events: { event: [Function] },
// _eventsCount: 1,
// _maxListeners: undefined }
});
myEmitter.emit('event', 'a', 'b');
It is possible to use ES6 Arrow Functions as listeners, however, when doing so,
the this
keyword will no longer reference the EventEmitter
instance:
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
console.log(a, b, this);
// Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');
Asynchronous vs. Synchronous#
The EventListener
calls all listeners synchronously in the order in which
they were registered. This is important to ensure the proper sequencing of
events and to avoid race conditions or logic errors. When appropriate,
listener functions can switch to an asynchronous mode of operation using
the setImmediate()
or process.nextTick()
methods:
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
setImmediate(() => {
console.log('this happens asynchronously');
});
});
myEmitter.emit('event', 'a', 'b');
Handling events only once#
When a listener is registered using the eventEmitter.on()
method, that
listener will be invoked every time the named event is emitted.
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2
Using the eventEmitter.once()
method, it is possible to register a listener
that is called at most once for a particular event. Once the event is emitted,
the listener is unregistered and then called.
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored
Error events#
When an error occurs within an EventEmitter
instance, the typical action is
for an 'error'
event to be emitted. These are treated as special cases
within Node.js.
If an EventEmitter
does not have at least one listener registered for the
'error'
event, and an 'error'
event is emitted, the error is thrown, a
stack trace is printed, and the Node.js process exits.
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js
To guard against crashing the Node.js process, a listener can be registered
on the process
object's uncaughtException
event or the domain
module
can be used. (Note, however, that the domain
module has been deprecated.)
const myEmitter = new MyEmitter();
process.on('uncaughtException', (err) => {
console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error
As a best practice, listeners should always be added for the 'error'
events.
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error
Class: EventEmitter#
The EventEmitter
class is defined and exposed by the events
module:
const EventEmitter = require('events');
All EventEmitters emit the event 'newListener'
when new listeners are
added and 'removeListener'
when existing listeners are removed.
Event: 'newListener'#
eventName
<any> The name of the event being listened forlistener
<Function> The event handler function
The EventEmitter
instance will emit its own 'newListener'
event before
a listener is added to its internal array of listeners.
Listeners registered for the 'newListener'
event will be passed the event
name and a reference to the listener being added.
The fact that the event is triggered before adding the listener has a subtle
but important side effect: any additional listeners registered to the same
name
within the 'newListener'
callback will be inserted before the
listener that is in the process of being added.
const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
if (event === 'event') {
// Insert a new listener in front
myEmitter.on('event', () => {
console.log('B');
});
}
});
myEmitter.on('event', () => {
console.log('A');
});
myEmitter.emit('event');
// Prints:
// B
// A
Event: 'removeListener'#
eventName
<any> The event namelistener
<Function> The event handler function
The 'removeListener'
event is emitted after the listener
is removed.
EventEmitter.listenerCount(emitter, eventName)#
Stability: 0 - Deprecated: Use emitter.listenerCount()
instead.
A class method that returns the number of listeners for the given eventName
registered on the given emitter
.
const myEmitter = new MyEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(EventEmitter.listenerCount(myEmitter, 'event'));
// Prints: 2
EventEmitter.defaultMaxListeners#
By default, a maximum of 10
listeners can be registered for any single
event. This limit can be changed for individual EventEmitter
instances
using the emitter.setMaxListeners(n)
method. To change the default
for all EventEmitter
instances, the EventEmitter.defaultMaxListeners
property can be used. If this value is not a positive number, a TypeError
will be thrown.
Take caution when setting the EventEmitter.defaultMaxListeners
because the
change effects all EventEmitter
instances, including those created before
the change is made. However, calling emitter.setMaxListeners(n)
still has
precedence over EventEmitter.defaultMaxListeners
.
Note that this is not a hard limit. The EventEmitter
instance will allow
more listeners to be added but will output a trace warning to stderr indicating
that a "possible EventEmitter memory leak" has been detected. For any single
EventEmitter
, the emitter.getMaxListeners()
and emitter.setMaxListeners()
methods can be used to temporarily avoid this warning:
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
// do stuff
emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});
The --trace-warnings
command line flag can be used to display the
stack trace for such warnings.
The emitted warning can be inspected with process.on('warning')
and will
have the additional emitter
, type
and count
properties, referring to
the event emitter instance, the event’s name and the number of attached
listeners, respectively.
Its name
property is set to 'MaxListenersExceededWarning'
.
emitter.addListener(eventName, listener)#
eventName
<any>listener
<Function>
Alias for emitter.on(eventName, listener)
.
emitter.emit(eventName[, ...args])#
eventName
<any>...args
<any>
Synchronously calls each of the listeners registered for the event named
eventName
, in the order they were registered, passing the supplied arguments
to each.
Returns true
if the event had listeners, false
otherwise.
emitter.eventNames()#
Returns an array listing the events for which the emitter has registered listeners. The values in the array will be strings or Symbols.
const EventEmitter = require('events');
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});
const sym = Symbol('symbol');
myEE.on(sym, () => {});
console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]
emitter.getMaxListeners()#
Returns the current max listener value for the EventEmitter
which is either
set by emitter.setMaxListeners(n)
or defaults to
EventEmitter.defaultMaxListeners
.
emitter.listenerCount(eventName)#
eventName
<any> The name of the event being listened for
Returns the number of listeners listening to the event named eventName
.
emitter.listeners(eventName)#
eventName
<any>
Returns a copy of the array of listeners for the event named eventName
.
server.on('connection', (stream) => {
console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]
emitter.on(eventName, listener)#
eventName
<any> The name of the event.listener
<Function> The callback function
Adds the listener
function to the end of the listeners array for the
event named eventName
. No checks are made to see if the listener
has
already been added. Multiple calls passing the same combination of eventName
and listener
will result in the listener
being added, and called, multiple
times.
server.on('connection', (stream) => {
console.log('someone connected!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The
emitter.prependListener()
method can be used as an alternative to add the
event listener to the beginning of the listeners array.
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
emitter.once(eventName, listener)#
eventName
<any> The name of the event.listener
<Function> The callback function
Adds a one time listener
function for the event named eventName
. The
next time eventName
is triggered, this listener is removed and then invoked.
server.once('connection', (stream) => {
console.log('Ah, we have our first user!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The
emitter.prependOnceListener()
method can be used as an alternative to add the
event listener to the beginning of the listeners array.
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
emitter.prependListener(eventName, listener)#
eventName
<any> The name of the event.listener
<Function> The callback function
Adds the listener
function to the beginning of the listeners array for the
event named eventName
. No checks are made to see if the listener
has
already been added. Multiple calls passing the same combination of eventName
and listener
will result in the listener
being added, and called, multiple
times.
server.prependListener('connection', (stream) => {
console.log('someone connected!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.prependOnceListener(eventName, listener)#
eventName
<any> The name of the event.listener
<Function> The callback function
Adds a one time listener
function for the event named eventName
to the
beginning of the listeners array. The next time eventName
is triggered, this
listener is removed, and then invoked.
server.prependOnceListener('connection', (stream) => {
console.log('Ah, we have our first user!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.removeAllListeners([eventName])#
eventName
<any>
Removes all listeners, or those of the specified eventName
.
Note that it is bad practice to remove listeners added elsewhere in the code,
particularly when the EventEmitter
instance was created by some other
component or module (e.g. sockets or file streams).
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.removeListener(eventName, listener)#
eventName
<any>listener
<Function>
Removes the specified listener
from the listener array for the event named
eventName
.
const callback = (stream) => {
console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);
removeListener
will remove, at most, one instance of a listener from the
listener array. If any single listener has been added multiple times to the
listener array for the specified eventName
, then removeListener
must be
called multiple times to remove each instance.
Note that once an event has been emitted, all listeners attached to it at the
time of emitting will be called in order. This implies that any removeListener()
or removeAllListeners()
calls after emitting and before the last listener
finishes execution will not remove them from emit()
in progress. Subsequent
events will behave as expected.
const myEmitter = new MyEmitter();
const callbackA = () => {
console.log('A');
myEmitter.removeListener('event', callbackB);
};
const callbackB = () => {
console.log('B');
};
myEmitter.on('event', callbackA);
myEmitter.on('event', callbackB);
// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
// A
// B
// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
// A
Because listeners are managed using an internal array, calling this will
change the position indices of any listener registered after the listener
being removed. This will not impact the order in which listeners are called,
but it means that any copies of the listener array as returned by
the emitter.listeners()
method will need to be recreated.
Returns a reference to the EventEmitter
, so that calls can be chained.
emitter.setMaxListeners(n)#
By default EventEmitters will print a warning if more than 10
listeners are
added for a particular event. This is a useful default that helps finding
memory leaks. Obviously, not all events should be limited to just 10 listeners.
The emitter.setMaxListeners()
method allows the limit to be modified for this
specific EventEmitter
instance. The value can be set to Infinity
(or 0
)
to indicate an unlimited number of listeners.
Returns a reference to the EventEmitter
, so that calls can be chained.
File System#
Stability: 2 - Stable
File I/O is provided by simple wrappers around standard POSIX functions. To
use this module do require('fs')
. All the methods have asynchronous and
synchronous forms.
The asynchronous form always takes a completion callback as its last argument.
The arguments passed to the completion callback depend on the method, but the
first argument is always reserved for an exception. If the operation was
completed successfully, then the first argument will be null
or undefined
.
When using the synchronous form any exceptions are immediately thrown.
Exceptions may be handled using try
/catch
, or they may be allowed to
bubble up.
Here is an example of the asynchronous version:
const fs = require('fs');
fs.unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
Here is the synchronous version:
const fs = require('fs');
fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
With the asynchronous methods there is no guaranteed ordering. So the following is prone to error:
fs.rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
It could be that fs.stat
is executed before fs.rename
.
The correct way to do this is to chain the callbacks.
fs.rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
fs.stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
});
In busy processes, the programmer is strongly encouraged to use the asynchronous versions of these calls. The synchronous versions will block the entire process until they complete--halting all connections.
The relative path to a filename can be used. Remember, however, that this path
will be relative to process.cwd()
.
While it is not recommended, most fs functions allow the callback argument to
be omitted, in which case a default callback is used that rethrows errors. To
get a trace to the original call site, set the NODE_DEBUG
environment
variable:
Note: Omitting the callback function on asynchronous fs functions is deprecated and may result in an error being thrown in the future.
$ cat script.js
function bad() {
require('fs').readFile('/');
}
bad();
$ env NODE_DEBUG=fs node script.js
fs.js:88
throw backtrace;
^
Error: EISDIR: illegal operation on a directory, read
<stack trace.>
Note: On Windows Node.js follows the concept of per-drive working directory.
This behavior can be observed when using a drive path without a backslash. For
example fs.readdirSync('c:\\')
can potentially return a different result than
fs.readdirSync('c:')
. For more information, see
this MSDN page.
WHATWG URL object support#
Stability: 1 - Experimental
For most fs
module functions, the path
or filename
argument may be passed
as a WHATWG URL
object. Only URL
objects using the file:
protocol
are supported.
const fs = require('fs');
const { URL } = require('url');
const fileUrl = new URL('file:///tmp/hello');
fs.readFileSync(fileUrl);
Note: file:
URLs are always absolute paths.
Using WHATWG URL
objects might introduce platform-specific behaviors.
On Windows, file:
URLs with a hostname convert to UNC paths, while file:
URLs with drive letters convert to local absolute paths. file:
URLs without a
hostname nor a drive letter will result in a throw :
// On Windows :
// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
// - WHATWG file URLs with drive letters convert to absolute path
// file:///C:/tmp/hello => C:\tmp\hello
fs.readFileSync(new URL('file:///C:/tmp/hello'));
// - WHATWG file URLs without hostname must have a drive letters
fs.readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
fs.readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute
Note: file:
URLs with drive letters must use :
as a separator just after
the drive letter. Using another separator will result in a throw.
On all other platforms, file:
URLs with a hostname are unsupported and will
result in a throw:
// On other platforms:
// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute
// - WHATWG file URLs convert to absolute path
// file:///tmp/hello => /tmp/hello
fs.readFileSync(new URL('file:///tmp/hello'));
A file:
URL having encoded slash characters will result in a throw on all
platforms:
// On Windows
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
// On POSIX
fs.readFileSync(new URL('file:///p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
/ characters */
On Windows, file:
URLs having encoded backslash will result in a throw:
// On Windows
fs.readFileSync(new URL('file:///C:/path/%5C'));
fs.readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
Buffer API#
fs
functions support passing and receiving paths as both strings
and Buffers. The latter is intended to make it possible to work with
filesystems that allow for non-UTF-8 filenames. For most typical
uses, working with paths as Buffers will be unnecessary, as the string
API converts to and from UTF-8 automatically.
Note: On certain file systems (such as NTFS and HFS+) filenames
will always be encoded as UTF-8. On such file systems, passing
non-UTF-8 encoded Buffers to fs
functions will not work as expected.
Class: fs.FSWatcher#
Objects returned from fs.watch()
are of this type.
The listener
callback provided to fs.watch()
receives the returned FSWatcher's
change
events.
The object itself emits these events:
Event: 'change'#
eventType
<string> The type of fs changefilename
<string> | <Buffer> The filename that changed (if relevant/available)
Emitted when something changes in a watched directory or file.
See more details in fs.watch()
.
The filename
argument may not be provided depending on operating system
support. If filename
is provided, it will be provided as a Buffer
if
fs.watch()
is called with its encoding
option set to 'buffer'
, otherwise
filename
will be a string.
// Example when handled through fs.watch listener
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
if (filename) {
console.log(filename);
// Prints: <Buffer ...>
}
});
Event: 'error'#
error
<Error>
Emitted when an error occurs.
watcher.close()#
Stop watching for changes on the given fs.FSWatcher
.
Class: fs.ReadStream#
ReadStream
is a Readable Stream.
Event: 'close'#
Emitted when the ReadStream
's underlying file descriptor has been closed
using the fs.close()
method.
Event: 'open'#
fd
<integer> Integer file descriptor used by the ReadStream.
Emitted when the ReadStream's file is opened.
readStream.bytesRead#
The number of bytes read so far.
readStream.path#
The path to the file the stream is reading from as specified in the first
argument to fs.createReadStream()
. If path
is passed as a string, then
readStream.path
will be a string. If path
is passed as a Buffer
, then
readStream.path
will be a Buffer
.
Class: fs.Stats#
Objects returned from fs.stat()
, fs.lstat()
and fs.fstat()
and
their synchronous counterparts are of this type.
stats.isFile()
stats.isDirectory()
stats.isBlockDevice()
stats.isCharacterDevice()
stats.isSymbolicLink()
(only valid withfs.lstat()
)stats.isFIFO()
stats.isSocket()
For a regular file util.inspect(stats)
would return a string very
similar to this:
Stats {
dev: 2114,
ino: 48064969,
mode: 33188,
nlink: 1,
uid: 85,
gid: 100,
rdev: 0,
size: 527,
blksize: 4096,
blocks: 8,
atimeMs: 1318289051000.1,
mtimeMs: 1318289051000.1,
ctimeMs: 1318289051000.1,
birthtimeMs: 1318289051000.1,
atime: Mon, 10 Oct 2011 23:24:11 GMT,
mtime: Mon, 10 Oct 2011 23:24:11 GMT,
ctime: Mon, 10 Oct 2011 23:24:11 GMT,
birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
Note: atimeMs
, mtimeMs
, ctimeMs
, birthtimeMs
are numbers
that hold the corresponding times in milliseconds. Their precision is platform
specific. atime
, mtime
, ctime
, and birthtime
are Date
object alternate representations of the various times. The Date
and number
values are not connected. Assigning a new number value, or mutating the Date
value, will not be reflected in the corresponding alternate representation.
Stat Time Values#
The times in the stat object have the following semantics:
atime
"Access Time" - Time when file data last accessed. Changed by the mknod(2), utimes(2), and read(2) system calls.mtime
"Modified Time" - Time when file data last modified. Changed by the mknod(2), utimes(2), and write(2) system calls.ctime
"Change Time" - Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2), read(2), and write(2) system calls.birthtime
"Birth Time" - Time of file creation. Set once when the file is created. On filesystems where birthtime is not available, this field may instead hold either thectime
or1970-01-01T00:00Z
(ie, unix epoch timestamp0
). Note that this value may be greater thanatime
ormtime
in this case. On Darwin and other FreeBSD variants, also set if theatime
is explicitly set to an earlier value than the currentbirthtime
using the utimes(2) system call.
Prior to Node v0.12, the ctime
held the birthtime
on Windows
systems. Note that as of v0.12, ctime
is not "creation time", and
on Unix systems, it never was.
Class: fs.WriteStream#
WriteStream
is a Writable Stream.
Event: 'close'#
Emitted when the WriteStream
's underlying file descriptor has been closed
using the fs.close()
method.
Event: 'open'#
fd
<integer> Integer file descriptor used by the WriteStream.
Emitted when the WriteStream's file is opened.
writeStream.bytesWritten#
The number of bytes written so far. Does not include data that is still queued for writing.
writeStream.path#
The path to the file the stream is writing to as specified in the first
argument to fs.createWriteStream()
. If path
is passed as a string, then
writeStream.path
will be a string. If path
is passed as a Buffer
, then
writeStream.path
will be a Buffer
.
fs.access(path[, mode], callback)#
path
<string> | <Buffer> | <URL>mode
<integer> Default:fs.constants.F_OK
callback
<Function>
Tests a user's permissions for the file or directory specified by path
.
The mode
argument is an optional integer that specifies the accessibility
checks to be performed. The following constants define the possible values of
mode
. It is possible to create a mask consisting of the bitwise OR of two or
more values.
fs.constants.F_OK
-path
is visible to the calling process. This is useful for determining if a file exists, but says nothing aboutrwx
permissions. Default if nomode
is specified.fs.constants.R_OK
-path
can be read by the calling process.fs.constants.W_OK
-path
can be written by the calling process.fs.constants.X_OK
-path
can be executed by the calling process. This has no effect on Windows (will behave likefs.constants.F_OK
).
The final argument, callback
, is a callback function that is invoked with
a possible error argument. If any of the accessibility checks fail, the error
argument will be populated. The following example checks if the file
/etc/passwd
can be read and written by the current process.
fs.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK, (err) => {
console.log(err ? 'no access!' : 'can read/write');
});
Using fs.access()
to check for the accessibility of a file before calling
fs.open()
, fs.readFile()
or fs.writeFile()
is not recommended. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file is not accessible.
For example:
write (NOT RECOMMENDED)
fs.access('myfile', (err) => {
if (!err) {
console.error('myfile already exists');
return;
}
fs.open('myfile', 'wx', (err, fd) => {
if (err) throw err;
writeMyData(fd);
});
});
write (RECOMMENDED)
fs.open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
writeMyData(fd);
});
read (NOT RECOMMENDED)
fs.access('myfile', (err) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
fs.open('myfile', 'r', (err, fd) => {
if (err) throw err;
readMyData(fd);
});
});
read (RECOMMENDED)
fs.open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
readMyData(fd);
});
The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the accessibility of a file only if the file won’t be used directly, for example when its accessibility is a signal from another process.
fs.accessSync(path[, mode])#
Synchronous version of fs.access()
. This throws if any accessibility
checks fail, and does nothing otherwise.
fs.appendFile(file, data[, options], callback)#
file
<string> | <Buffer> | <number> filename or file descriptordata
<string> | <Buffer>options
<Object> | <string>callback
<Function>
Asynchronously append data to a file, creating the file if it does not yet exist.
data
can be a string or a buffer.
Example:
fs.appendFile('message.txt', 'data to append', (err) => {
if (err) throw err;
console.log('The "data to append" was appended to file!');
});
If options
is a string, then it specifies the encoding. Example:
fs.appendFile('message.txt', 'data to append', 'utf8', callback);
Any specified file descriptor has to have been opened for appending.
Note: If a file descriptor is specified as the file
, it will not be closed
automatically.
fs.appendFileSync(file, data[, options])#
file
<string> | <Buffer> | <number> filename or file descriptordata
<string> | <Buffer>options
<Object> | <string>
The synchronous version of fs.appendFile()
. Returns undefined
.
fs.chmod(path, mode, callback)#
path
<string> | <Buffer> | <URL>mode
<integer>callback
<Function>
Asynchronous chmod(2). No arguments other than a possible exception are given to the completion callback.
fs.chmodSync(path, mode)#
Synchronous chmod(2). Returns undefined
.
fs.chown(path, uid, gid, callback)#
Asynchronous chown(2). No arguments other than a possible exception are given to the completion callback.
fs.chownSync(path, uid, gid)#
Synchronous chown(2). Returns undefined
.
fs.close(fd, callback)#
fd
<integer>callback
<Function>
Asynchronous close(2). No arguments other than a possible exception are given to the completion callback.
fs.closeSync(fd)#
fd
<integer>
Synchronous close(2). Returns undefined
.
fs.constants#
Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants.
fs.createReadStream(path[, options])#
Returns a new ReadStream
object. (See Readable Stream).
Be aware that, unlike the default value set for highWaterMark
on a
readable stream (16 kb), the stream returned by this method has a
default value of 64 kb for the same parameter.
options
is an object or string with the following defaults:
const defaults = {
flags: 'r',
encoding: null,
fd: null,
mode: 0o666,
autoClose: true
};
options
can include start
and end
values to read a range of bytes from
the file instead of the entire file. Both start
and end
are inclusive and
start counting at 0. If fd
is specified and start
is omitted or undefined
,
fs.createReadStream()
reads sequentially from the current file position.
The encoding
can be any one of those accepted by Buffer
.
If fd
is specified, ReadStream
will ignore the path
argument and will use
the specified file descriptor. This means that no 'open'
event will be
emitted. Note that fd
should be blocking; non-blocking fd
s should be passed
to net.Socket
.
If autoClose
is false, then the file descriptor won't be closed, even if
there's an error. It is the application's responsibility to close it and make
sure there's no file descriptor leak. If autoClose
is set to true (default
behavior), on error
or end
the file descriptor will be closed
automatically.
mode
sets the file mode (permission and sticky bits), but only if the
file was created.
An example to read the last 10 bytes of a file which is 100 bytes long:
fs.createReadStream('sample.txt', { start: 90, end: 99 });
If options
is a string, then it specifies the encoding.
fs.createWriteStream(path[, options])#
Returns a new WriteStream
object. (See Writable Stream).
options
is an object or string with the following defaults:
const defaults = {
flags: 'w',
defaultEncoding: 'utf8',
fd: null,
mode: 0o666,
autoClose: true
};
options
may also include a start
option to allow writing data at
some position past the beginning of the file. Modifying a file rather
than replacing it may require a flags
mode of r+
rather than the
default mode w
. The defaultEncoding
can be any one of those accepted by
Buffer
.
If autoClose
is set to true (default behavior) on error
or end
the file descriptor will be closed automatically. If autoClose
is false,
then the file descriptor won't be closed, even if there's an error.
It is the application's responsibility to close it and make sure there's no
file descriptor leak.
Like ReadStream
, if fd
is specified, WriteStream
will ignore the
path
argument and will use the specified file descriptor. This means that no
'open'
event will be emitted. Note that fd
should be blocking; non-blocking
fd
s should be passed to net.Socket
.
If options
is a string, then it specifies the encoding.
fs.exists(path, callback)#
Stability: 0 - Deprecated: Usefs.stat()
orfs.access()
instead.
path
<string> | <Buffer> | <URL>callback
<Function>
Test whether or not the given path exists by checking with the file system.
Then call the callback
argument with either true or false. Example:
fs.exists('/etc/passwd', (exists) => {
console.log(exists ? 'it\'s there' : 'no passwd!');
});
Note that the parameter to this callback is not consistent with other
Node.js callbacks. Normally, the first parameter to a Node.js callback is
an err
parameter, optionally followed by other parameters. The
fs.exists()
callback has only one boolean parameter. This is one reason
fs.access()
is recommended instead of fs.exists()
.
Using fs.exists()
to check for the existence of a file before calling
fs.open()
, fs.readFile()
or fs.writeFile()
is not recommended. Doing
so introduces a race condition, since other processes may change the file's
state between the two calls. Instead, user code should open/read/write the
file directly and handle the error raised if the file does not exist.
For example:
write (NOT RECOMMENDED)
fs.exists('myfile', (exists) => {
if (exists) {
console.error('myfile already exists');
} else {
fs.open('myfile', 'wx', (err, fd) => {
if (err) throw err;
writeMyData(fd);
});
}
});
write (RECOMMENDED)
fs.open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
writeMyData(fd);
});
read (NOT RECOMMENDED)
fs.exists('myfile', (exists) => {
if (exists) {
fs.open('myfile', 'r', (err, fd) => {
readMyData(fd);
});
} else {
console.error('myfile does not exist');
}
});
read (RECOMMENDED)
fs.open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
readMyData(fd);
});
The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the existence of a file only if the file won’t be used directly, for example when its existence is a signal from another process.
fs.existsSync(path)#
Synchronous version of fs.exists()
.
Returns true
if the file exists, false
otherwise.
Note that fs.exists()
is deprecated, but fs.existsSync()
is not.
(The callback
parameter to fs.exists()
accepts parameters that are
inconsistent with other Node.js callbacks. fs.existsSync()
does not use
a callback.)
fs.fchmod(fd, mode, callback)#
fd
<integer>mode
<integer>callback
<Function>
Asynchronous fchmod(2). No arguments other than a possible exception are given to the completion callback.
fs.fchmodSync(fd, mode)#
Synchronous fchmod(2). Returns undefined
.
fs.fchown(fd, uid, gid, callback)#
fd
<integer>uid
<integer>gid
<integer>callback
<Function>
Asynchronous fchown(2). No arguments other than a possible exception are given to the completion callback.
fs.fchownSync(fd, uid, gid)#
Synchronous fchown(2). Returns undefined
.
fs.fdatasync(fd, callback)#
fd
<integer>callback
<Function>
Asynchronous fdatasync(2). No arguments other than a possible exception are given to the completion callback.
fs.fdatasyncSync(fd)#
fd
<integer>
Synchronous fdatasync(2). Returns undefined
.
fs.fstat(fd, callback)#
fd
<integer>callback
<Function>
Asynchronous fstat(2). The callback gets two arguments (err, stats)
where
stats
is an fs.Stats
object. fstat()
is identical to stat()
,
except that the file to be stat-ed is specified by the file descriptor fd
.
fs.fstatSync(fd)#
fd
<integer>
Synchronous fstat(2). Returns an instance of fs.Stats
.
fs.fsync(fd, callback)#
fd
<integer>callback
<Function>
Asynchronous fsync(2). No arguments other than a possible exception are given to the completion callback.
fs.fsyncSync(fd)#
fd
<integer>
Synchronous fsync(2). Returns undefined
.
fs.ftruncate(fd[, len], callback)#
fd
<integer>len
<integer> Default:0
callback
<Function>
Asynchronous ftruncate(2). No arguments other than a possible exception are given to the completion callback.
If the file referred to by the file descriptor was larger than len
bytes, only
the first len
bytes will be retained in the file.
For example, the following program retains only the first four bytes of the file
console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js
// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');
// truncate the file to first four bytes
fs.ftruncate(fd, 4, (err) => {
assert.ifError(err);
console.log(fs.readFileSync('temp.txt', 'utf8'));
});
// Prints: Node
If the file previously was shorter than len
bytes, it is extended, and the
extended part is filled with null bytes ('\0'). For example,
console.log(fs.readFileSync('temp.txt', 'utf-8'));
// Prints: Node.js
// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');
// truncate the file to 10 bytes, whereas the actual size is 7 bytes
fs.ftruncate(fd, 10, (err) => {
assert.ifError(err);
console.log(fs.readFileSync('temp.txt'));
});
// Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00>
// ('Node.js\0\0\0' in UTF8)
The last three bytes are null bytes ('\0'), to compensate the over-truncation.
fs.ftruncateSync(fd[, len])#
Synchronous ftruncate(2). Returns undefined
.
fs.futimes(fd, atime, mtime, callback)#
fd
<integer>atime
<integer>mtime
<integer>callback
<Function>
Change the file timestamps of a file referenced by the supplied file descriptor.
Note: This function does not work on AIX versions before 7.1, it will return
the error UV_ENOSYS
.
fs.futimesSync(fd, atime, mtime)#
Synchronous version of fs.futimes()
. Returns undefined
.
fs.lchmod(path, mode, callback)#
path
<string> | <Buffer>mode
<integer>callback
<Function>
Asynchronous lchmod(2). No arguments other than a possible exception are given to the completion callback.
Only available on macOS.
fs.lchmodSync(path, mode)#
Synchronous lchmod(2). Returns undefined
.
fs.lchown(path, uid, gid, callback)#
path
<string> | <Buffer>uid
<integer>gid
<integer>callback
<Function>
Asynchronous lchown(2). No arguments other than a possible exception are given to the completion callback.
fs.lchownSync(path, uid, gid)#
Synchronous lchown(2). Returns undefined
.
fs.link(existingPath, newPath, callback)#
Asynchronous link(2). No arguments other than a possible exception are given to the completion callback.
fs.linkSync(existingPath, newPath)#
Synchronous link(2). Returns undefined
.
fs.lstat(path, callback)#
path
<string> | <Buffer> | <URL>callback
<Function>
Asynchronous lstat(2). The callback gets two arguments (err, stats)
where
stats
is a fs.Stats
object. lstat()
is identical to stat()
,
except that if path
is a symbolic link, then the link itself is stat-ed,
not the file that it refers to.
fs.lstatSync(path)#
Synchronous lstat(2). Returns an instance of fs.Stats
.
fs.mkdir(path[, mode], callback)#
path
<string> | <Buffer> | <URL>mode
<integer> Default:0o777
callback
<Function>
Asynchronous mkdir(2). No arguments other than a possible exception are given
to the completion callback. mode
defaults to 0o777
.
fs.mkdirSync(path[, mode])#
Synchronous mkdir(2). Returns undefined
.
fs.mkdtemp(prefix[, options], callback)#
prefix
<string>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Creates a unique temporary directory.
Generates six random characters to be appended behind a required
prefix
to create a unique temporary directory.
The created folder path is passed as a string to the callback's second parameter.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use.
Example:
fs.mkdtemp('/tmp/foo-', (err, folder) => {
if (err) throw err;
console.log(folder);
// Prints: /tmp/foo-itXde2
});
Note: The fs.mkdtemp()
method will append the six randomly selected
characters directly to the prefix
string. For instance, given a directory
/tmp
, if the intention is to create a temporary directory within /tmp
,
the prefix
must end with a trailing platform-specific path separator
(require('path').sep
).
// The parent directory for the new temporary directory
const tmpDir = '/tmp';
// This method is *INCORRECT*:
fs.mkdtemp(tmpDir, (err, folder) => {
if (err) throw err;
console.log(folder);
// Will print something similar to `/tmpabc123`.
// Note that a new temporary directory is created
// at the file system root rather than *within*
// the /tmp directory.
});
// This method is *CORRECT*:
const { sep } = require('path');
fs.mkdtemp(`${tmpDir}${sep}`, (err, folder) => {
if (err) throw err;
console.log(folder);
// Will print something similar to `/tmp/abc123`.
// A new temporary directory is created within
// the /tmp directory.
});
fs.mkdtempSync(prefix[, options])#
The synchronous version of fs.mkdtemp()
. Returns the created
folder path.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use.
fs.open(path, flags[, mode], callback)#
path
<string> | <Buffer> | <URL>flags
<string> | <number>mode
<integer> Default:0o666
callback
<Function>
Asynchronous file open. See open(2). flags
can be:
'r'
- Open file for reading. An exception occurs if the file does not exist.'r+'
- Open file for reading and writing. An exception occurs if the file does not exist.'rs+'
- Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.
Note that this doesn't turn
fs.open()
into a synchronous blocking call. If synchronous operation is desiredfs.openSync()
should be used.'w'
- Open file for writing. The file is created (if it does not exist) or truncated (if it exists).'wx'
- Like'w'
but fails ifpath
exists.'w+'
- Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).'wx+'
- Like'w+'
but fails ifpath
exists.'a'
- Open file for appending. The file is created if it does not exist.'ax'
- Like'a'
but fails ifpath
exists.'a+'
- Open file for reading and appending. The file is created if it does not exist.'ax+'
- Like'a+'
but fails ifpath
exists.
mode
sets the file mode (permission and sticky bits), but only if the file was
created. It defaults to 0o666
, readable and writable.
The callback gets two arguments (err, fd)
.
The exclusive flag 'x'
(O_EXCL
flag in open(2)) ensures that path
is newly
created. On POSIX systems, path
is considered to exist even if it is a symlink
to a non-existent file. The exclusive flag may or may not work with network file
systems.
flags
can also be a number as documented by open(2); commonly used constants
are available from fs.constants
. On Windows, flags are translated to
their equivalent ones where applicable, e.g. O_WRONLY
to FILE_GENERIC_WRITE
,
or O_EXCL|O_CREAT
to CREATE_NEW
, as accepted by CreateFileW.
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
Note: The behavior of fs.open()
is platform-specific for some flags. As
such, opening a directory on macOS and Linux with the 'a+'
flag - see example
below - will return an error. In contrast, on Windows and FreeBSD, a file
descriptor will be returned.
// macOS and Linux
fs.open('<directory>', 'a+', (err, fd) => {
// => [Error: EISDIR: illegal operation on a directory, open <directory>]
});
// Windows and FreeBSD
fs.open('<directory>', 'a+', (err, fd) => {
// => null, <fd>
});
Some characters (< > : " / \ | ? *
) are reserved under Windows as documented
by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains
a colon, Node.js will open a file system stream, as described by
this MSDN page.
Functions based on fs.open()
exhibit this behavior as well. eg.
fs.writeFile()
, fs.readFile()
, etc.
fs.openSync(path, flags[, mode])#
Synchronous version of fs.open()
. Returns an integer representing the file
descriptor.
fs.read(fd, buffer, offset, length, position, callback)#
fd
<integer>buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>callback
<Function>
Read data from the file specified by fd
.
buffer
is the buffer that the data will be written to.
offset
is the offset in the buffer to start writing at.
length
is an integer specifying the number of bytes to read.
position
is an integer specifying where to begin reading from in the file.
If position
is null
, data will be read from the current file position.
The callback is given the three arguments, (err, bytesRead, buffer)
.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise for an object with bytesRead
and buffer
properties.
fs.readdir(path[, options], callback)#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Asynchronous readdir(3). Reads the contents of a directory.
The callback gets two arguments (err, files)
where files
is an array of
the names of the files in the directory excluding '.'
and '..'
.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the filenames passed to the callback. If the encoding
is set to 'buffer'
,
the filenames returned will be passed as Buffer
objects.
fs.readdirSync(path[, options])#
Synchronous readdir(3). Returns an array of filenames excluding '.'
and
'..'
.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the filenames passed to the callback. If the encoding
is set to 'buffer'
,
the filenames returned will be passed as Buffer
objects.
fs.readFile(path[, options], callback)#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions
<Object> | <string>callback
<Function>
Asynchronously reads the entire contents of a file. Example:
fs.readFile('/etc/passwd', (err, data) => {
if (err) throw err;
console.log(data);
});
The callback is passed two arguments (err, data)
, where data
is the
contents of the file.
If no encoding is specified, then the raw buffer is returned.
If options
is a string, then it specifies the encoding. Example:
fs.readFile('/etc/passwd', 'utf8', callback);
Note: When the path is a directory, the behavior of
fs.readFile()
and fs.readFileSync()
is platform-specific. On macOS,
Linux, and Windows, an error will be returned. On FreeBSD, a representation
of the directory's contents will be returned.
// macOS, Linux and Windows
fs.readFile('<directory>', (err, data) => {
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
});
// FreeBSD
fs.readFile('<directory>', (err, data) => {
// => null, <data>
});
Any specified file descriptor has to support reading.
Note: If a file descriptor is specified as the path
, it will not be closed
automatically.
fs.readFileSync(path[, options])#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions
<Object> | <string>
Synchronous version of fs.readFile()
. Returns the contents of the path
.
If the encoding
option is specified then this function returns a
string. Otherwise it returns a buffer.
Note: Similar to fs.readFile()
, when the path is a directory, the
behavior of fs.readFileSync()
is platform-specific.
// macOS, Linux and Windows
fs.readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
// FreeBSD
fs.readFileSync('<directory>'); // => null, <data>
fs.readlink(path[, options], callback)#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Asynchronous readlink(2). The callback gets two arguments (err,
linkString)
.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the link path passed to the callback. If the encoding
is set to 'buffer'
,
the link path returned will be passed as a Buffer
object.
fs.readlinkSync(path[, options])#
Synchronous readlink(2). Returns the symbolic link's string value.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the link path passed to the callback. If the encoding
is set to 'buffer'
,
the link path returned will be passed as a Buffer
object.
fs.readSync(fd, buffer, offset, length, position)#
fd
<integer>buffer
<string> | <Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>
Synchronous version of fs.read()
. Returns the number of bytesRead
.
fs.realpath(path[, options], callback)#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Asynchronous realpath(3). The callback
gets two arguments (err,
resolvedPath)
. May use process.cwd
to resolve relative paths.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the path passed to the callback. If the encoding
is set to 'buffer'
,
the path returned will be passed as a Buffer
object.
Note: If path
resolves to a socket or a pipe, the function will return a
system dependent name for that object.
fs.realpathSync(path[, options])#
Synchronous realpath(3). Returns the resolved path.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use for
the returned value. If the encoding
is set to 'buffer'
, the path returned
will be passed as a Buffer
object.
Note: If path
resolves to a socket or a pipe, the function will return a
system dependent name for that object.
fs.rename(oldPath, newPath, callback)#
Asynchronous rename(2). No arguments other than a possible exception are given to the completion callback.
fs.renameSync(oldPath, newPath)#
Synchronous rename(2). Returns undefined
.
fs.rmdir(path, callback)#
path
<string> | <Buffer> | <URL>callback
<Function>
Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.
Note: Using fs.rmdir()
on a file (not a directory) results in an ENOENT
error on Windows and an ENOTDIR
error on POSIX.
fs.rmdirSync(path)#
Synchronous rmdir(2). Returns undefined
.
Note: Using fs.rmdirSync()
on a file (not a directory) results in an ENOENT
error on Windows and an ENOTDIR
error on POSIX.
fs.stat(path, callback)#
path
<string> | <Buffer> | <URL>callback
<Function>
Asynchronous stat(2). The callback gets two arguments (err, stats)
where
stats
is an fs.Stats
object.
In case of an error, the err.code
will be one of Common System Errors.
Using fs.stat()
to check for the existence of a file before calling
fs.open()
, fs.readFile()
or fs.writeFile()
is not recommended.
Instead, user code should open/read/write the file directly and handle the
error raised if the file is not available.
To check if a file exists without manipulating it afterwards, fs.access()
is recommended.
fs.statSync(path)#
Synchronous stat(2). Returns an instance of fs.Stats
.
fs.symlink(target, path[, type], callback)#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> Default:'file'
callback
<Function>
Asynchronous symlink(2). No arguments other than a possible exception are given
to the completion callback. The type
argument can be set to 'dir'
,
'file'
, or 'junction'
(default is 'file'
) and is only available on
Windows (ignored on other platforms). Note that Windows junction points require
the destination path to be absolute. When using 'junction'
, the target
argument will automatically be normalized to absolute path.
Here is an example below:
fs.symlink('./foo', './new-port', callback);
It creates a symbolic link named "new-port" that points to "foo".
fs.symlinkSync(target, path[, type])#
Synchronous symlink(2). Returns undefined
.
fs.truncate(path[, len], callback)#
path
<string> | <Buffer>len
<integer> Default:0
callback
<Function>
Asynchronous truncate(2). No arguments other than a possible exception are
given to the completion callback. A file descriptor can also be passed as the
first argument. In this case, fs.ftruncate()
is called.
fs.truncateSync(path[, len])#
Synchronous truncate(2). Returns undefined
. A file descriptor can also be
passed as the first argument. In this case, fs.ftruncateSync()
is called.
fs.unlink(path, callback)#
path
<string> | <Buffer> | <URL>callback
<Function>
Asynchronous unlink(2). No arguments other than a possible exception are given to the completion callback.
fs.unlinkSync(path)#
Synchronous unlink(2). Returns undefined
.
fs.unwatchFile(filename[, listener])#
filename
<string> | <Buffer>listener
<Function> | <undefined> Default:undefined
Stop watching for changes on filename
. If listener
is specified, only that
particular listener is removed. Otherwise, all listeners are removed,
effectively stopping watching of filename
.
Calling fs.unwatchFile()
with a filename that is not being watched is a
no-op, not an error.
Note: fs.watch()
is more efficient than fs.watchFile()
and
fs.unwatchFile()
. fs.watch()
should be used instead of fs.watchFile()
and fs.unwatchFile()
when possible.
fs.utimes(path, atime, mtime, callback)#
Change file timestamps of the file referenced by the supplied path.
Note: The arguments atime
and mtime
of the following related functions
follow these rules:
- The value should be a Unix timestamp in seconds. For example,
Date.now()
returns milliseconds, so it should be divided by 1000 before passing it in. - If the value is a numeric string like
'123456789'
, the value will get converted to the corresponding number. - If the value is
NaN
,Infinity
or-Infinity
, an Error will be thrown.
fs.utimesSync(path, atime, mtime)#
Synchronous version of fs.utimes()
. Returns undefined
.
fs.watch(filename[, options][, listener])#
filename
<string> | <Buffer> | <URL>options
<string> | <Object>persistent
<boolean> Indicates whether the process should continue to run as long as files are being watched. Default:true
recursive
<boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See Caveats). Default:false
encoding
<string> Specifies the character encoding to be used for the filename passed to the listener. Default:'utf8'
listener
<Function> | <undefined> Default:undefined
Watch for changes on filename
, where filename
is either a file or a
directory. The returned object is a fs.FSWatcher
.
The second argument is optional. If options
is provided as a string, it
specifies the encoding
. Otherwise options
should be passed as an object.
The listener callback gets two arguments (eventType, filename)
. eventType
is either
'rename'
or 'change'
, and filename
is the name of the file which triggered
the event.
Note that on most platforms, 'rename'
is emitted whenever a filename appears
or disappears in the directory.
Also note the listener callback is attached to the 'change'
event fired by
fs.FSWatcher
, but it is not the same thing as the 'change'
value of
eventType
.
Caveats#
The fs.watch
API is not 100% consistent across platforms, and is
unavailable in some situations.
The recursive option is only supported on macOS and Windows.
Availability#
This feature depends on the underlying operating system providing a way to be notified of filesystem changes.
- On Linux systems, this uses
inotify
- On BSD systems, this uses
kqueue
- On macOS, this uses
kqueue
for files andFSEvents
for directories. - On SunOS systems (including Solaris and SmartOS), this uses
event ports
. - On Windows systems, this feature depends on
ReadDirectoryChangesW
. - On Aix systems, this feature depends on
AHAFS
, which must be enabled.
If the underlying functionality is not available for some reason, then
fs.watch
will not be able to function. For example, watching files or
directories can be unreliable, and in some cases impossible, on network file
systems (NFS, SMB, etc), or host file systems when using virtualization software
such as Vagrant, Docker, etc.
It is still possible to use fs.watchFile()
, which uses stat polling, but
this method is slower and less reliable.
Inodes#
On Linux and macOS systems, fs.watch()
resolves the path to an inode and
watches the inode. If the watched path is deleted and recreated, it is assigned
a new inode. The watch will emit an event for the delete but will continue
watching the original inode. Events for the new inode will not be emitted.
This is expected behavior.
In AIX, save and close of a file being watched causes two notifications - one for adding new content, and one for truncation. Moreover, save and close operations on some platforms cause inode changes that force watch operations to become invalid and ineffective. AIX retains inode for the lifetime of a file, that way though this is different from Linux / macOS, this improves the usability of file watching. This is expected behavior.
Filename Argument#
Providing filename
argument in the callback is only supported on Linux,
macOS, Windows, and AIX. Even on supported platforms, filename
is not always
guaranteed to be provided. Therefore, don't assume that filename
argument is
always provided in the callback, and have some fallback logic if it is null.
fs.watch('somedir', (eventType, filename) => {
console.log(`event type is: ${eventType}`);
if (filename) {
console.log(`filename provided: ${filename}`);
} else {
console.log('filename not provided');
}
});
fs.watchFile(filename[, options], listener)#
filename
<string> | <Buffer> | <URL>options
<Object>listener
<Function>
Watch for changes on filename
. The callback listener
will be called each
time the file is accessed.
The options
argument may be omitted. If provided, it should be an object. The
options
object may contain a boolean named persistent
that indicates
whether the process should continue to run as long as files are being watched.
The options
object may specify an interval
property indicating how often the
target should be polled in milliseconds. The default is
{ persistent: true, interval: 5007 }
.
The listener
gets two arguments the current stat object and the previous
stat object:
fs.watchFile('message.text', (curr, prev) => {
console.log(`the current mtime is: ${curr.mtime}`);
console.log(`the previous mtime was: ${prev.mtime}`);
});
These stat objects are instances of fs.Stat
.
To be notified when the file was modified, not just accessed, it is necessary
to compare curr.mtime
and prev.mtime
.
Note: When an fs.watchFile
operation results in an ENOENT
error, it
will invoke the listener once, with all the fields zeroed (or, for dates, the
Unix Epoch). In Windows, blksize
and blocks
fields will be undefined
,
instead of zero. If the file is created later on, the listener will be called
again, with the latest stat objects. This is a change in functionality since
v0.10.
Note: fs.watch()
is more efficient than fs.watchFile
and
fs.unwatchFile
. fs.watch
should be used instead of fs.watchFile
and
fs.unwatchFile
when possible.
fs.write(fd, buffer[, offset[, length[, position]]], callback)#
fd
<integer>buffer
<Buffer> | <Uint8Array>offset
<integer>length
<integer>position
<integer>callback
<Function>
Write buffer
to the file specified by fd
.
offset
determines the part of the buffer to be written, and length
is
an integer specifying the number of bytes to write.
position
refers to the offset from the beginning of the file where this data
should be written. If typeof position !== 'number'
, the data will be written
at the current position. See pwrite(2).
The callback will be given three arguments (err, bytesWritten, buffer)
where
bytesWritten
specifies how many bytes were written from buffer
.
If this method is invoked as its util.promisify()
ed version, it returns
a Promise for an object with bytesWritten
and buffer
properties.
Note that it is unsafe to use fs.write
multiple times on the same file
without waiting for the callback. For this scenario,
fs.createWriteStream
is strongly recommended.
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
fs.write(fd, string[, position[, encoding]], callback)#
fd
<integer>string
<string>position
<integer>encoding
<string>callback
<Function>
Write string
to the file specified by fd
. If string
is not a string, then
the value will be coerced to one.
position
refers to the offset from the beginning of the file where this data
should be written. If typeof position !== 'number'
the data will be written at
the current position. See pwrite(2).
encoding
is the expected string encoding.
The callback will receive the arguments (err, written, string)
where written
specifies how many bytes the passed string required to be written. Note that
bytes written is not the same as string characters. See Buffer.byteLength
.
Unlike when writing buffer
, the entire string must be written. No substring
may be specified. This is because the byte offset of the resulting data may not
be the same as the string offset.
Note that it is unsafe to use fs.write
multiple times on the same file
without waiting for the callback. For this scenario,
fs.createWriteStream
is strongly recommended.
On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
fs.writeFile(file, data[, options], callback)#
file
<string> | <Buffer> | <integer> filename or file descriptordata
<string> | <Buffer> | <Uint8Array>options
<Object> | <string>callback
<Function>
Asynchronously writes data to a file, replacing the file if it already exists.
data
can be a string or a buffer.
The encoding
option is ignored if data
is a buffer. It defaults
to 'utf8'
.
Example:
fs.writeFile('message.txt', 'Hello Node.js', (err) => {
if (err) throw err;
console.log('The file has been saved!');
});
If options
is a string, then it specifies the encoding. Example:
fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
Any specified file descriptor has to support writing.
Note that it is unsafe to use fs.writeFile
multiple times on the same file
without waiting for the callback. For this scenario,
fs.createWriteStream
is strongly recommended.
Note: If a file descriptor is specified as the file
, it will not be closed
automatically.
fs.writeFileSync(file, data[, options])#
file
<string> | <Buffer> | <integer> filename or file descriptordata
<string> | <Buffer> | <Uint8Array>options
<Object> | <string>
The synchronous version of fs.writeFile()
. Returns undefined
.
fs.writeSync(fd, buffer[, offset[, length[, position]]])#
fs.writeSync(fd, string[, position[, encoding]])#
Synchronous versions of fs.write()
. Returns the number of bytes written.
FS Constants#
The following constants are exported by fs.constants
.
Note: Not every constant will be available on every operating system.
File Access Constants#
The following constants are meant for use with fs.access()
.
Constant | Description |
---|---|
F_OK |
Flag indicating that the file is visible to the calling process. |
R_OK |
Flag indicating that the file can be read by the calling process. |
W_OK |
Flag indicating that the file can be written by the calling process. |
X_OK |
Flag indicating that the file can be executed by the calling process. |
File Open Constants#
The following constants are meant for use with fs.open()
.
Constant | Description |
---|---|
O_RDONLY |
Flag indicating to open a file for read-only access. |
O_WRONLY |
Flag indicating to open a file for write-only access. |
O_RDWR |
Flag indicating to open a file for read-write access. |
O_CREAT |
Flag indicating to create the file if it does not already exist. |
O_EXCL |
Flag indicating that opening a file should fail if the
O_CREAT flag is set and the file already exists. |
O_NOCTTY |
Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one). |
O_TRUNC |
Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero. |
O_APPEND |
Flag indicating that data will be appended to the end of the file. |
O_DIRECTORY |
Flag indicating that the open should fail if the path is not a directory. |
O_NOATIME |
Flag indicating reading accesses to the file system will no longer
result in an update to the atime information associated with the file.
This flag is available on Linux operating systems only. |
O_NOFOLLOW |
Flag indicating that the open should fail if the path is a symbolic link. |
O_SYNC |
Flag indicating that the file is opened for synchronous I/O. |
O_SYMLINK |
Flag indicating to open the symbolic link itself rather than the resource it is pointing to. |
O_DIRECT |
When set, an attempt will be made to minimize caching effects of file I/O. |
O_NONBLOCK |
Flag indicating to open the file in nonblocking mode when possible. |
File Type Constants#
The following constants are meant for use with the fs.Stats
object's
mode
property for determining a file's type.
Constant | Description |
---|---|
S_IFMT |
Bit mask used to extract the file type code. |
S_IFREG |
File type constant for a regular file. |
S_IFDIR |
File type constant for a directory. |
S_IFCHR |
File type constant for a character-oriented device file. |
S_IFBLK |
File type constant for a block-oriented device file. |
S_IFIFO |
File type constant for a FIFO/pipe. |
S_IFLNK |
File type constant for a symbolic link. |
S_IFSOCK |
File type constant for a socket. |
File Mode Constants#
The following constants are meant for use with the fs.Stats
object's
mode
property for determining the access permissions for a file.
Constant | Description |
---|---|
S_IRWXU |
File mode indicating readable, writable and executable by owner. |
S_IRUSR |
File mode indicating readable by owner. |
S_IWUSR |
File mode indicating writable by owner. |
S_IXUSR |
File mode indicating executable by owner. |
S_IRWXG |
File mode indicating readable, writable and executable by group. |
S_IRGRP |
File mode indicating readable by group. |
S_IWGRP |
File mode indicating writable by group. |
S_IXGRP |
File mode indicating executable by group. |
S_IRWXO |
File mode indicating readable, writable and executable by others. |
S_IROTH |
File mode indicating readable by others. |
S_IWOTH |
File mode indicating writable by others. |
S_IXOTH |
File mode indicating executable by others. |
Global Objects#
These objects are available in all modules. The following variables may appear to be global but are not. They exist only in the scope of modules, see the module system documentation:
The objects listed here are specific to Node.js. There are a number of built-in objects that are part of the JavaScript language itself, which are also globally accessible.
Class: Buffer#
Used to handle binary data. See the buffer section.
__dirname#
This variable may appear to be global but is not. See __dirname
.
__filename#
This variable may appear to be global but is not. See __filename
.
clearImmediate(immediateObject)#
clearImmediate
is described in the timers section.
clearInterval(intervalObject)#
clearInterval
is described in the timers section.
clearTimeout(timeoutObject)#
clearTimeout
is described in the timers section.
console#
Used to print to stdout and stderr. See the console
section.
exports#
This variable may appear to be global but is not. See exports
.
global#
- <Object> The global namespace object.
In browsers, the top-level scope is the global scope. This means that
within the browser var something
will define a new global variable. In
Node.js this is different. The top-level scope is not the global scope;
var something
inside a Node.js module will be local to that module.
module#
This variable may appear to be global but is not. See module
.
process#
The process object. See the process
object section.
require()#
This variable may appear to be global but is not. See require()
.
setImmediate(callback[, ...args])#
setImmediate
is described in the timers section.
setInterval(callback, delay[, ...args])#
setInterval
is described in the timers section.
setTimeout(callback, delay[, ...args])#
setTimeout
is described in the timers section.
HTTP#
Stability: 2 - Stable
To use the HTTP server and client one must require('http')
.
The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses--the user is able to stream data.
HTTP message headers are represented by an object like this:
{ 'content-length': '123',
'content-type': 'text/plain',
'connection': 'keep-alive',
'host': 'mysite.com',
'accept': '*/*' }
Keys are lowercased. Values are not modified.
In order to support the full spectrum of possible HTTP applications, Node.js's HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body.
See message.headers
for details on how duplicate headers are handled.
The raw headers as they were received are retained in the rawHeaders
property, which is an array of [key, value, key2, value2, ...]
. For
example, the previous message header object might have a rawHeaders
list like the following:
[ 'ConTent-Length', '123456',
'content-LENGTH', '123',
'content-type', 'text/plain',
'CONNECTION', 'keep-alive',
'Host', 'mysite.com',
'accepT', '*/*' ]
Class: http.Agent#
An Agent
is responsible for managing connection persistence
and reuse for HTTP clients. It maintains a queue of pending requests
for a given host and port, reusing a single socket connection for each
until the queue is empty, at which time the socket is either destroyed
or put into a pool where it is kept to be used again for requests to the
same host and port. Whether it is destroyed or pooled depends on the
keepAlive
option.
Pooled connections have TCP Keep-Alive enabled for them, but servers may
still close idle connections, in which case they will be removed from the
pool and a new connection will be made when a new HTTP request is made for
that host and port. Servers may also refuse to allow multiple requests
over the same connection, in which case the connection will have to be
remade for every request and cannot be pooled. The Agent
will still make
the requests to that server, but each one will occur over a new connection.
When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref()).
It is good practice, to destroy()
an Agent
instance when it is no
longer in use, because unused sockets consume OS resources.
Sockets are removed from an agent's pool when the socket emits either
a 'close'
event or an 'agentRemove'
event. When intending to keep one
HTTP request open for a long time without keeping it in the pool, something
like the following may be done:
http.get(options, (res) => {
// Do stuff
}).on('socket', (socket) => {
socket.emit('agentRemove');
});
An agent may also be used for an individual request. By providing
{agent: false}
as an option to the http.get()
or http.request()
functions, a one-time use Agent
with default options will be used
for the client connection.
agent:false
:
http.get({
hostname: 'localhost',
port: 80,
path: '/',
agent: false // create a new agent just for this one request
}, (res) => {
// Do stuff with response
});
new Agent(options)#
options
<Object> Set of configurable options to set on the agent. Can have the following fields:keepAlive
<boolean> Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Defaults tofalse
keepAliveMsecs
<number> When using thekeepAlive
option, specifies the initial delay for TCP Keep-Alive packets. Ignored when thekeepAlive
option isfalse
orundefined
. Defaults to1000
.maxSockets
<number> Maximum number of sockets to allow per host. Defaults toInfinity
.maxFreeSockets
<number> Maximum number of sockets to leave open in a free state. Only relevant ifkeepAlive
is set totrue
. Defaults to256
.
The default http.globalAgent
that is used by http.request()
has all
of these values set to their respective defaults.
To configure any of them, a custom http.Agent
instance must be created.
const http = require('http');
const keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);
agent.createConnection(options[, callback])#
options
<Object> Options containing connection details. Checknet.createConnection()
for the format of the optionscallback
<Function> Callback function that receives the created socket- Returns: <net.Socket>
Produces a socket/stream to be used for HTTP requests.
By default, this function is the same as net.createConnection()
. However,
custom agents may override this method in case greater flexibility is desired.
A socket/stream can be supplied in one of two ways: by returning the
socket/stream from this function, or by passing the socket/stream to callback
.
callback
has a signature of (err, stream)
.
agent.keepSocketAlive(socket)#
socket
<net.Socket>
Called when socket
is detached from a request and could be persisted by the
Agent. Default behavior is to:
socket.unref();
socket.setKeepAlive(agent.keepAliveMsecs);
This method can be overridden by a particular Agent
subclass. If this
method returns a falsy value, the socket will be destroyed instead of persisting
it for use with the next request.
agent.reuseSocket(socket, request)#
socket
<net.Socket>request
<http.ClientRequest>
Called when socket
is attached to request
after being persisted because of
the keep-alive options. Default behavior is to:
socket.ref();
This method can be overridden by a particular Agent
subclass.
agent.destroy()#
Destroy any sockets that are currently in use by the agent.
It is usually not necessary to do this. However, if using an
agent with keepAlive
enabled, then it is best to explicitly shut down
the agent when it will no longer be used. Otherwise,
sockets may hang open for quite a long time before the server
terminates them.
agent.freeSockets#
An object which contains arrays of sockets currently awaiting use by
the agent when keepAlive
is enabled. Do not modify.
agent.getName(options)#
Get a unique name for a set of request options, to determine whether a
connection can be reused. For an HTTP agent, this returns
host:port:localAddress
. For an HTTPS agent, the name includes the
CA, cert, ciphers, and other HTTPS/TLS-specific options that determine
socket reusability.
agent.maxFreeSockets#
By default set to 256. For agents with keepAlive
enabled, this
sets the maximum number of sockets that will be left open in the free
state.
agent.maxSockets#
By default set to Infinity. Determines how many concurrent sockets the agent can have open per origin. Origin is either a 'host:port' or 'host:port:localAddress' combination.
agent.requests#
An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.
agent.sockets#
An object which contains arrays of sockets currently in use by the agent. Do not modify.
Class: http.ClientRequest#
This object is created internally and returned from http.request()
. It
represents an in-progress request whose header has already been queued. The
header is still mutable using the setHeader(name, value)
, getHeader(name)
,
removeHeader(name)
API. The actual header will be sent along with the first
data chunk or when closing the connection.
To get the response, add a listener for 'response'
to the request object.
'response'
will be emitted from the request object when the response
headers have been received. The 'response'
event is executed with one
argument which is an instance of http.IncomingMessage
.
During the 'response'
event, one can add listeners to the
response object; particularly to listen for the 'data'
event.
If no 'response'
handler is added, then the response will be
entirely discarded. However, if a 'response'
event handler is added,
then the data from the response object must be consumed, either by
calling response.read()
whenever there is a 'readable'
event, or
by adding a 'data'
handler, or by calling the .resume()
method.
Until the data is consumed, the 'end'
event will not fire. Also, until
the data is read it will consume memory that can eventually lead to a
'process out of memory' error.
Note: Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.
The request implements the Writable Stream interface. This is an
EventEmitter
with the following events:
Event: 'abort'#
Emitted when the request has been aborted by the client. This event is only
emitted on the first call to abort()
.
Event: 'aborted'#
Emitted when the request has been aborted by the server and the network socket has closed.
Event: 'connect'#
response
<http.IncomingMessage>socket
<net.Socket>head
<Buffer>
Emitted each time a server responds to a request with a CONNECT
method. If this
event is not being listened for, clients receiving a CONNECT
method will have
their connections closed.
A client and server pair demonstrating how to listen for the 'connect'
event:
const http = require('http');
const net = require('net');
const url = require('url');
// Create an HTTP tunneling proxy
const proxy = http.createServer((req, res) => {
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('okay');
});
proxy.on('connect', (req, cltSocket, head) => {
// connect to an origin server
const srvUrl = url.parse(`http://${req.url}`);
const srvSocket = net.connect(srvUrl.port, srvUrl.hostname, () => {
cltSocket.write('HTTP/1.1 200 Connection Established\r\n' +
'Proxy-agent: Node.js-Proxy\r\n' +
'\r\n');
srvSocket.write(head);
srvSocket.pipe(cltSocket);
cltSocket.pipe(srvSocket);
});
});
// now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {
// make a request to a tunneling proxy
const options = {
port: 1337,
hostname: '127.0.0.1',
method: 'CONNECT',
path: 'www.google.com:80'
};
const req = http.request(options);
req.end();
req.on('connect', (res, socket, head) => {
console.log('got connected!');
// make a request over an HTTP tunnel
socket.write('GET / HTTP/1.1\r\n' +
'Host: www.google.com:80\r\n' +
'Connection: close\r\n' +
'\r\n');
socket.on('data', (chunk) => {
console.log(chunk.toString());
});
socket.on('end', () => {
proxy.close();
});
});
});
Event: 'continue'#
Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body.
Event: 'response'#
response
<http.IncomingMessage>
Emitted when a response is received to this request. This event is emitted only once.
Event: 'socket'#
socket
<net.Socket>
Emitted after a socket is assigned to this request.
Event: 'upgrade'#
response
<http.IncomingMessage>socket
<net.Socket>head
<Buffer>
Emitted each time a server responds to a request with an upgrade. If this event is not being listened for, clients receiving an upgrade header will have their connections closed.
A client server pair demonstrating how to listen for the 'upgrade'
event.
const http = require('http');
// Create an HTTP server
const srv = http.createServer((req, res) => {
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('okay');
});
srv.on('upgrade', (req, socket, head) => {
socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
'Upgrade: WebSocket\r\n' +
'Connection: Upgrade\r\n' +
'\r\n');
socket.pipe(socket); // echo back
});
// now that server is running
srv.listen(1337, '127.0.0.1', () => {
// make a request
const options = {
port: 1337,
hostname: '127.0.0.1',
headers: {
'Connection': 'Upgrade',
'Upgrade': 'websocket'
}
};
const req = http.request(options);
req.end();
req.on('upgrade', (res, socket, upgradeHead) => {
console.log('got upgraded!');
socket.end();
process.exit(0);
});
});
request.abort()#
Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.
request.aborted#
If a request has been aborted, this value is the time when the request was aborted, in milliseconds since 1 January 1970 00:00:00 UTC.
request.connection#
See request.socket
request.end([data[, encoding]][, callback])#
data
<string> | <Buffer>encoding
<string>callback
<Function>
Finishes sending the request. If any parts of the body are
unsent, it will flush them to the stream. If the request is
chunked, this will send the terminating '0\r\n\r\n'
.
If data
is specified, it is equivalent to calling
request.write(data, encoding)
followed by request.end(callback)
.
If callback
is specified, it will be called when the request stream
is finished.
request.flushHeaders()#
Flush the request headers.
For efficiency reasons, Node.js normally buffers the request headers until
request.end()
is called or the first chunk of request data is written. It
then tries to pack the request headers and data into a single TCP packet.
That's usually desired (it saves a TCP round-trip), but not when the first
data is not sent until possibly much later. request.flushHeaders()
bypasses
the optimization and kickstarts the request.
request.setNoDelay([noDelay])#
noDelay
<boolean>
Once a socket is assigned to this request and is connected
socket.setNoDelay()
will be called.
request.setSocketKeepAlive([enable][, initialDelay])#
Once a socket is assigned to this request and is connected
socket.setKeepAlive()
will be called.
request.setTimeout(timeout[, callback])#
timeout
<number> Milliseconds before a request is considered to be timed out.callback
<Function> Optional function to be called when a timeout occurs. Same as binding to thetimeout
event.
Once a socket is assigned to this request and is connected
socket.setTimeout()
will be called.
Returns request
.
request.socket#
Reference to the underlying socket. Usually users will not want to access
this property. In particular, the socket will not emit 'readable'
events
because of how the protocol parser attaches to the socket. After
response.end()
, the property is nulled. The socket
may also be accessed
via request.connection
.
Example:
const http = require('http');
const server = http.createServer((req, res) => {
const ip = req.socket.remoteAddress;
const port = req.socket.remotePort;
res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);
request.write(chunk[, encoding][, callback])#
chunk
<string> | <Buffer>encoding
<string>callback
<Function>
Sends a chunk of the body. By calling this method
many times, a request body can be sent to a
server--in that case it is suggested to use the
['Transfer-Encoding', 'chunked']
header line when
creating the request.
The encoding
argument is optional and only applies when chunk
is a string.
Defaults to 'utf8'
.
The callback
argument is optional and will be called when this chunk of data
is flushed.
Returns request
.
Class: http.Server#
This class inherits from net.Server
and has the following additional events:
Event: 'checkContinue'#
request
<http.IncomingMessage>response
<http.ServerResponse>
Emitted each time a request with an HTTP Expect: 100-continue
is received.
If this event is not listened for, the server will automatically respond
with a 100 Continue
as appropriate.
Handling this event involves calling response.writeContinue()
if the client
should continue to send the request body, or generating an appropriate HTTP
response (e.g. 400 Bad Request) if the client should not continue to send the
request body.
Note that when this event is emitted and handled, the 'request'
event will
not be emitted.
Event: 'checkExpectation'#
request
<http.ClientRequest>response
<http.ServerResponse>
Emitted each time a request with an HTTP Expect
header is received, where the
value is not 100-continue
. If this event is not listened for, the server will
automatically respond with a 417 Expectation Failed
as appropriate.
Note that when this event is emitted and handled, the 'request'
event will
not be emitted.
Event: 'clientError'#
exception
<Error>socket
<net.Socket>
If a client connection emits an 'error'
event, it will be forwarded here.
Listener of this event is responsible for closing/destroying the underlying
socket. For example, one may wish to more gracefully close the socket with an
HTTP '400 Bad Request' response instead of abruptly severing the connection.
Default behavior is to destroy the socket immediately on malformed request.
socket
is the net.Socket
object that the error originated from.
const http = require('http');
const server = http.createServer((req, res) => {
res.end();
});
server.on('clientError', (err, socket) => {
socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);
When the 'clientError'
event occurs, there is no request
or response
object, so any HTTP response sent, including response headers and payload,
must be written directly to the socket
object. Care must be taken to
ensure the response is a properly formatted HTTP response message.
Event: 'close'#
Emitted when the server closes.
Event: 'connect'#
request
<http.IncomingMessage> Arguments for the HTTP request, as it is in the'request'
eventsocket
<net.Socket> Network socket between the server and clienthead
<Buffer> The first packet of the tunneling stream (may be empty)
Emitted each time a client requests an HTTP CONNECT
method. If this event is
not listened for, then clients requesting a CONNECT
method will have their
connections closed.
After this event is emitted, the request's socket will not have a 'data'
event listener, meaning it will need to be bound in order to handle data
sent to the server on that socket.
Event: 'connection'#
socket
<net.Socket>
When a new TCP stream is established. socket
is an object of type
net.Socket
. Usually users will not want to access this event. In
particular, the socket will not emit 'readable'
events because of how
the protocol parser attaches to the socket. The socket
can also be
accessed at request.connection
.
Event: 'request'#
request
<http.IncomingMessage>response
<http.ServerResponse>
Emitted each time there is a request. Note that there may be multiple requests per connection (in the case of HTTP Keep-Alive connections).
Event: 'upgrade'#
request
<http.IncomingMessage> Arguments for the HTTP request, as it is in the'request'
eventsocket
<net.Socket> Network socket between the server and clienthead
<Buffer> The first packet of the upgraded stream (may be empty)
Emitted each time a client requests an HTTP upgrade. If this event is not listened for, then clients requesting an upgrade will have their connections closed.
After this event is emitted, the request's socket will not have a 'data'
event listener, meaning it will need to be bound in order to handle data
sent to the server on that socket.
server.close([callback])#
callback
<Function>
Stops the server from accepting new connections. See net.Server.close()
.
server.listen(handle[, callback])#
handle
<Object>callback
<Function>
The handle
object can be set to either a server or socket (anything
with an underlying _handle
member), or a {fd: <n>}
object.
This will cause the server to accept connections on the specified handle, but it is presumed that the file descriptor or handle has already been bound to a port or domain socket.
Listening on a file descriptor is not supported on Windows.
This function is asynchronous. callback
will be added as a listener for the
'listening'
event. See also net.Server.listen()
.
Returns server
.
Note: The server.listen()
method may be called multiple times. Each
subsequent call will re-open the server using the provided options.
server.listen(path[, callback])#
path
<string>callback
<Function>
Start a UNIX socket server listening for connections on the given path
.
This function is asynchronous. callback
will be added as a listener for the
'listening'
event. See also net.Server.listen(path)
.
Note: The server.listen()
method may be called multiple times. Each
subsequent call will re-open the server using the provided options.
server.listen([port][, hostname][, backlog][, callback])#
port
<number>hostname
<string>backlog
<number>callback
<Function>
Begin accepting connections on the specified port
and hostname
. If the
hostname
is omitted, the server will accept connections on the
unspecified IPv6 address (::
) when IPv6 is available, or the
unspecified IPv4 address (0.0.0.0
) otherwise.
Note: In most operating systems, listening to the
unspecified IPv6 address (::
) may cause the net.Server
to also listen on
the unspecified IPv4 address (0.0.0.0
).
Omit the port argument, or use a port value of 0
, to have the operating system
assign a random port, which can be retrieved by using server.address().port
after the 'listening'
event has been emitted.
To listen to a unix socket, supply a filename instead of port and hostname.
backlog
is the maximum length of the queue of pending connections.
The actual length will be determined by the OS through sysctl settings such as
tcp_max_syn_backlog
and somaxconn
on linux. The default value of this
parameter is 511 (not 512).
This function is asynchronous. callback
will be added as a listener for the
'listening'
event. See also net.Server.listen(port)
.
Note: The server.listen()
method may be called multiple times. Each
subsequent call will re-open the server using the provided options.
server.listening#
A Boolean indicating whether or not the server is listening for connections.
server.maxHeadersCount#
- <number> Defaults to 2000.
Limits maximum incoming headers count, equal to 2000 by default. If set to 0 - no limit will be applied.
server.setTimeout([msecs][, callback])#
msecs
<number> Defaults to 120000 (2 minutes).callback
<Function>
Sets the timeout value for sockets, and emits a 'timeout'
event on
the Server object, passing the socket as an argument, if a timeout
occurs.
If there is a 'timeout'
event listener on the Server object, then it
will be called with the timed-out socket as an argument.
By default, the Server's timeout value is 2 minutes, and sockets are
destroyed automatically if they time out. However, if a callback is assigned
to the Server's 'timeout'
event, timeouts must be handled explicitly.
Returns server
.
server.timeout#
- <number> Timeout in milliseconds. Defaults to 120000 (2 minutes).
The number of milliseconds of inactivity before a socket is presumed to have timed out.
A value of 0 will disable the timeout behavior on incoming connections.
Note: The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.
server.keepAliveTimeout#
- <number> Timeout in milliseconds. Defaults to 5000 (5 seconds).
The number of milliseconds of inactivity a server needs to wait for additional
incoming data, after it has finished writing the last response, before a socket
will be destroyed. If the server receives new data before the keep-alive
timeout has fired, it will reset the regular inactivity timeout, i.e.,
server.timeout
.
A value of 0 will disable the keep-alive timeout behavior on incoming connections.
Note: The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.
Class: http.ServerResponse#
This object is created internally by an HTTP server--not by the user. It is
passed as the second parameter to the 'request'
event.
The response implements, but does not inherit from, the Writable Stream
interface. This is an EventEmitter
with the following events:
Event: 'close'#
Indicates that the underlying connection was terminated before
response.end()
was called or able to flush.
Event: 'finish'#
Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.
After this event, no more events will be emitted on the response object.
response.addTrailers(headers)#
headers
<Object>
This method adds HTTP trailing headers (a header but at the end of the message) to the response.
Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded.
Note that HTTP requires the Trailer
header to be sent in order to
emit trailers, with a list of the header fields in its value. E.g.,
response.writeHead(200, { 'Content-Type': 'text/plain',
'Trailer': 'Content-MD5' });
response.write(fileData);
response.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
response.end();
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
response.connection#
See response.socket
.
response.end([data][, encoding][, callback])#
data
<string> | <Buffer>encoding
<string>callback
<Function>
This method signals to the server that all of the response headers and body
have been sent; that server should consider this message complete.
The method, response.end()
, MUST be called on each response.
If data
is specified, it is equivalent to calling
response.write(data, encoding)
followed by response.end(callback)
.
If callback
is specified, it will be called when the response stream
is finished.
response.finished#
Boolean value that indicates whether the response has completed. Starts
as false
. After response.end()
executes, the value will be true
.
response.getHeader(name)#
Reads out a header that's already been queued but not sent to the client. Note that the name is case insensitive.
Example:
const contentType = response.getHeader('content-type');
response.getHeaderNames()#
- Returns: <Array>
Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.
Example:
response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']
response.getHeaders()#
- Returns: <Object>
Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.
Note: The object returned by the response.getHeaders()
method does not
prototypically inherit from the JavaScript Object
. This means that typical
Object
methods such as obj.toString()
, obj.hasOwnProperty()
, and others
are not defined and will not work.
Example:
response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
const headers = response.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
response.hasHeader(name)#
Returns true
if the header identified by name
is currently set in the
outgoing headers. Note that the header name matching is case-insensitive.
Example:
const hasContentType = response.hasHeader('content-type');
response.headersSent#
Boolean (read-only). True if headers were sent, false otherwise.
response.removeHeader(name)#
name
<string>
Removes a header that's queued for implicit sending.
Example:
response.removeHeader('Content-Encoding');
response.sendDate#
When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.
This should only be disabled for testing; HTTP requires the Date header in responses.
response.setHeader(name, value)#
name
<string>value
<string> | <string[]>
Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name.
Example:
response.setHeader('Content-Type', 'text/html');
or
response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
When headers have been set with response.setHeader()
, they will be merged with
any headers passed to response.writeHead()
, with the headers passed to
response.writeHead()
given precedence.
// returns content-type = text/plain
const server = http.createServer((req, res) => {
res.setHeader('Content-Type', 'text/html');
res.setHeader('X-Foo', 'bar');
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('ok');
});
response.setTimeout(msecs[, callback])#
msecs
<number>callback
<Function>
Sets the Socket's timeout value to msecs
. If a callback is
provided, then it is added as a listener on the 'timeout'
event on
the response object.
If no 'timeout'
listener is added to the request, the response, or
the server, then sockets are destroyed when they time out. If a handler is
assigned to the request, the response, or the server's 'timeout'
events,
timed out sockets must be handled explicitly.
Returns response
.
response.socket#
Reference to the underlying socket. Usually users will not want to access
this property. In particular, the socket will not emit 'readable'
events
because of how the protocol parser attaches to the socket. After
response.end()
, the property is nulled. The socket
may also be accessed
via response.connection
.
Example:
const http = require('http');
const server = http.createServer((req, res) => {
const ip = req.socket.remoteAddress;
const port = req.socket.remotePort;
res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);
response.statusCode#
When using implicit headers (not calling response.writeHead()
explicitly),
this property controls the status code that will be sent to the client when
the headers get flushed.
Example:
response.statusCode = 404;
After response header was sent to the client, this property indicates the status code which was sent out.
response.statusMessage#
When using implicit headers (not calling response.writeHead()
explicitly), this property
controls the status message that will be sent to the client when the headers get
flushed. If this is left as undefined
then the standard message for the status
code will be used.
Example:
response.statusMessage = 'Not found';
After response header was sent to the client, this property indicates the status message which was sent out.
response.write(chunk[, encoding][, callback])#
chunk
<string> | <Buffer>encoding
<string>callback
<Function>- Returns: <boolean>
If this method is called and response.writeHead()
has not been called,
it will switch to implicit header mode and flush the implicit headers.
This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.
Note that in the http
module, the response body is omitted when the
request is a HEAD request. Similarly, the 204
and 304
responses
must not include a message body.
chunk
can be a string or a buffer. If chunk
is a string,
the second parameter specifies how to encode it into a byte stream.
By default the encoding
is 'utf8'
. callback
will be called when this chunk
of data is flushed.
Note: This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.
The first time response.write()
is called, it will send the buffered
header information and the first chunk of the body to the client. The second
time response.write()
is called, Node.js assumes data will be streamed,
and sends the new data separately. That is, the response is buffered up to the
first chunk of the body.
Returns true
if the entire data was flushed successfully to the kernel
buffer. Returns false
if all or part of the data was queued in user memory.
'drain'
will be emitted when the buffer is free again.
response.writeContinue()#
Sends a HTTP/1.1 100 Continue message to the client, indicating that
the request body should be sent. See the 'checkContinue'
event on Server
.
response.writeHead(statusCode[, statusMessage][, headers])#
Sends a response header to the request. The status code is a 3-digit HTTP
status code, like 404
. The last argument, headers
, are the response headers.
Optionally one can give a human-readable statusMessage
as the second
argument.
Example:
const body = 'hello world';
response.writeHead(200, {
'Content-Length': Buffer.byteLength(body),
'Content-Type': 'text/plain' });
This method must only be called once on a message and it must
be called before response.end()
is called.
If response.write()
or response.end()
are called before calling
this, the implicit/mutable headers will be calculated and call this function.
When headers have been set with response.setHeader()
, they will be merged with
any headers passed to response.writeHead()
, with the headers passed to
response.writeHead()
given precedence.
// returns content-type = text/plain
const server = http.createServer((req, res) => {
res.setHeader('Content-Type', 'text/html');
res.setHeader('X-Foo', 'bar');
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('ok');
});
Note that Content-Length is given in bytes not characters. The above example
works because the string 'hello world'
contains only single byte characters.
If the body contains higher coded characters then Buffer.byteLength()
should be used to determine the number of bytes in a given encoding.
And Node.js does not check whether Content-Length and the length of the body
which has been transmitted are equal or not.
Attempting to set a header field name or value that contains invalid characters
will result in a TypeError
being thrown.
Class: http.IncomingMessage#
An IncomingMessage
object is created by http.Server
or
http.ClientRequest
and passed as the first argument to the 'request'
and 'response'
event respectively. It may be used to access response status,
headers and data.
It implements the Readable Stream interface, as well as the following additional events, methods, and properties.
Event: 'aborted'#
Emitted when the request has been aborted by the client and the network socket has closed.
Event: 'close'#
Indicates that the underlying connection was closed.
Just like 'end'
, this event occurs only once per response.
message.destroy([error])#
error
<Error>
Calls destroy()
on the socket that received the IncomingMessage
. If error
is provided, an 'error'
event is emitted and error
is passed as an argument
to any listeners on the event.
message.headers#
The request/response headers object.
Key-value pairs of header names and values. Header names are lower-cased. Example:
// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
// host: '127.0.0.1:8000',
// accept: '*/*' }
console.log(request.headers);
Duplicates in raw headers are handled in the following ways, depending on the header name:
- Duplicates of
age
,authorization
,content-length
,content-type
,etag
,expires
,from
,host
,if-modified-since
,if-unmodified-since
,last-modified
,location
,max-forwards
,proxy-authorization
,referer
,retry-after
, oruser-agent
are discarded. set-cookie
is always an array. Duplicates are added to the array.- For all other headers, the values are joined together with ', '.
message.httpVersion#
In case of server request, the HTTP version sent by the client. In the case of
client response, the HTTP version of the connected-to server.
Probably either '1.1'
or '1.0'
.
Also message.httpVersionMajor
is the first integer and
message.httpVersionMinor
is the second.
message.method#
Only valid for request obtained from http.Server
.
The request method as a string. Read only. Example:
'GET'
, 'DELETE'
.
message.rawHeaders#
The raw request/response headers list exactly as they were received.
Note that the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.
Header names are not lowercased, and duplicates are not merged.
// Prints something like:
//
// [ 'user-agent',
// 'this is invalid because there can be only one',
// 'User-Agent',
// 'curl/7.22.0',
// 'Host',
// '127.0.0.1:8000',
// 'ACCEPT',
// '*/*' ]
console.log(request.rawHeaders);
message.rawTrailers#
The raw request/response trailer keys and values exactly as they were
received. Only populated at the 'end'
event.
message.setTimeout(msecs, callback)#
msecs
<number>callback
<Function>
Calls message.connection.setTimeout(msecs, callback)
.
Returns message
.
message.socket#
The net.Socket
object associated with the connection.
With HTTPS support, use request.socket.getPeerCertificate()
to obtain the
client's authentication details.
message.statusCode#
Only valid for response obtained from http.ClientRequest
.
The 3-digit HTTP response status code. E.G. 404
.
message.statusMessage#
Only valid for response obtained from http.ClientRequest
.
The HTTP response status message (reason phrase). E.G. OK
or Internal Server Error
.
message.trailers#
The request/response trailers object. Only populated at the 'end'
event.
message.url#
Only valid for request obtained from http.Server
.
Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:
GET /status?name=ryan HTTP/1.1\r\n
Accept: text/plain\r\n
\r\n
Then request.url
will be:
'/status?name=ryan'
To parse the url into its parts require('url').parse(request.url)
can be used. Example:
$ node
> require('url').parse('/status?name=ryan')
Url {
protocol: null,
slashes: null,
auth: null,
host: null,
port: null,
hostname: null,
hash: null,
search: '?name=ryan',
query: 'name=ryan',
pathname: '/status',
path: '/status?name=ryan',
href: '/status?name=ryan' }
To extract the parameters from the query string, the
require('querystring').parse
function can be used, or
true
can be passed as the second argument to require('url').parse
.
Example:
$ node
> require('url').parse('/status?name=ryan', true)
Url {
protocol: null,
slashes: null,
auth: null,
host: null,
port: null,
hostname: null,
hash: null,
search: '?name=ryan',
query: { name: 'ryan' },
pathname: '/status',
path: '/status?name=ryan',
href: '/status?name=ryan' }
http.METHODS#
A list of the HTTP methods that are supported by the parser.
http.STATUS_CODES#
A collection of all the standard HTTP response status codes, and the
short description of each. For example, http.STATUS_CODES[404] === 'Not
Found'
.
http.createServer([requestListener])#
requestListener
<Function>Returns: <http.Server>
Returns a new instance of http.Server
.
The requestListener
is a function which is automatically
added to the 'request'
event.
http.get(options[, callback])#
options
<Object> | <string> | <URL> Accepts the sameoptions
ashttp.request()
, with themethod
always set toGET
. Properties that are inherited from the prototype are ignored.callback
<Function>- Returns: <http.ClientRequest>
Since most requests are GET requests without bodies, Node.js provides this
convenience method. The only difference between this method and
http.request()
is that it sets the method to GET and calls req.end()
automatically. Note that response data must be consumed in the callback
for reasons stated in http.ClientRequest
section.
The callback
is invoked with a single argument that is an instance of
http.IncomingMessage
JSON Fetching Example:
http.get('http://nodejs.org/dist/index.json', (res) => {
const { statusCode } = res;
const contentType = res.headers['content-type'];
let error;
if (statusCode !== 200) {
error = new Error('Request Failed.\n' +
`Status Code: ${statusCode}`);
} else if (!/^application\/json/.test(contentType)) {
error = new Error('Invalid content-type.\n' +
`Expected application/json but received ${contentType}`);
}
if (error) {
console.error(error.message);
// consume response data to free up memory
res.resume();
return;
}
res.setEncoding('utf8');
let rawData = '';
res.on('data', (chunk) => { rawData += chunk; });
res.on('end', () => {
try {
const parsedData = JSON.parse(rawData);
console.log(parsedData);
} catch (e) {
console.error(e.message);
}
});
}).on('error', (e) => {
console.error(`Got error: ${e.message}`);
});
http.globalAgent#
Global instance of Agent
which is used as the default for all HTTP client
requests.
http.request(options[, callback])#
options
<Object> | <string> | <URL>protocol
<string> Protocol to use. Defaults tohttp:
.host
<string> A domain name or IP address of the server to issue the request to. Defaults tolocalhost
.hostname
<string> Alias forhost
. To supporturl.parse()
,hostname
is preferred overhost
.family
<number> IP address family to use when resolvinghost
andhostname
. Valid values are4
or6
. When unspecified, both IP v4 and v6 will be used.port
<number> Port of remote server. Defaults to 80.localAddress
<string> Local interface to bind for network connections.socketPath
<string> Unix Domain Socket (use one of host:port or socketPath).method
<string> A string specifying the HTTP request method. Defaults to'GET'
.path
<string> Request path. Defaults to'/'
. Should include query string if any. E.G.'/index.html?page=12'
. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future.headers
<Object> An object containing request headers.auth
<string> Basic authentication i.e.'user:password'
to compute an Authorization header.agent
<http.Agent> | <boolean> ControlsAgent
behavior. Possible values:undefined
(default): usehttp.globalAgent
for this host and port.Agent
object: explicitly use the passed inAgent
.false
: causes a newAgent
with default values to be used.
createConnection
<Function> A function that produces a socket/stream to use for the request when theagent
option is not used. This can be used to avoid creating a customAgent
class just to override the defaultcreateConnection
function. Seeagent.createConnection()
for more details.timeout
<number>: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.
callback
<Function>- Returns: <http.ClientRequest>
Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.
options
can be an object, a string, or a URL
object. If options
is a
string, it is automatically parsed with url.parse()
. If it is a URL
object, it will be automatically converted to an ordinary options
object.
The optional callback
parameter will be added as a one time listener for
the 'response'
event.
http.request()
returns an instance of the http.ClientRequest
class. The ClientRequest
instance is a writable stream. If one needs to
upload a file with a POST request, then write to the ClientRequest
object.
Example:
const postData = querystring.stringify({
'msg': 'Hello World!'
});
const options = {
hostname: 'www.google.com',
port: 80,
path: '/upload',
method: 'POST',
headers: {
'Content-Type': 'application/x-www-form-urlencoded',
'Content-Length': Buffer.byteLength(postData)
}
};
const req = http.request(options, (res) => {
console.log(`STATUS: ${res.statusCode}`);
console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
res.setEncoding('utf8');
res.on('data', (chunk) => {
console.log(`BODY: ${chunk}`);
});
res.on('end', () => {
console.log('No more data in response.');
});
});
req.on('error', (e) => {
console.error(`problem with request: ${e.message}`);
});
// write data to request body
req.write(postData);
req.end();
Note that in the example req.end()
was called. With http.request()
one
must always call req.end()
to signify the end of the request -
even if there is no data being written to the request body.
If any error is encountered during the request (be that with DNS resolution,
TCP level errors, or actual HTTP parse errors) an 'error'
event is emitted
on the returned request object. As with all 'error'
events, if no listeners
are registered the error will be thrown.
There are a few special headers that should be noted.
Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request.
Sending a 'Content-Length' header will disable the default chunked encoding.
Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', both a timeout and a listener for the
continue
event should be set. See RFC2616 Section 8.2.3 for more information.Sending an Authorization header will override using the
auth
option to compute basic authentication.
Example using a URL
as options
:
const { URL } = require('url');
const options = new URL('http://abc:xyz@example.com');
const req = http.request(options, (res) => {
// ...
});
HTTPS#
Stability: 2 - Stable
HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.
Class: https.Agent#
An Agent object for HTTPS similar to http.Agent
. See https.request()
for more information.
Class: https.Server#
This class is a subclass of tls.Server
and emits events same as
http.Server
. See http.Server
for more information.
server.setTimeout([msecs][, callback])#
msecs
<number> Defaults to 120000 (2 minutes).callback
<Function>
server.timeout#
- <number> Defaults to 120000 (2 minutes).
See http.Server#timeout
.
server.keepAliveTimeout#
- <number> Defaults to 5000 (5 seconds).
See http.Server#keepAliveTimeout
.
https.createServer([options][, requestListener])#
options
<Object> Acceptsoptions
fromtls.createServer()
andtls.createSecureContext()
.requestListener
<Function> A listener to be added to therequest
event.
Example:
// curl -k https://localhost:8000/
const https = require('https');
const fs = require('fs');
const options = {
key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};
https.createServer(options, (req, res) => {
res.writeHead(200);
res.end('hello world\n');
}).listen(8000);
Or
const https = require('https');
const fs = require('fs');
const options = {
pfx: fs.readFileSync('test/fixtures/test_cert.pfx'),
passphrase: 'sample'
};
https.createServer(options, (req, res) => {
res.writeHead(200);
res.end('hello world\n');
}).listen(8000);
server.close([callback])#
callback
<Function>
See http.close()
for details.
server.listen(handle[, callback])#
handle
<Object>callback
<Function>
server.listen(path[, callback])#
path
<string>callback
<Function>
server.listen([port][, host][, backlog][, callback])#
port
<number>hostname
<string>backlog
<number>callback
<Function>
See http.listen()
for details.
https.get(options[, callback])#
options
<Object> | <string> | <URL> Accepts the sameoptions
ashttps.request()
, with themethod
always set toGET
.callback
<Function>
Like http.get()
but for HTTPS.
options
can be an object, a string, or a URL
object. If options
is a
string, it is automatically parsed with url.parse()
. If it is a URL
object, it will be automatically converted to an ordinary options
object.
Example:
const https = require('https');
https.get('https://encrypted.google.com/', (res) => {
console.log('statusCode:', res.statusCode);
console.log('headers:', res.headers);
res.on('data', (d) => {
process.stdout.write(d);
});
}).on('error', (e) => {
console.error(e);
});
https.globalAgent#
Global instance of https.Agent
for all HTTPS client requests.
https.request(options[, callback])#
options
<Object> | <string> | <URL> Accepts alloptions
fromhttp.request()
, with some differences in default values:protocol
Defaults tohttps:
port
Defaults to443
.agent
Defaults tohttps.globalAgent
.
callback
<Function>
Makes a request to a secure web server.
The following additional options
from tls.connect()
are also accepted when using a
custom Agent
:
pfx
, key
, passphrase
, cert
, ca
, ciphers
, rejectUnauthorized
, secureProtocol
, servername
options
can be an object, a string, or a URL
object. If options
is a
string, it is automatically parsed with url.parse()
. If it is a URL
object, it will be automatically converted to an ordinary options
object.
Example:
const https = require('https');
const options = {
hostname: 'encrypted.google.com',
port: 443,
path: '/',
method: 'GET'
};
const req = https.request(options, (res) => {
console.log('statusCode:', res.statusCode);
console.log('headers:', res.headers);
res.on('data', (d) => {
process.stdout.write(d);
});
});
req.on('error', (e) => {
console.error(e);
});
req.end();
Example using options from tls.connect()
:
const options = {
hostname: 'encrypted.google.com',
port: 443,
path: '/',
method: 'GET',
key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};
options.agent = new https.Agent(options);
const req = https.request(options, (res) => {
// ...
});
Alternatively, opt out of connection pooling by not using an Agent
.
Example:
const options = {
hostname: 'encrypted.google.com',
port: 443,
path: '/',
method: 'GET',
key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
agent: false
};
const req = https.request(options, (res) => {
// ...
});
Example using a URL
as options
:
const { URL } = require('url');
const options = new URL('https://abc:xyz@example.com');
const req = https.request(options, (res) => {
// ...
});
Inspector#
Stability: 1 - Experimental
The inspector
module provides an API for interacting with the V8 inspector.
It can be accessed using:
const inspector = require('inspector');
inspector.open([port[, host[, wait]]])#
- port <number> Port to listen on for inspector connections. Optional, defaults to what was specified on the CLI.
- host <string> Host to listen on for inspector connections. Optional, defaults to what was specified on the CLI.
- wait <boolean> Block until a client has connected. Optional, defaults to false.
Activate inspector on host and port. Equivalent to node
--inspect=[[host:]port]
, but can be done programatically after node has
started.
If wait is true
, will block until a client has connected to the inspect port
and flow control has been passed to the debugger client.
inspector.close()#
Deactivate the inspector. Blocks until there are no active connections.
inspector.url()#
Return the URL of the active inspector, or undefined
if there is none.
Class: inspector.Session#
The inspector.Session
is used for dispatching messages to the V8 inspector
back-end and receiving message responses and notifications.
Constructor: new inspector.Session()#
Create a new instance of the inspector.Session
class. The inspector session
needs to be connected through session.connect()
before the messages
can be dispatched to the inspector backend.
inspector.Session
is an EventEmitter
with the following events:
Event: 'inspectorNotification'#
- <Object> The notification message object
Emitted when any notification from the V8 Inspector is received.
session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed
It is also possible to subscribe only to notifications with specific method:
Event: <inspector-protocol-method>#
- <Object> The notification message object
Emitted when an inspector notification is received that has its method field set
to the <inspector-protocol-method>
value.
The following snippet installs a listener on the Debugger.paused
event, and prints the reason for program suspension whenever program
execution is suspended (through breakpoints, for example):
session.on('Debugger.paused', ({ params }) => {
console.log(params.hitBreakpoints);
});
// [ '/node/test/inspector/test-bindings.js:11:0' ]
session.connect()#
Connects a session to the inspector back-end. An exception will be thrown if there is already a connected session established either through the API or by a front-end connected to the Inspector WebSocket port.
session.post(method[, params][, callback])#
- method <string>
- params <Object>
- callback <Function>
Posts a message to the inspector back-end. callback
will be notified when
a response is received. callback
is a function that accepts two optional
arguments - error and message-specific result.
session.post('Runtime.evaluate', { expression: '2 + 2' },
(error, { result }) => console.log(result));
// Output: { type: 'number', value: 4, description: '4' }
The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.
Node inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.
session.disconnect()#
Immediately close the session. All pending message callbacks will be called
with an error. session.connect()
will need to be called to be able to send
messages again. Reconnected session will lose all inspector state, such as
enabled agents or configured breakpoints.
Internationalization Support#
Node.js has many features that make it easier to write internationalized programs. Some of them are:
- Locale-sensitive or Unicode-aware functions in the ECMAScript Language Specification:
- All functionality described in the ECMAScript Internationalization API
Specification (aka ECMA-402):
Intl
object- Locale-sensitive methods like
String.prototype.localeCompare()
andDate.prototype.toLocaleString()
- The WHATWG URL parser's internationalized domain names (IDNs) support
require('buffer').transcode()
- More accurate REPL line editing
Node.js (and its underlying V8 engine) uses ICU to implement these features in native C/C++ code. However, some of them require a very large ICU data file in order to support all locales of the world. Because it is expected that most Node.js users will make use of only a small portion of ICU functionality, only a subset of the full ICU data set is provided by Node.js by default. Several options are provided for customizing and expanding the ICU data set either when building or running Node.js.
Options for building Node.js#
To control how ICU is used in Node.js, four configure
options are available
during compilation. Additional details on how to compile Node.js are documented
in BUILDING.md.
--with-intl=none
/--without-intl
--with-intl=system-icu
--with-intl=small-icu
(default)--with-intl=full-icu
An overview of available Node.js and JavaScript features for each configure
option:
none |
system-icu |
small-icu |
full-icu |
|
---|---|---|---|---|
String.prototype.normalize() |
none (function is no-op) | full | full | full |
String.prototype.to*Case() |
full | full | full | full |
Intl |
none (object does not exist) | partial/full (depends on OS) | partial (English-only) | full |
String.prototype.localeCompare() |
partial (not locale-aware) | full | full | full |
String.prototype.toLocale*Case() |
partial (not locale-aware) | full | full | full |
Number.prototype.toLocaleString() |
partial (not locale-aware) | partial/full (depends on OS) | partial (English-only) | full |
Date.prototype.toLocale*String() |
partial (not locale-aware) | partial/full (depends on OS) | partial (English-only) | full |
WHATWG URL Parser | partial (no IDN support) | full | full | full |
require('buffer').transcode() |
none (function does not exist) | full | full | full |
REPL | partial (inaccurate line editing) | full | full | full |
Note: The "(not locale-aware)" designation denotes that the function carries
out its operation just like the non-Locale
version of the function, if one
exists. For example, under none
mode, Date.prototype.toLocaleString()
's
operation is identical to that of Date.prototype.toString()
.
Disable all internationalization features (none
)#
If this option is chosen, most internationalization features mentioned above
will be unavailable in the resulting node
binary.
Build with a pre-installed ICU (system-icu
)#
Node.js can link against an ICU build already installed on the system. In fact, most Linux distributions already come with ICU installed, and this option would make it possible to reuse the same set of data used by other components in the OS.
Functionalities that only require the ICU library itself, such as
String.prototype.normalize()
and the WHATWG URL parser, are fully
supported under system-icu
. Features that require ICU locale data in
addition, such as Intl.DateTimeFormat
may be fully or partially
supported, depending on the completeness of the ICU data installed on the
system.
Embed a limited set of ICU data (small-icu
)#
This option makes the resulting binary link against the ICU library statically,
and includes a subset of ICU data (typically only the English locale) within
the node
executable.
Functionalities that only require the ICU library itself, such as
String.prototype.normalize()
and the WHATWG URL parser, are fully
supported under small-icu
. Features that require ICU locale data in addition,
such as Intl.DateTimeFormat
, generally only work with the English locale:
const january = new Date(9e8);
const english = new Intl.DateTimeFormat('en', { month: 'long' });
const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
console.log(english.format(january));
// Prints "January"
console.log(spanish.format(january));
// Prints "M01" on small-icu
// Should print "enero"
This mode provides a good balance between features and binary size, and it is
the default behavior if no --with-intl
flag is passed. The official binaries
are also built in this mode.
Providing ICU data at runtime#
If the small-icu
option is used, one can still provide additional locale data
at runtime so that the JS methods would work for all ICU locales. Assuming the
data file is stored at /some/directory
, it can be made available to ICU
through either:
The
NODE_ICU_DATA
environmental variable:env NODE_ICU_DATA=/some/directory node
The
--icu-data-dir
CLI parameter:node --icu-data-dir=/some/directory
(If both are specified, the --icu-data-dir
CLI parameter takes precedence.)
ICU is able to automatically find and load a variety of data formats, but the
data must be appropriate for the ICU version, and the file correctly named.
The most common name for the data file is icudt5X[bl].dat
, where 5X
denotes
the intended ICU version, and b
or l
indicates the system's endianness.
Check "ICU Data" article in the ICU User Guide for other supported formats
and more details on ICU data in general.
The full-icu npm module can greatly simplify ICU data installation by
detecting the ICU version of the running node
executable and downloading the
appropriate data file. After installing the module through npm i full-icu
,
the data file will be available at ./node_modules/full-icu
. This path can be
then passed either to NODE_ICU_DATA
or --icu-data-dir
as shown above to
enable full Intl
support.
Embed the entire ICU (full-icu
)#
This option makes the resulting binary link against ICU statically and include a full set of ICU data. A binary created this way has no further external dependencies and supports all locales, but might be rather large. See BUILDING.md on how to compile a binary using this mode.
Detecting internationalization support#
To verify that ICU is enabled at all (system-icu
, small-icu
, or
full-icu
), simply checking the existence of Intl
should suffice:
const hasICU = typeof Intl === 'object';
Alternatively, checking for process.versions.icu
, a property defined only
when ICU is enabled, works too:
const hasICU = typeof process.versions.icu === 'string';
To check for support for a non-English locale (i.e. full-icu
or
system-icu
), Intl.DateTimeFormat
can be a good distinguishing factor:
const hasFullICU = (() => {
try {
const january = new Date(9e8);
const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
return spanish.format(january) === 'enero';
} catch (err) {
return false;
}
})();
For more verbose tests for Intl
support, the following resources may be found
to be helpful:
- btest402: Generally used to check whether Node.js with
Intl
support is built correctly. - Test262: ECMAScript's official conformance test suite includes a section dedicated to ECMA-402.
Modules#
Stability: 2 - Stable
Node.js has a simple module loading system. In Node.js, files and modules are in one-to-one correspondence (each file is treated as a separate module).
As an example, consider a file named foo.js
:
const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);
On the first line, foo.js
loads the module circle.js
that is in the same
directory as foo.js
.
Here are the contents of circle.js
:
const { PI } = Math;
exports.area = (r) => PI * r ** 2;
exports.circumference = (r) => 2 * PI * r;
The module circle.js
has exported the functions area()
and
circumference()
. Functions and objects are added to the root of a module
by specifying additional properties on the special exports
object.
Variables local to the module will be private, because the module is wrapped
in a function by Node.js (see module wrapper).
In this example, the variable PI
is private to circle.js
.
The module.exports
property can be assigned a new value (such as a function
or object).
Below, bar.js
makes use of the square
module, which exports a constructor:
const square = require('./square.js');
const mySquare = square(2);
console.log(`The area of my square is ${mySquare.area()}`);
The square
module is defined in square.js
:
// assigning to exports will not modify module, must use module.exports
module.exports = (width) => {
return {
area: () => width ** 2
};
};
The module system is implemented in the require('module')
module.
Accessing the main module#
When a file is run directly from Node.js, require.main
is set to its
module
. That means that it is possible to determine whether a file has been
run directly by testing require.main === module
.
For a file foo.js
, this will be true
if run via node foo.js
, but
false
if run by require('./foo')
.
Because module
provides a filename
property (normally equivalent to
__filename
), the entry point of the current application can be obtained
by checking require.main.filename
.
Addenda: Package Manager Tips#
The semantics of Node.js's require()
function were designed to be general
enough to support a number of reasonable directory structures. Package manager
programs such as dpkg
, rpm
, and npm
will hopefully find it possible to
build native packages from Node.js modules without modification.
Below we give a suggested directory structure that could work:
Let's say that we wanted to have the folder at
/usr/lib/node/<some-package>/<some-version>
hold the contents of a
specific version of a package.
Packages can depend on one another. In order to install package foo
, it
may be necessary to install a specific version of package bar
. The bar
package may itself have dependencies, and in some cases, these may even collide
or form cyclic dependencies.
Since Node.js looks up the realpath
of any modules it loads (that is,
resolves symlinks), and then looks for their dependencies in the node_modules
folders as described here, this
situation is very simple to resolve with the following architecture:
/usr/lib/node/foo/1.2.3/
- Contents of thefoo
package, version 1.2.3./usr/lib/node/bar/4.3.2/
- Contents of thebar
package thatfoo
depends on./usr/lib/node/foo/1.2.3/node_modules/bar
- Symbolic link to/usr/lib/node/bar/4.3.2/
./usr/lib/node/bar/4.3.2/node_modules/*
- Symbolic links to the packages thatbar
depends on.
Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use.
When the code in the foo
package does require('bar')
, it will get the
version that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar
.
Then, when the code in the bar
package calls require('quux')
, it'll get
the version that is symlinked into
/usr/lib/node/bar/4.3.2/node_modules/quux
.
Furthermore, to make the module lookup process even more optimal, rather
than putting packages directly in /usr/lib/node
, we could put them in
/usr/lib/node_modules/<name>/<version>
. Then Node.js will not bother
looking for missing dependencies in /usr/node_modules
or /node_modules
.
In order to make modules available to the Node.js REPL, it might be useful to
also add the /usr/lib/node_modules
folder to the $NODE_PATH
environment
variable. Since the module lookups using node_modules
folders are all
relative, and based on the real path of the files making the calls to
require()
, the packages themselves can be anywhere.
All Together...#
To get the exact filename that will be loaded when require()
is called, use
the require.resolve()
function.
Putting together all of the above, here is the high-level algorithm
in pseudocode of what require.resolve()
does:
require(X) from module at path Y
1. If X is a core module,
a. return the core module
b. STOP
2. If X begins with '/'
a. set Y to be the filesystem root
3. If X begins with './' or '/' or '../'
a. LOAD_AS_FILE(Y + X)
b. LOAD_AS_DIRECTORY(Y + X)
4. LOAD_NODE_MODULES(X, dirname(Y))
5. THROW "not found"
LOAD_AS_FILE(X)
1. If X is a file, load X as JavaScript text. STOP
2. If X.js is a file, load X.js as JavaScript text. STOP
3. If X.json is a file, parse X.json to a JavaScript Object. STOP
4. If X.node is a file, load X.node as binary addon. STOP
LOAD_INDEX(X)
1. If X/index.js is a file, load X/index.js as JavaScript text. STOP
2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
3. If X/index.node is a file, load X/index.node as binary addon. STOP
LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
a. Parse X/package.json, and look for "main" field.
b. let M = X + (json main field)
c. LOAD_AS_FILE(M)
d. LOAD_INDEX(M)
2. LOAD_INDEX(X)
LOAD_NODE_MODULES(X, START)
1. let DIRS=NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
a. LOAD_AS_FILE(DIR/X)
b. LOAD_AS_DIRECTORY(DIR/X)
NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I >= 0,
a. if PARTS[I] = "node_modules" CONTINUE
b. DIR = path join(PARTS[0 .. I] + "node_modules")
c. DIRS = DIRS + DIR
d. let I = I - 1
5. return DIRS
Caching#
Modules are cached after the first time they are loaded. This means
(among other things) that every call to require('foo')
will get
exactly the same object returned, if it would resolve to the same file.
Multiple calls to require('foo')
may not cause the module code to be
executed multiple times. This is an important feature. With it,
"partially done" objects can be returned, thus allowing transitive
dependencies to be loaded even when they would cause cycles.
To have a module execute code multiple times, export a function, and call that function.
Module Caching Caveats#
Modules are cached based on their resolved filename. Since modules may
resolve to a different filename based on the location of the calling
module (loading from node_modules
folders), it is not a guarantee
that require('foo')
will always return the exact same object, if it
would resolve to different files.
Additionally, on case-insensitive file systems or operating systems, different
resolved filenames can point to the same file, but the cache will still treat
them as different modules and will reload the file multiple times. For example,
require('./foo')
and require('./FOO')
return two different objects,
irrespective of whether or not ./foo
and ./FOO
are the same file.
Core Modules#
Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation.
The core modules are defined within Node.js's source and are located in the
lib/
folder.
Core modules are always preferentially loaded if their identifier is
passed to require()
. For instance, require('http')
will always
return the built in HTTP module, even if there is a file by that name.
Cycles#
When there are circular require()
calls, a module might not have finished
executing when it is returned.
Consider this situation:
a.js
:
console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');
b.js
:
console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');
main.js
:
console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done=%j, b.done=%j', a.done, b.done);
When main.js
loads a.js
, then a.js
in turn loads b.js
. At that
point, b.js
tries to load a.js
. In order to prevent an infinite
loop, an unfinished copy of the a.js
exports object is returned to the
b.js
module. b.js
then finishes loading, and its exports
object is
provided to the a.js
module.
By the time main.js
has loaded both modules, they're both finished.
The output of this program would thus be:
$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done=true, b.done=true
Careful planning is required to allow cyclic module dependencies to work correctly within an application.
File Modules#
If the exact filename is not found, then Node.js will attempt to load the
required filename with the added extensions: .js
, .json
, and finally
.node
.
.js
files are interpreted as JavaScript text files, and .json
files are
parsed as JSON text files. .node
files are interpreted as compiled addon
modules loaded with dlopen
.
A required module prefixed with '/'
is an absolute path to the file. For
example, require('/home/marco/foo.js')
will load the file at
/home/marco/foo.js
.
A required module prefixed with './'
is relative to the file calling
require()
. That is, circle.js
must be in the same directory as foo.js
for
require('./circle')
to find it.
Without a leading '/', './', or '../' to indicate a file, the module must
either be a core module or is loaded from a node_modules
folder.
If the given path does not exist, require()
will throw an Error
with its
code
property set to 'MODULE_NOT_FOUND'
.
Folders as Modules#
It is convenient to organize programs and libraries into self-contained
directories, and then provide a single entry point to that library.
There are three ways in which a folder may be passed to require()
as
an argument.
The first is to create a package.json
file in the root of the folder,
which specifies a main
module. An example package.json file might
look like this:
{ "name" : "some-library",
"main" : "./lib/some-library.js" }
If this was in a folder at ./some-library
, then
require('./some-library')
would attempt to load
./some-library/lib/some-library.js
.
This is the extent of Node.js's awareness of package.json files.
Note: If the file specified by the "main"
entry of package.json
is
missing and can not be resolved, Node.js will report the entire module as
missing with the default error:
Error: Cannot find module 'some-library'
If there is no package.json file present in the directory, then Node.js
will attempt to load an index.js
or index.node
file out of that
directory. For example, if there was no package.json file in the above
example, then require('./some-library')
would attempt to load:
./some-library/index.js
./some-library/index.node
Loading from node_modules
Folders#
If the module identifier passed to require()
is not a
core module, and does not begin with '/'
, '../'
, or
'./'
, then Node.js starts at the parent directory of the current module, and
adds /node_modules
, and attempts to load the module from that location. Node
will not append node_modules
to a path already ending in node_modules
.
If it is not found there, then it moves to the parent directory, and so on, until the root of the file system is reached.
For example, if the file at '/home/ry/projects/foo.js'
called
require('bar.js')
, then Node.js would look in the following locations, in
this order:
/home/ry/projects/node_modules/bar.js
/home/ry/node_modules/bar.js
/home/node_modules/bar.js
/node_modules/bar.js
This allows programs to localize their dependencies, so that they do not clash.
It is possible to require specific files or sub modules distributed with a
module by including a path suffix after the module name. For instance
require('example-module/path/to/file')
would resolve path/to/file
relative to where example-module
is located. The suffixed path follows the
same module resolution semantics.
Loading from the global folders#
If the NODE_PATH
environment variable is set to a colon-delimited list
of absolute paths, then Node.js will search those paths for modules if they
are not found elsewhere.
Note: On Windows, NODE_PATH
is delimited by semicolons instead of colons.
NODE_PATH
was originally created to support loading modules from
varying paths before the current module resolution algorithm was frozen.
NODE_PATH
is still supported, but is less necessary now that the Node.js
ecosystem has settled on a convention for locating dependent modules.
Sometimes deployments that rely on NODE_PATH
show surprising behavior
when people are unaware that NODE_PATH
must be set. Sometimes a
module's dependencies change, causing a different version (or even a
different module) to be loaded as the NODE_PATH
is searched.
Additionally, Node.js will search in the following locations:
- 1:
$HOME/.node_modules
- 2:
$HOME/.node_libraries
- 3:
$PREFIX/lib/node
Where $HOME
is the user's home directory, and $PREFIX
is Node.js's
configured node_prefix
.
These are mostly for historic reasons.
Note: It is strongly encouraged to place dependencies in the local
node_modules
folder. These will be loaded faster, and more reliably.
The module wrapper#
Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:
(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});
By doing this, Node.js achieves a few things:
- It keeps top-level variables (defined with
var
,const
orlet
) scoped to the module rather than the global object. - It helps to provide some global-looking variables that are actually specific
to the module, such as:
- The
module
andexports
objects that the implementor can use to export values from the module. - The convenience variables
__filename
and__dirname
, containing the module's absolute filename and directory path.
- The
The module scope#
__dirname#
The directory name of the current module. This the same as the
path.dirname()
of the __filename
.
Example: running node example.js
from /Users/mjr
console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr
__filename#
The file name of the current module. This is the resolved absolute path of the current module file.
For a main program this is not necessarily the same as the file name used in the command line.
See __dirname
for the directory name of the current module.
Examples:
Running node example.js
from /Users/mjr
console.log(__filename);
// Prints: /Users/mjr/example.js
console.log(__dirname);
// Prints: /Users/mjr
Given two modules: a
and b
, where b
is a dependency of
a
and there is a directory structure of:
/Users/mjr/app/a.js
/Users/mjr/app/node_modules/b/b.js
References to __filename
within b.js
will return
/Users/mjr/app/node_modules/b/b.js
while references to __filename
within
a.js
will return /Users/mjr/app/a.js
.
exports#
A reference to the module.exports
that is shorter to type.
See the section about the exports shortcut for details on when to use
exports
and when to use module.exports
.
module#
A reference to the current module, see the section about the
module
object. In particular, module.exports
is used for defining what
a module exports and makes available through require()
.
require()#
To require modules.
require.cache#
Modules are cached in this object when they are required. By deleting a key
value from this object, the next require
will reload the module. Note that
this does not apply to native addons, for which reloading will result in an
Error.
require.extensions#
Stability: 0 - Deprecated
Instruct require
on how to handle certain file extensions.
Process files with the extension .sjs
as .js
:
require.extensions['.sjs'] = require.extensions['.js'];
Deprecated In the past, this list has been used to load non-JavaScript modules into Node.js by compiling them on-demand. However, in practice, there are much better ways to do this, such as loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.
Since the module system is locked, this feature will probably never go away. However, it may have subtle bugs and complexities that are best left untouched.
Note that the number of file system operations that the module system
has to perform in order to resolve a require(...)
statement to a
filename scales linearly with the number of registered extensions.
In other words, adding extensions slows down the module loader and should be discouraged.
require.resolve()#
Use the internal require()
machinery to look up the location of a module,
but rather than loading the module, just return the resolved filename.
The module
Object#
In each module, the module
free variable is a reference to the object
representing the current module. For convenience, module.exports
is
also accessible via the exports
module-global. module
is not actually
a global but rather local to each module.
module.children#
The module objects required by this one.
module.exports#
The module.exports
object is created by the Module system. Sometimes this is
not acceptable; many want their module to be an instance of some class. To do
this, assign the desired export object to module.exports
. Note that assigning
the desired object to exports
will simply rebind the local exports
variable,
which is probably not what is desired.
For example suppose we were making a module called a.js
const EventEmitter = require('events');
module.exports = new EventEmitter();
// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(() => {
module.exports.emit('ready');
}, 1000);
Then in another file we could do
const a = require('./a');
a.on('ready', () => {
console.log('module a is ready');
});
Note that assignment to module.exports
must be done immediately. It cannot be
done in any callbacks. This does not work:
x.js:
setTimeout(() => {
module.exports = { a: 'hello' };
}, 0);
y.js:
const x = require('./x');
console.log(x.a);
exports shortcut#
The exports
variable is available within a module's file-level scope, and is
assigned the value of module.exports
before the module is evaluated.
It allows a shortcut, so that module.exports.f = ...
can be written more
succinctly as exports.f = ...
. However, be aware that like any variable, if a
new value is assigned to exports
, it is no longer bound to module.exports
:
module.exports.hello = true; // Exported from require of module
exports = { hello: false }; // Not exported, only available in the module
When the module.exports
property is being completely replaced by a new
object, it is common to also reassign exports
, for example:
module.exports = exports = function Constructor() {
// ... etc.
};
To illustrate the behavior, imagine this hypothetical implementation of
require()
, which is quite similar to what is actually done by require()
:
function require(/* ... */) {
const module = { exports: {} };
((module, exports) => {
// Module code here. In this example, define a function.
function someFunc() {}
exports = someFunc;
// At this point, exports is no longer a shortcut to module.exports, and
// this module will still export an empty default object.
module.exports = someFunc;
// At this point, the module will now export someFunc, instead of the
// default object.
})(module, module.exports);
return module.exports;
}
module.filename#
The fully resolved filename to the module.
module.id#
The identifier for the module. Typically this is the fully resolved filename.
module.loaded#
Whether or not the module is done loading, or is in the process of loading.
module.parent#
- <Object> Module object
The module that first required this one.
module.paths#
The search paths for the module.
module.require(id)#
The module.require
method provides a way to load a module as if
require()
was called from the original module.
Note: In order to do this, it is necessary to get a reference to the
module
object. Since require()
returns the module.exports
, and the
module
is typically only available within a specific module's code, it must
be explicitly exported in order to be used.
Net#
Stability: 2 - Stable
The net
module provides an asynchronous network API for creating stream-based
TCP or IPC servers (net.createServer()
) and clients
(net.createConnection()
).
It can be accessed using:
const net = require('net');
IPC Support#
The net
module supports IPC with named pipes on Windows, and UNIX domain
sockets on other operating systems.
Identifying paths for IPC connections#
net.connect()
, net.createConnection()
, server.listen()
and
socket.connect()
take a path
parameter to identify IPC endpoints.
On UNIX, the local domain is also known as the UNIX domain. The path is a
filesystem path name. It gets truncated to sizeof(sockaddr_un.sun_path) - 1
,
which varies on different operating system between 91 and 107 bytes.
The typical values are 107 on Linux and 103 on macOS. The path is
subject to the same naming conventions and permissions checks as would be done
on file creation. It will be visible in the filesystem, and will persist until
unlinked.
On Windows, the local domain is implemented using a named pipe. The path must
refer to an entry in \\?\pipe\
or \\.\pipe\
. Any characters are permitted,
but the latter may do some processing of pipe names, such as resolving ..
sequences. Despite appearances, the pipe name space is flat. Pipes will not
persist, they are removed when the last reference to them is closed. Do not
forget JavaScript string escaping requires paths to be specified with
double-backslashes, such as:
net.createServer().listen(
path.join('\\\\?\\pipe', process.cwd(), 'myctl'));
Class: net.Server#
This class is used to create a TCP or IPC server.
new net.Server([options][, connectionListener])#
- Returns: <net.Server>
See net.createServer([options][, connectionListener])
.
net.Server
is an EventEmitter
with the following events:
Event: 'close'#
Emitted when the server closes. Note that if connections exist, this event is not emitted until all connections are ended.
Event: 'connection'#
- <net.Socket> The connection object
Emitted when a new connection is made. socket
is an instance of
net.Socket
.
Event: 'error'#
Emitted when an error occurs. Unlike net.Socket
, the 'close'
event will not be emitted directly following this event unless
server.close()
is manually called. See the example in discussion of
server.listen()
.
Event: 'listening'#
Emitted when the server has been bound after calling server.listen()
.
server.address()#
Returns the bound address, the address family name, and port of the server
as reported by the operating system if listening on an IP socket.
Useful to find which port was assigned when getting an OS-assigned address.
Returns an object with port
, family
, and address
properties:
{ port: 12346, family: 'IPv4', address: '127.0.0.1' }
For a server listening on a pipe or UNIX domain socket, the name is returned as a string.
Example:
const server = net.createServer((socket) => {
socket.end('goodbye\n');
}).on('error', (err) => {
// handle errors here
throw err;
});
// grab an arbitrary unused port.
server.listen(() => {
console.log('opened server on', server.address());
});
Don't call server.address()
until the 'listening'
event has been emitted.
server.close([callback])#
- Returns: <net.Server>
Stops the server from accepting new connections and keeps existing
connections. This function is asynchronous, the server is finally
closed when all connections are ended and the server emits a 'close'
event.
The optional callback
will be called once the 'close'
event occurs. Unlike
that event, it will be called with an Error as its only argument if the server
was not open when it was closed.
Returns server
.
server.connections#
Stability: 0 - Deprecated: Use server.getConnections()
instead.
The number of concurrent connections on the server.
This becomes null
when sending a socket to a child with
child_process.fork()
. To poll forks and get current number of active
connections use asynchronous server.getConnections()
instead.
server.getConnections(callback)#
- Returns <net.Server>
Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.
Callback should take two arguments err
and count
.
server.listen()#
Start a server listening for connections. A net.Server
can be a TCP or
a IPC server depending on what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback])
for IPC serversserver.listen([port][, host][, backlog][, callback])
for TCP servers
This function is asynchronous. When the server starts listening, the
'listening'
event will be emitted. The last parameter callback
will be added as a listener for the 'listening'
event.
All listen()
methods can take a backlog
parameter to specify the maximum
length of the queue of pending connections. The actual length will be determined
by the OS through sysctl settings such as tcp_max_syn_backlog
and somaxconn
on Linux. The default value of this parameter is 511 (not 512).
Note:
All
net.Socket
are set toSO_REUSEADDR
(See socket(7) for details).The
server.listen()
method may be called multiple times. Each subsequent call will re-open the server using the provided options.
One of the most common errors raised when listening is EADDRINUSE
.
This happens when another server is already listening on the requested
port
/ path
/ handle
. One way to handle this would be to retry
after a certain amount of time:
server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.log('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});
server.listen(handle[, backlog][, callback])#
handle
<Object>backlog
<number> Common parameter ofserver.listen()
functionscallback
<Function> Common parameter ofserver.listen()
functions- Returns: <net.Server>
Start a server listening for connections on a given handle
that has
already been bound to a port, a UNIX domain socket, or a Windows named pipe.
The handle
object can be either a server, a socket (anything with an
underlying _handle
member), or an object with an fd
member that is a
valid file descriptor.
Note: Listening on a file descriptor is not supported on Windows.
server.listen(options[, callback])#
options
<Object> Required. Supports the following properties:port
<number>host
<string>path
<string> Will be ignored ifport
is specified. See Identifying paths for IPC connections.backlog
<number> Common parameter ofserver.listen()
functionsexclusive
<boolean> Default tofalse
callback
<Function> Common parameter ofserver.listen()
functions- Returns: <net.Server>
If port
is specified, it behaves the same as
server.listen([port][, hostname][, backlog][, callback])
.
Otherwise, if path
is specified, it behaves the same as
server.listen(path[, backlog][, callback])
.
If none of them is specified, an error will be thrown.
If exclusive
is false
(default), then cluster workers will use the same
underlying handle, allowing connection handling duties to be shared. When
exclusive
is true
, the handle is not shared, and attempted port sharing
results in an error. An example which listens on an exclusive port is
shown below.
server.listen({
host: 'localhost',
port: 80,
exclusive: true
});
server.listen(path[, backlog][, callback])#
path
<String> Path the server should listen to. See Identifying paths for IPC connections.backlog
<number> Common parameter ofserver.listen()
functionscallback
<Function> Common parameter ofserver.listen()
functions- Returns: <net.Server>
Start a IPC server listening for connections on the given path
.
server.listen([port][, host][, backlog][, callback])#
port
<number>host
<string>backlog
<number> Common parameter ofserver.listen()
functionscallback
<Function> Common parameter ofserver.listen()
functions- Returns: <net.Server>
Start a TCP server listening for connections on the given port
and host
.
If port
is omitted or is 0, the operating system will assign an arbitrary
unused port, which can be retrieved by using server.address().port
after the 'listening'
event has been emitted.
If host
is omitted, the server will accept connections on the
unspecified IPv6 address (::
) when IPv6 is available, or the
unspecified IPv4 address (0.0.0.0
) otherwise.
Note: In most operating systems, listening to the
unspecified IPv6 address (::
) may cause the net.Server
to also listen on
the unspecified IPv4 address (0.0.0.0
).
server.listening#
A Boolean indicating whether or not the server is listening for connections.
server.maxConnections#
Set this property to reject connections when the server's connection count gets high.
It is not recommended to use this option once a socket has been sent to a child
with child_process.fork()
.
server.ref()#
- Returns: <net.Server>
Opposite of unref
, calling ref
on a previously unref
d server will not
let the program exit if it's the only server left (the default behavior). If
the server is ref
d calling ref
again will have no effect.
server.unref()#
- Returns: <net.Server>
Calling unref
on a server will allow the program to exit if this is the only
active server in the event system. If the server is already unref
d calling
unref
again will have no effect.
Class: net.Socket#
This class is an abstraction of a TCP socket or a streaming IPC endpoint
(uses named pipes on Windows, and UNIX domain sockets otherwise). A
net.Socket
is also a duplex stream, so it can be both readable and
writable, and it is also a EventEmitter
.
A net.Socket
can be created by the user and used directly to interact with
a server. For example, it is returned by net.createConnection()
,
so the user can use it to talk to the server.
It can also be created by Node.js and passed to the user when a connection
is received. For example, it is passed to the listeners of a
'connection'
event emitted on a net.Server
, so the user can use
it to interact with the client.
new net.Socket(options)#
Creates a new socket object.
options
<Object> Available options are:fd
: <number> If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.allowHalfOpen
<boolean> Indicates whether half-opened TCP connections are allowed. Seenet.createServer()
and the'end'
event for details. Defaults tofalse
.readable
<boolean> Allow reads on the socket when anfd
is passed, otherwise ignored. Defaults tofalse
.writable
<boolean> Allow writes on the socket when anfd
is passed, otherwise ignored. Defaults tofalse
.
- Returns: <net.Socket>
The newly created socket can be either a TCP socket or a streaming IPC
endpoint, depending on what it connect()
to.
Event: 'close'#
had_error
<boolean>true
if the socket had a transmission error.
Emitted once the socket is fully closed. The argument had_error
is a boolean
which says if the socket was closed due to a transmission error.
Event: 'connect'#
Emitted when a socket connection is successfully established.
See net.createConnection()
.
Event: 'data'#
Emitted when data is received. The argument data
will be a Buffer
or
String
. Encoding of data is set by socket.setEncoding()
.
(See the Readable Stream section for more information.)
Note that the data will be lost if there is no listener when a Socket
emits a 'data'
event.
Event: 'drain'#
Emitted when the write buffer becomes empty. Can be used to throttle uploads.
See also: the return values of socket.write()
Event: 'end'#
Emitted when the other end of the socket sends a FIN packet, thus ending the readable side of the socket.
By default (allowHalfOpen
is false
) the socket will send a FIN packet
back and destroy its file descriptor once it has written out its pending
write queue. However, if allowHalfOpen
is set to true
, the socket will
not automatically end()
its writable side, allowing the
user to write arbitrary amounts of data. The user must call
end()
explicitly to close the connection (i.e. sending a
FIN packet back).
Event: 'error'#
Emitted when an error occurs. The 'close'
event will be called directly
following this event.
Event: 'lookup'#
Emitted after resolving the hostname but before connecting. Not applicable to UNIX sockets.
err
<Error> | <null> The error object. Seedns.lookup()
.address
<string> The IP address.family
<string> | <null> The address type. Seedns.lookup()
.host
<string> The hostname.
Event: 'timeout'#
Emitted if the socket times out from inactivity. This is only to notify that the socket has been idle. The user must manually close the connection.
See also: socket.setTimeout()
socket.address()#
Returns the bound address, the address family name and port of the
socket as reported by the operating system. Returns an object with
three properties, e.g.
{ port: 12346, family: 'IPv4', address: '127.0.0.1' }
socket.bufferSize#
net.Socket
has the property that socket.write()
always works. This is to
help users get up and running quickly. The computer cannot always keep up
with the amount of data that is written to a socket - the network connection
simply might be too slow. Node.js will internally queue up the data written to a
socket and send it out over the wire when it is possible. (Internally it is
polling on the socket's file descriptor for being writable).
The consequence of this internal buffering is that memory may grow. This property shows the number of characters currently buffered to be written. (Number of characters is approximately equal to the number of bytes to be written, but the buffer may contain strings, and the strings are lazily encoded, so the exact number of bytes is not known.)
Users who experience large or growing bufferSize
should attempt to
"throttle" the data flows in their program with
socket.pause()
and socket.resume()
.
socket.bytesRead#
The amount of received bytes.
socket.bytesWritten#
The amount of bytes sent.
socket.connect()#
Initiate a connection on a given socket.
Possible signatures:
- socket.connect(options[, connectListener])
- socket.connect(path[, connectListener]) for IPC connections.
- socket.connect(port[, host][, connectListener]) for TCP connections.
- Returns: <net.Socket> The socket itself.
This function is asynchronous. When the connection is established, the
'connect'
event will be emitted. If there is a problem connecting,
instead of a 'connect'
event, an 'error'
event will be emitted with
the error passed to the 'error'
listener.
The last parameter connectListener
, if supplied, will be added as a listener
for the 'connect'
event once.
socket.connect(options[, connectListener])#
options
<Object>connectListener
<Function> Common parameter ofsocket.connect()
methods. Will be added as a listener for the'connect'
event once.- Returns: <net.Socket> The socket itself.
Initiate a connection on a given socket. Normally this method is not needed,
the socket should be created and opened with net.createConnection()
. Use
this only when implementing a custom Socket.
For TCP connections, available options
are:
port
<number> Required. Port the socket should connect to.host
<string> Host the socket should connect to. Defaults to'localhost'
.localAddress
<string> Local address the socket should connect from.localPort
<number> Local port the socket should connect from.family
<number>: Version of IP stack, can be either 4 or 6. Defaults to 4.hints
<number> Optionaldns.lookup()
hints.lookup
<Function> Custom lookup function. Defaults todns.lookup()
.
For IPC connections, available options
are:
path
<string> Required. Path the client should connect to. See Identifying paths for IPC connections.
Returns socket
.
socket.connect(path[, connectListener])#
path
<string> Path the client should connect to. See Identifying paths for IPC connections.connectListener
<Function> Common parameter ofsocket.connect()
methods. Will be added as a listener for the'connect'
event once.- Returns: <net.Socket> The socket itself.
Initiate an IPC connection on the given socket.
Alias to
socket.connect(options[, connectListener])
called with { path: path }
as options
.
Returns socket
.
socket.connect(port[, host][, connectListener])#
port
<number> Port the client should connect to.host
<string> Host the client should connect to.connectListener
<Function> Common parameter ofsocket.connect()
methods. Will be added as a listener for the'connect'
event once.- Returns: <net.Socket> The socket itself.
Initiate a TCP connection on the given socket.
Alias to
socket.connect(options[, connectListener])
called with {port: port, host: host}
as options
.
Returns socket
.
socket.connecting#
If true
-
socket.connect(options[, connectListener])
was called and haven't yet finished. Will be set to false
before emitting
connect
event and/or calling
socket.connect(options[, connectListener])
's
callback.
socket.destroy([exception])#
- Returns: <net.Socket>
Ensures that no more I/O activity happens on this socket. Only necessary in case of errors (parse error or so).
If exception
is specified, an 'error'
event will be emitted and any
listeners for that event will receive exception
as an argument.
socket.destroyed#
A Boolean value that indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.
socket.end([data][, encoding])#
- Returns: <net.Socket> The socket itself.
Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.
If data
is specified, it is equivalent to calling
socket.write(data, encoding)
followed by socket.end()
.
socket.localAddress#
The string representation of the local IP address the remote client is
connecting on. For example, in a server listening on '0.0.0.0'
, if a client
connects on '192.168.1.1'
, the value of socket.localAddress
would be
'192.168.1.1'
.
socket.localPort#
The numeric representation of the local port. For example,
80
or 21
.
socket.pause()#
- Returns: <net.Socket> The socket itself.
Pauses the reading of data. That is, 'data'
events will not be emitted.
Useful to throttle back an upload.
socket.ref()#
- Returns: <net.Socket> The socket itself.
Opposite of unref
, calling ref
on a previously unref
d socket will not
let the program exit if it's the only socket left (the default behavior). If
the socket is ref
d calling ref
again will have no effect.
socket.remoteAddress#
The string representation of the remote IP address. For example,
'74.125.127.100'
or '2001:4860:a005::68'
. Value may be undefined
if
the socket is destroyed (for example, if the client disconnected).
socket.remoteFamily#
The string representation of the remote IP family. 'IPv4'
or 'IPv6'
.
socket.remotePort#
The numeric representation of the remote port. For example,
80
or 21
.
socket.resume()#
- Returns: <net.Socket> The socket itself.
Resumes reading after a call to socket.pause()
.
socket.setEncoding([encoding])#
- Returns: <net.Socket> The socket itself.
Set the encoding for the socket as a Readable Stream. See
stream.setEncoding()
for more information.
socket.setKeepAlive([enable][, initialDelay])#
- Returns: <net.Socket> The socket itself.
Enable/disable keep-alive functionality, and optionally set the initial
delay before the first keepalive probe is sent on an idle socket.
enable
defaults to false
.
Set initialDelay
(in milliseconds) to set the delay between the last
data packet received and the first keepalive probe. Setting 0 for
initialDelay will leave the value unchanged from the default
(or previous) setting. Defaults to 0
.
socket.setNoDelay([noDelay])#
- Returns: <net.Socket> The socket itself.
Disables the Nagle algorithm. By default TCP connections use the Nagle
algorithm, they buffer data before sending it off. Setting true
for
noDelay
will immediately fire off data each time socket.write()
is called.
noDelay
defaults to true
.
socket.setTimeout(timeout[, callback])#
- Returns: <net.Socket> The socket itself.
Sets the socket to timeout after timeout
milliseconds of inactivity on
the socket. By default net.Socket
do not have a timeout.
When an idle timeout is triggered the socket will receive a 'timeout'
event but the connection will not be severed. The user must manually call
socket.end()
or socket.destroy()
to end the connection.
socket.setTimeout(3000);
socket.on('timeout', () => {
console.log('socket timeout');
socket.end();
});
If timeout
is 0, then the existing idle timeout is disabled.
The optional callback
parameter will be added as a one time listener for the
'timeout'
event.
socket.unref()#
- Returns: <net.Socket> The socket itself.
Calling unref
on a socket will allow the program to exit if this is the only
active socket in the event system. If the socket is already unref
d calling
unref
again will have no effect.
socket.write(data[, encoding][, callback])#
Sends data on the socket. The second parameter specifies the encoding in the case of a string--it defaults to UTF8 encoding.
Returns true
if the entire data was flushed successfully to the kernel
buffer. Returns false
if all or part of the data was queued in user memory.
'drain'
will be emitted when the buffer is again free.
The optional callback
parameter will be executed when the data is finally
written out - this may not be immediately.
net.connect()#
Aliases to
net.createConnection()
.
Possible signatures:
net.connect(options[, connectListener])
net.connect(path[, connectListener])
for IPC connections.net.connect(port[, host][, connectListener])
for TCP connections.
net.connect(options[, connectListener])#
Alias to
net.createConnection(options[, connectListener])
.
net.connect(path[, connectListener])#
Alias to
net.createConnection(path[, connectListener])
.
net.connect(port[, host][, connectListener])#
Alias to
net.createConnection(port[, host][, connectListener])
.
net.createConnection()#
A factory function, which creates a new net.Socket
,
immediately initiates connection with socket.connect()
,
then returns the net.Socket
that starts the connection.
When the connection is established, a 'connect'
event will be emitted
on the returned socket. The last parameter connectListener
, if supplied,
will be added as a listener for the 'connect'
event once.
Possible signatures:
net.createConnection(options[, connectListener])
net.createConnection(path[, connectListener])
for IPC connections.net.createConnection(port[, host][, connectListener])
for TCP connections.
Note: The net.connect()
function is an alias to this function.
net.createConnection(options[, connectListener])#
options
<Object> Required. Will be passed to both thenew net.Socket([options])
call and thesocket.connect(options[, connectListener])
method.connectListener
<Function> Common parameter of thenet.createConnection()
functions. If supplied, will be added as a listener for the'connect'
event on the returned socket once.- Returns: <net.Socket> The newly created socket used to start the connection.
For available options, see
new net.Socket([options])
and socket.connect(options[, connectListener])
.
Additional options:
timeout
<number> If set, will be used to callsocket.setTimeout(timeout)
after the socket is created, but before it starts the connection.
Following is an example of a client of the echo server described
in the net.createServer()
section:
const net = require('net');
const client = net.createConnection({ port: 8124 }, () => {
//'connect' listener
console.log('connected to server!');
client.write('world!\r\n');
});
client.on('data', (data) => {
console.log(data.toString());
client.end();
});
client.on('end', () => {
console.log('disconnected from server');
});
To connect on the socket /tmp/echo.sock
the second line would just be
changed to
const client = net.createConnection({ path: '/tmp/echo.sock' });
net.createConnection(path[, connectListener])#
path
<string> Path the socket should connect to. Will be passed tosocket.connect(path[, connectListener])
. See Identifying paths for IPC connections.connectListener
<Function> Common parameter of thenet.createConnection()
functions, an "once" listener for the'connect'
event on the initiating socket. Will be passed tosocket.connect(path[, connectListener])
.- Returns: <net.Socket> The newly created socket used to start the connection.
Initiates an IPC connection.
This function creates a new net.Socket
with all options set to default,
immediately initiates connection with
socket.connect(path[, connectListener])
,
then returns the net.Socket
that starts the connection.
net.createConnection(port[, host][, connectListener])#
port
<number> Port the socket should connect to. Will be passed tosocket.connect(port[, host][, connectListener])
.host
<string> Host the socket should connect to. Defaults to'localhost'
. Will be passed tosocket.connect(port[, host][, connectListener])
.connectListener
<Function> Common parameter of thenet.createConnection()
functions, an "once" listener for the'connect'
event on the initiating socket. Will be passed tosocket.connect(path[, connectListener])
.- Returns: <net.Socket> The newly created socket used to start the connection.
Initiates a TCP connection.
This function creates a new net.Socket
with all options set to default,
immediately initiates connection with
socket.connect(port[, host][, connectListener])
,
then returns the net.Socket
that starts the connection.
net.createServer([options][, connectionListener])#
Creates a new TCP or IPC server.
options
<Object>connectionListener
<Function> Automatically set as a listener for the'connection'
event- Returns: <net.Server>
If allowHalfOpen
is set to true
, when the other end of the socket
sends a FIN packet, the server will only send a FIN packet back when
socket.end()
is explicitly called, until then the connection is
half-closed (non-readable but still writable). See 'end'
event
and RFC 1122 for more information.
If pauseOnConnect
is set to true
, then the socket associated with each
incoming connection will be paused, and no data will be read from its handle.
This allows connections to be passed between processes without any data being
read by the original process. To begin reading data from a paused socket, call
socket.resume()
.
The server can be a TCP server or a IPC server, depending on what it
listen()
to.
Here is an example of an TCP echo server which listens for connections on port 8124:
const net = require('net');
const server = net.createServer((c) => {
// 'connection' listener
console.log('client connected');
c.on('end', () => {
console.log('client disconnected');
});
c.write('hello\r\n');
c.pipe(c);
});
server.on('error', (err) => {
throw err;
});
server.listen(8124, () => {
console.log('server bound');
});
Test this by using telnet
:
$ telnet localhost 8124
To listen on the socket /tmp/echo.sock
the third line from the last would
just be changed to
server.listen('/tmp/echo.sock', () => {
console.log('server bound');
});
Use nc
to connect to a UNIX domain socket server:
$ nc -U /tmp/echo.sock
net.isIP(input)#
Tests if input is an IP address. Returns 0 for invalid strings, returns 4 for IP version 4 addresses, and returns 6 for IP version 6 addresses.
net.isIPv4(input)#
Returns true if input is a version 4 IP address, otherwise returns false.
net.isIPv6(input)#
Returns true if input is a version 6 IP address, otherwise returns false.
OS#
Stability: 2 - Stable
The os
module provides a number of operating system-related utility methods.
It can be accessed using:
const os = require('os');
os.EOL#
A string constant defining the operating system-specific end-of-line marker:
\n
on POSIX\r\n
on Windows
os.arch()#
- Returns: <string>
The os.arch()
method returns a string identifying the operating system CPU
architecture for which the Node.js binary was compiled.
The current possible values are: 'arm'
, 'arm64'
, 'ia32'
, 'mips'
,
'mipsel'
, 'ppc'
, 'ppc64'
, 's390'
, 's390x'
, 'x32'
, 'x64'
, and
'x86'
.
Equivalent to process.arch
.
os.constants#
Returns an object containing commonly used operating system specific constants for error codes, process signals, and so on. The specific constants currently defined are described in OS Constants.
os.cpus()#
- Returns: <Array>
The os.cpus()
method returns an array of objects containing information about
each CPU/core installed.
The properties included on each object include:
model
<string>speed
<number> (in MHz)times
<Object>user
<number> The number of milliseconds the CPU has spent in user mode.nice
<number> The number of milliseconds the CPU has spent in nice mode.sys
<number> The number of milliseconds the CPU has spent in sys mode.idle
<number> The number of milliseconds the CPU has spent in idle mode.irq
<number> The number of milliseconds the CPU has spent in irq mode.
For example:
[
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 252020,
nice: 0,
sys: 30340,
idle: 1070356870,
irq: 0
}
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 306960,
nice: 0,
sys: 26980,
idle: 1071569080,
irq: 0
}
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 248450,
nice: 0,
sys: 21750,
idle: 1070919370,
irq: 0
}
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 256880,
nice: 0,
sys: 19430,
idle: 1070905480,
irq: 20
}
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 511580,
nice: 20,
sys: 40900,
idle: 1070842510,
irq: 0
}
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 291660,
nice: 0,
sys: 34360,
idle: 1070888000,
irq: 10
}
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 308260,
nice: 0,
sys: 55410,
idle: 1071129970,
irq: 880
}
},
{
model: 'Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz',
speed: 2926,
times: {
user: 266450,
nice: 1480,
sys: 34920,
idle: 1072572010,
irq: 30
}
}
]
Note: Because nice
values are UNIX-specific, on Windows the nice
values
of all processors are always 0.
os.endianness()#
- Returns: <string>
The os.endianness()
method returns a string identifying the endianness of the
CPU for which the Node.js binary was compiled.
Possible values are:
'BE'
for big endian'LE'
for little endian.
os.freemem()#
- Returns: <integer>
The os.freemem()
method returns the amount of free system memory in bytes as
an integer.
os.homedir()#
- Returns: <string>
The os.homedir()
method returns the home directory of the current user as a
string.
os.hostname()#
- Returns: <string>
The os.hostname()
method returns the hostname of the operating system as a
string.
os.loadavg()#
- Returns: <Array>
The os.loadavg()
method returns an array containing the 1, 5, and 15 minute
load averages.
The load average is a measure of system activity, calculated by the operating system and expressed as a fractional number. As a rule of thumb, the load average should ideally be less than the number of logical CPUs in the system.
The load average is a UNIX-specific concept with no real equivalent on
Windows platforms. On Windows, the return value is always [0, 0, 0]
.
os.networkInterfaces()#
- Returns: <Object>
The os.networkInterfaces()
method returns an object containing only network
interfaces that have been assigned a network address.
Each key on the returned object identifies a network interface. The associated value is an array of objects that each describe an assigned network address.
The properties available on the assigned network address object include:
address
<string> The assigned IPv4 or IPv6 addressnetmask
<string> The IPv4 or IPv6 network maskfamily
<string> EitherIPv4
orIPv6
mac
<string> The MAC address of the network interfaceinternal
<boolean>true
if the network interface is a loopback or similar interface that is not remotely accessible; otherwisefalse
scopeid
<number> The numeric IPv6 scope ID (only specified whenfamily
isIPv6
)
{
lo: [
{
address: '127.0.0.1',
netmask: '255.0.0.0',
family: 'IPv4',
mac: '00:00:00:00:00:00',
internal: true
},
{
address: '::1',
netmask: 'ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff',
family: 'IPv6',
mac: '00:00:00:00:00:00',
internal: true
}
],
eth0: [
{
address: '192.168.1.108',
netmask: '255.255.255.0',
family: 'IPv4',
mac: '01:02:03:0a:0b:0c',
internal: false
},
{
address: 'fe80::a00:27ff:fe4e:66a1',
netmask: 'ffff:ffff:ffff:ffff::',
family: 'IPv6',
mac: '01:02:03:0a:0b:0c',
internal: false
}
]
}
os.platform()#
- Returns: <string>
The os.platform()
method returns a string identifying the operating system
platform as set during compile time of Node.js.
Currently possible values are:
'aix'
'darwin'
'freebsd'
'linux'
'openbsd'
'sunos'
'win32'
Equivalent to process.platform
.
Note: The value 'android'
may also be returned if the Node.js is built on
the Android operating system. However, Android support in Node.js is considered
to be experimental at this time.
os.release()#
- Returns: <string>
The os.release()
method returns a string identifying the operating system
release.
Note: On POSIX systems, the operating system release is determined by
calling uname(3). On Windows, GetVersionExW()
is used. Please see
https://en.wikipedia.org/wiki/Uname#Examples for more information.
os.tmpdir()#
- Returns: <string>
The os.tmpdir()
method returns a string specifying the operating system's
default directory for temporary files.
os.totalmem()#
- Returns: <integer>
The os.totalmem()
method returns the total amount of system memory in bytes
as an integer.
os.type()#
- Returns: <string>
The os.type()
method returns a string identifying the operating system name
as returned by uname(3). For example 'Linux'
on Linux, 'Darwin'
on macOS and
'Windows_NT'
on Windows.
Please see https://en.wikipedia.org/wiki/Uname#Examples for additional information about the output of running uname(3) on various operating systems.
os.uptime()#
- Returns: <integer>
The os.uptime()
method returns the system uptime in number of seconds.
Note: On Windows the returned value includes fractions of a second.
Use Math.floor()
to get whole seconds.
os.userInfo(options)#
options
<Object>encoding
<string> Character encoding used to interpret resulting strings. Ifencoding
is set to'buffer'
, theusername
,shell
, andhomedir
values will beBuffer
instances. (Default: 'utf8')
- Returns: <Object>
The os.userInfo()
method returns information about the currently effective
user -- on POSIX platforms, this is typically a subset of the password file. The
returned object includes the username
, uid
, gid
, shell
, and homedir
.
On Windows, the uid
and gid
fields are -1
, and shell
is null
.
The value of homedir
returned by os.userInfo()
is provided by the operating
system. This differs from the result of os.homedir()
, which queries several
environment variables for the home directory before falling back to the
operating system response.
OS Constants#
The following constants are exported by os.constants
.
Note: Not all constants will be available on every operating system.
Signal Constants#
The following signal constants are exported by os.constants.signals
:
Constant | Description |
---|---|
SIGHUP |
Sent to indicate when a controlling terminal is closed or a parent process exits. |
SIGINT |
Sent to indicate when a user wishes to interrupt a process
((Ctrl+C) ). |
SIGQUIT |
Sent to indicate when a user wishes to terminate a process and perform a core dump. |
SIGILL |
Sent to a process to notify that it has attempted to perform an illegal, malformed, unknown or privileged instruction. |
SIGTRAP |
Sent to a process when an exception has occurred. |
SIGABRT |
Sent to a process to request that it abort. |
SIGIOT |
Synonym for SIGABRT |
SIGBUS |
Sent to a process to notify that it has caused a bus error. |
SIGFPE |
Sent to a process to notify that it has performed an illegal arithmetic operation. |
SIGKILL |
Sent to a process to terminate it immediately. |
SIGUSR1 SIGUSR2 |
Sent to a process to identify user-defined conditions. |
SIGSEGV |
Sent to a process to notify of a segmentation fault. |
SIGPIPE |
Sent to a process when it has attempted to write to a disconnected pipe. |
SIGALRM |
Sent to a process when a system timer elapses. |
SIGTERM |
Sent to a process to request termination. |
SIGCHLD |
Sent to a process when a child process terminates. |
SIGSTKFLT |
Sent to a process to indicate a stack fault on a coprocessor. |
SIGCONT |
Sent to instruct the operating system to continue a paused process. |
SIGSTOP |
Sent to instruct the operating system to halt a process. |
SIGTSTP |
Sent to a process to request it to stop. |
SIGBREAK |
Sent to indicate when a user wishes to interrupt a process. |
SIGTTIN |
Sent to a process when it reads from the TTY while in the background. |
SIGTTOU |
Sent to a process when it writes to the TTY while in the background. |
SIGURG |
Sent to a process when a socket has urgent data to read. |
SIGXCPU |
Sent to a process when it has exceeded its limit on CPU usage. |
SIGXFSZ |
Sent to a process when it grows a file larger than the maximum allowed. |
SIGVTALRM |
Sent to a process when a virtual timer has elapsed. |
SIGPROF |
Sent to a process when a system timer has elapsed. |
SIGWINCH |
Sent to a process when the controlling terminal has changed its size. |
SIGIO |
Sent to a process when I/O is available. |
SIGPOLL |
Synonym for SIGIO |
SIGLOST |
Sent to a process when a file lock has been lost. |
SIGPWR |
Sent to a process to notify of a power failure. |
SIGINFO |
Synonym for SIGPWR |
SIGSYS |
Sent to a process to notify of a bad argument. |
SIGUNUSED |
Synonym for SIGSYS |
Error Constants#
The following error constants are exported by os.constants.errno
:
POSIX Error Constants#
Constant | Description |
---|---|
E2BIG |
Indicates that the list of arguments is longer than expected. |
EACCES |
Indicates that the operation did not have sufficient permissions. |
EADDRINUSE |
Indicates that the network address is already in use. |
EADDRNOTAVAIL |
Indicates that the network address is currently unavailable for use. |
EAFNOSUPPORT |
Indicates that the network address family is not supported. |
EAGAIN |
Indicates that there is currently no data available and to try the operation again later. |
EALREADY |
Indicates that the socket already has a pending connection in progress. |
EBADF |
Indicates that a file descriptor is not valid. |
EBADMSG |
Indicates an invalid data message. |
EBUSY |
Indicates that a device or resource is busy. |
ECANCELED |
Indicates that an operation was canceled. |
ECHILD |
Indicates that there are no child processes. |
ECONNABORTED |
Indicates that the network connection has been aborted. |
ECONNREFUSED |
Indicates that the network connection has been refused. |
ECONNRESET |
Indicates that the network connection has been reset. |
EDEADLK |
Indicates that a resource deadlock has been avoided. |
EDESTADDRREQ |
Indicates that a destination address is required. |
EDOM |
Indicates that an argument is out of the domain of the function. |
EDQUOT |
Indicates that the disk quota has been exceeded. |
EEXIST |
Indicates that the file already exists. |
EFAULT |
Indicates an invalid pointer address. |
EFBIG |
Indicates that the file is too large. |
EHOSTUNREACH |
Indicates that the host is unreachable. |
EIDRM |
Indicates that the identifier has been removed. |
EILSEQ |
Indicates an illegal byte sequence. |
EINPROGRESS |
Indicates that an operation is already in progress. |
EINTR |
Indicates that a function call was interrupted. |
EINVAL |
Indicates that an invalid argument was provided. |
EIO |
Indicates an otherwise unspecified I/O error. |
EISCONN |
Indicates that the socket is connected. |
EISDIR |
Indicates that the path is a directory. |
ELOOP |
Indicates too many levels of symbolic links in a path. |
EMFILE |
Indicates that there are too many open files. |
EMLINK |
Indicates that there are too many hard links to a file. |
EMSGSIZE |
Indicates that the provided message is too long. |
EMULTIHOP |
Indicates that a multihop was attempted. |
ENAMETOOLONG |
Indicates that the filename is too long. |
ENETDOWN |
Indicates that the network is down. |
ENETRESET |
Indicates that the connection has been aborted by the network. |
ENETUNREACH |
Indicates that the network is unreachable. |
ENFILE |
Indicates too many open files in the system. |
ENOBUFS |
Indicates that no buffer space is available. |
ENODATA |
Indicates that no message is available on the stream head read queue. |
ENODEV |
Indicates that there is no such device. |
ENOENT |
Indicates that there is no such file or directory. |
ENOEXEC |
Indicates an exec format error. |
ENOLCK |
Indicates that there are no locks available. |
ENOLINK |
Indications that a link has been severed. |
ENOMEM |
Indicates that there is not enough space. |
ENOMSG |
Indicates that there is no message of the desired type. |
ENOPROTOOPT |
Indicates that a given protocol is not available. |
ENOSPC |
Indicates that there is no space available on the device. |
ENOSR |
Indicates that there are no stream resources available. |
ENOSTR |
Indicates that a given resource is not a stream. |
ENOSYS |
Indicates that a function has not been implemented. |
ENOTCONN |
Indicates that the socket is not connected. |
ENOTDIR |
Indicates that the path is not a directory. |
ENOTEMPTY |
Indicates that the directory is not empty. |
ENOTSOCK |
Indicates that the given item is not a socket. |
ENOTSUP |
Indicates that a given operation is not supported. |
ENOTTY |
Indicates an inappropriate I/O control operation. |
ENXIO |
Indicates no such device or address. |
EOPNOTSUPP |
Indicates that an operation is not supported on the socket.
Note that while ENOTSUP and EOPNOTSUPP have the same value on Linux,
according to POSIX.1 these error values should be distinct.) |
EOVERFLOW |
Indicates that a value is too large to be stored in a given data type. |
EPERM |
Indicates that the operation is not permitted. |
EPIPE |
Indicates a broken pipe. |
EPROTO |
Indicates a protocol error. |
EPROTONOSUPPORT |
Indicates that a protocol is not supported. |
EPROTOTYPE |
Indicates the wrong type of protocol for a socket. |
ERANGE |
Indicates that the results are too large. |
EROFS |
Indicates that the file system is read only. |
ESPIPE |
Indicates an invalid seek operation. |
ESRCH |
Indicates that there is no such process. |
ESTALE |
Indicates that the file handle is stale. |
ETIME |
Indicates an expired timer. |
ETIMEDOUT |
Indicates that the connection timed out. |
ETXTBSY |
Indicates that a text file is busy. |
EWOULDBLOCK |
Indicates that the operation would block. |
EXDEV |
Indicates an improper link. |
Windows Specific Error Constants#
The following error codes are specific to the Windows operating system:
Constant | Description |
---|---|
WSAEINTR |
Indicates an interrupted function call. |
WSAEBADF |
Indicates an invalid file handle. |
WSAEACCES |
Indicates insufficient permissions to complete the operation. |
WSAEFAULT |
Indicates an invalid pointer address. |
WSAEINVAL |
Indicates that an invalid argument was passed. |
WSAEMFILE |
Indicates that there are too many open files. |
WSAEWOULDBLOCK |
Indicates that a resource is temporarily unavailable. |
WSAEINPROGRESS |
Indicates that an operation is currently in progress. |
WSAEALREADY |
Indicates that an operation is already in progress. |
WSAENOTSOCK |
Indicates that the resource is not a socket. |
WSAEDESTADDRREQ |
Indicates that a destination address is required. |
WSAEMSGSIZE |
Indicates that the message size is too long. |
WSAEPROTOTYPE |
Indicates the wrong protocol type for the socket. |
WSAENOPROTOOPT |
Indicates a bad protocol option. |
WSAEPROTONOSUPPORT |
Indicates that the protocol is not supported. |
WSAESOCKTNOSUPPORT |
Indicates that the socket type is not supported. |
WSAEOPNOTSUPP |
Indicates that the operation is not supported. |
WSAEPFNOSUPPORT |
Indicates that the protocol family is not supported. |
WSAEAFNOSUPPORT |
Indicates that the address family is not supported. |
WSAEADDRINUSE |
Indicates that the network address is already in use. |
WSAEADDRNOTAVAIL |
Indicates that the network address is not available. |
WSAENETDOWN |
Indicates that the network is down. |
WSAENETUNREACH |
Indicates that the network is unreachable. |
WSAENETRESET |
Indicates that the network connection has been reset. |
WSAECONNABORTED |
Indicates that the connection has been aborted. |
WSAECONNRESET |
Indicates that the connection has been reset by the peer. |
WSAENOBUFS |
Indicates that there is no buffer space available. |
WSAEISCONN |
Indicates that the socket is already connected. |
WSAENOTCONN |
Indicates that the socket is not connected. |
WSAESHUTDOWN |
Indicates that data cannot be sent after the socket has been shutdown. |
WSAETOOMANYREFS |
Indicates that there are too many references. |
WSAETIMEDOUT |
Indicates that the connection has timed out. |
WSAECONNREFUSED |
Indicates that the connection has been refused. |
WSAELOOP |
Indicates that a name cannot be translated. |
WSAENAMETOOLONG |
Indicates that a name was too long. |
WSAEHOSTDOWN |
Indicates that a network host is down. |
WSAEHOSTUNREACH |
Indicates that there is no route to a network host. |
WSAENOTEMPTY |
Indicates that the directory is not empty. |
WSAEPROCLIM |
Indicates that there are too many processes. |
WSAEUSERS |
Indicates that the user quota has been exceeded. |
WSAEDQUOT |
Indicates that the disk quota has been exceeded. |
WSAESTALE |
Indicates a stale file handle reference. |
WSAEREMOTE |
Indicates that the item is remote. |
WSASYSNOTREADY |
Indicates that the network subsystem is not ready. |
WSAVERNOTSUPPORTED |
Indicates that the winsock.dll version is out of range. |
WSANOTINITIALISED |
Indicates that successful WSAStartup has not yet been performed. |
WSAEDISCON |
Indicates that a graceful shutdown is in progress. |
WSAENOMORE |
Indicates that there are no more results. |
WSAECANCELLED |
Indicates that an operation has been canceled. |
WSAEINVALIDPROCTABLE |
Indicates that the procedure call table is invalid. |
WSAEINVALIDPROVIDER |
Indicates an invalid service provider. |
WSAEPROVIDERFAILEDINIT |
Indicates that the service provider failed to initialized. |
WSASYSCALLFAILURE |
Indicates a system call failure. |
WSASERVICE_NOT_FOUND |
Indicates that a service was not found. |
WSATYPE_NOT_FOUND |
Indicates that a class type was not found. |
WSA_E_NO_MORE |
Indicates that there are no more results. |
WSA_E_CANCELLED |
Indicates that the call was canceled. |
WSAEREFUSED |
Indicates that a database query was refused. |
libuv Constants#
Constant | Description |
---|---|
UV_UDP_REUSEADDR |
Path#
Stability: 2 - Stable
The path
module provides utilities for working with file and directory paths.
It can be accessed using:
const path = require('path');
Windows vs. POSIX#
The default operation of the path
module varies based on the operating system
on which a Node.js application is running. Specifically, when running on a
Windows operating system, the path
module will assume that Windows-style
paths are being used.
For example, using the path.basename()
function with the Windows file path
C:\temp\myfile.html
, will yield different results when running on POSIX than
when run on Windows:
On POSIX:
path.basename('C:\\temp\\myfile.html');
// Returns: 'C:\\temp\\myfile.html'
On Windows:
path.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'
To achieve consistent results when working with Windows file paths on any
operating system, use path.win32
:
On POSIX and Windows:
path.win32.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'
To achieve consistent results when working with POSIX file paths on any
operating system, use path.posix
:
On POSIX and Windows:
path.posix.basename('/tmp/myfile.html');
// Returns: 'myfile.html'
Note: On Windows Node.js follows the concept of per-drive working directory.
This behavior can be observed when using a drive path without a backslash. For
example path.resolve('c:\\')
can potentially return a different result than
path.resolve('c:')
. For more information, see
this MSDN page.
path.basename(path[, ext])#
The path.basename()
methods returns the last portion of a path
, similar to
the Unix basename
command. Trailing directory separators are ignored, see
path.sep
.
For example:
path.basename('/foo/bar/baz/asdf/quux.html');
// Returns: 'quux.html'
path.basename('/foo/bar/baz/asdf/quux.html', '.html');
// Returns: 'quux'
A TypeError
is thrown if path
is not a string or if ext
is given
and is not a string.
path.delimiter#
Provides the platform-specific path delimiter:
;
for Windows:
for POSIX
For example, on POSIX:
console.log(process.env.PATH);
// Prints: '/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin'
process.env.PATH.split(path.delimiter);
// Returns: ['/usr/bin', '/bin', '/usr/sbin', '/sbin', '/usr/local/bin']
On Windows:
console.log(process.env.PATH);
// Prints: 'C:\Windows\system32;C:\Windows;C:\Program Files\node\'
process.env.PATH.split(path.delimiter);
// Returns ['C:\\Windows\\system32', 'C:\\Windows', 'C:\\Program Files\\node\\']
path.dirname(path)#
The path.dirname()
method returns the directory name of a path
, similar to
the Unix dirname
command. Trailing directory separators are ignored, see
path.sep
.
For example:
path.dirname('/foo/bar/baz/asdf/quux');
// Returns: '/foo/bar/baz/asdf'
A TypeError
is thrown if path
is not a string.
path.extname(path)#
The path.extname()
method returns the extension of the path
, from the last
occurrence of the .
(period) character to end of string in the last portion of
the path
. If there is no .
in the last portion of the path
, or if the
first character of the basename of path
(see path.basename()
) is .
, then
an empty string is returned.
For example:
path.extname('index.html');
// Returns: '.html'
path.extname('index.coffee.md');
// Returns: '.md'
path.extname('index.');
// Returns: '.'
path.extname('index');
// Returns: ''
path.extname('.index');
// Returns: ''
A TypeError
is thrown if path
is not a string.
path.format(pathObject)#
The path.format()
method returns a path string from an object. This is the
opposite of path.parse()
.
When providing properties to the pathObject
remember that there are
combinations where one property has priority over another:
pathObject.root
is ignored ifpathObject.dir
is providedpathObject.ext
andpathObject.name
are ignored ifpathObject.base
exists
For example, on POSIX:
// If `dir`, `root` and `base` are provided,
// `${dir}${path.sep}${base}`
// will be returned. `root` is ignored.
path.format({
root: '/ignored',
dir: '/home/user/dir',
base: 'file.txt'
});
// Returns: '/home/user/dir/file.txt'
// `root` will be used if `dir` is not specified.
// If only `root` is provided or `dir` is equal to `root` then the
// platform separator will not be included. `ext` will be ignored.
path.format({
root: '/',
base: 'file.txt',
ext: 'ignored'
});
// Returns: '/file.txt'
// `name` + `ext` will be used if `base` is not specified.
path.format({
root: '/',
name: 'file',
ext: '.txt'
});
// Returns: '/file.txt'
On Windows:
path.format({
dir: 'C:\\path\\dir',
base: 'file.txt'
});
// Returns: 'C:\\path\\dir\\file.txt'
path.isAbsolute(path)#
The path.isAbsolute()
method determines if path
is an absolute path.
If the given path
is a zero-length string, false
will be returned.
For example on POSIX:
path.isAbsolute('/foo/bar'); // true
path.isAbsolute('/baz/..'); // true
path.isAbsolute('qux/'); // false
path.isAbsolute('.'); // false
On Windows:
path.isAbsolute('//server'); // true
path.isAbsolute('\\\\server'); // true
path.isAbsolute('C:/foo/..'); // true
path.isAbsolute('C:\\foo\\..'); // true
path.isAbsolute('bar\\baz'); // false
path.isAbsolute('bar/baz'); // false
path.isAbsolute('.'); // false
A TypeError
is thrown if path
is not a string.
path.join([...paths])#
The path.join()
method joins all given path
segments together using the
platform specific separator as a delimiter, then normalizes the resulting path.
Zero-length path
segments are ignored. If the joined path string is a
zero-length string then '.'
will be returned, representing the current
working directory.
For example:
path.join('/foo', 'bar', 'baz/asdf', 'quux', '..');
// Returns: '/foo/bar/baz/asdf'
path.join('foo', {}, 'bar');
// throws 'TypeError: Path must be a string. Received {}'
A TypeError
is thrown if any of the path segments is not a string.
path.normalize(path)#
The path.normalize()
method normalizes the given path
, resolving '..'
and
'.'
segments.
When multiple, sequential path segment separation characters are found (e.g.
/
on POSIX and either \
or /
on Windows), they are replaced by a single
instance of the platform specific path segment separator (/
on POSIX and
\
on Windows). Trailing separators are preserved.
If the path
is a zero-length string, '.'
is returned, representing the
current working directory.
For example on POSIX:
path.normalize('/foo/bar//baz/asdf/quux/..');
// Returns: '/foo/bar/baz/asdf'
On Windows:
path.normalize('C:\\temp\\\\foo\\bar\\..\\');
// Returns: 'C:\\temp\\foo\\'
Since Windows recognizes multiple path separators, both separators will be
replaced by instances of the Windows preferred separator (\
):
path.win32.normalize('C:////temp\\\\/\\/\\/foo/bar');
// Returns: 'C:\\temp\\foo\\bar'
A TypeError
is thrown if path
is not a string.
path.parse(path)#
The path.parse()
method returns an object whose properties represent
significant elements of the path
. Trailing directory separators are ignored,
see path.sep
.
The returned object will have the following properties:
For example on POSIX:
path.parse('/home/user/dir/file.txt');
// Returns:
// { root: '/',
// dir: '/home/user/dir',
// base: 'file.txt',
// ext: '.txt',
// name: 'file' }
┌─────────────────────┬────────────┐
│ dir │ base │
├──────┬ ├──────┬─────┤
│ root │ │ name │ ext │
" / home/user/dir / file .txt "
└──────┴──────────────┴──────┴─────┘
(all spaces in the "" line should be ignored -- they are purely for formatting)
On Windows:
path.parse('C:\\path\\dir\\file.txt');
// Returns:
// { root: 'C:\\',
// dir: 'C:\\path\\dir',
// base: 'file.txt',
// ext: '.txt',
// name: 'file' }
┌─────────────────────┬────────────┐
│ dir │ base │
├──────┬ ├──────┬─────┤
│ root │ │ name │ ext │
" C:\ path\dir \ file .txt "
└──────┴──────────────┴──────┴─────┘
(all spaces in the "" line should be ignored -- they are purely for formatting)
A TypeError
is thrown if path
is not a string.
path.posix#
The path.posix
property provides access to POSIX specific implementations
of the path
methods.
path.relative(from, to)#
The path.relative()
method returns the relative path from from
to to
based
on the current working directory. If from
and to
each resolve to the same
path (after calling path.resolve()
on each), a zero-length string is returned.
If a zero-length string is passed as from
or to
, the current working
directory will be used instead of the zero-length strings.
For example on POSIX:
path.relative('/data/orandea/test/aaa', '/data/orandea/impl/bbb');
// Returns: '../../impl/bbb'
On Windows:
path.relative('C:\\orandea\\test\\aaa', 'C:\\orandea\\impl\\bbb');
// Returns: '..\\..\\impl\\bbb'
A TypeError
is thrown if either from
or to
is not a string.
path.resolve([...paths])#
The path.resolve()
method resolves a sequence of paths or path segments into
an absolute path.
The given sequence of paths is processed from right to left, with each
subsequent path
prepended until an absolute path is constructed.
For instance, given the sequence of path segments: /foo
, /bar
, baz
,
calling path.resolve('/foo', '/bar', 'baz')
would return /bar/baz
.
If after processing all given path
segments an absolute path has not yet
been generated, the current working directory is used.
The resulting path is normalized and trailing slashes are removed unless the path is resolved to the root directory.
Zero-length path
segments are ignored.
If no path
segments are passed, path.resolve()
will return the absolute path
of the current working directory.
For example:
path.resolve('/foo/bar', './baz');
// Returns: '/foo/bar/baz'
path.resolve('/foo/bar', '/tmp/file/');
// Returns: '/tmp/file'
path.resolve('wwwroot', 'static_files/png/', '../gif/image.gif');
// if the current working directory is /home/myself/node,
// this returns '/home/myself/node/wwwroot/static_files/gif/image.gif'
A TypeError
is thrown if any of the arguments is not a string.
path.sep#
Provides the platform-specific path segment separator:
\
on Windows/
on POSIX
For example on POSIX:
'foo/bar/baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']
On Windows:
'foo\\bar\\baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']
Note: On Windows, both the forward slash (/
) and backward slash (\
) are
accepted as path segment separators; however, the path
methods only add
backward slashes (\
).
path.win32#
The path.win32
property provides access to Windows-specific implementations
of the path
methods.
Process#
The process
object is a global
that provides information about, and control
over, the current Node.js process. As a global, it is always available to
Node.js applications without using require()
.
Process Events#
The process
object is an instance of EventEmitter
.
Event: 'beforeExit'#
The 'beforeExit'
event is emitted when Node.js empties its event loop and has
no additional work to schedule. Normally, the Node.js process will exit when
there is no work scheduled, but a listener registered on the 'beforeExit'
event can make asynchronous calls, and thereby cause the Node.js process to
continue.
The listener callback function is invoked with the value of
process.exitCode
passed as the only argument.
The 'beforeExit'
event is not emitted for conditions causing explicit
termination, such as calling process.exit()
or uncaught exceptions.
The 'beforeExit'
should not be used as an alternative to the 'exit'
event
unless the intention is to schedule additional work.
Event: 'disconnect'#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'disconnect'
event will be emitted when
the IPC channel is closed.
Event: 'exit'#
The 'exit'
event is emitted when the Node.js process is about to exit as a
result of either:
- The
process.exit()
method being called explicitly; - The Node.js event loop no longer having any additional work to perform.
There is no way to prevent the exiting of the event loop at this point, and once
all 'exit'
listeners have finished running the Node.js process will terminate.
The listener callback function is invoked with the exit code specified either
by the process.exitCode
property, or the exitCode
argument passed to the
process.exit()
method, as the only argument.
For example:
process.on('exit', (code) => {
console.log(`About to exit with code: ${code}`);
});
Listener functions must only perform synchronous operations. The Node.js
process will exit immediately after calling the 'exit'
event listeners
causing any additional work still queued in the event loop to be abandoned.
In the following example, for instance, the timeout will never occur:
process.on('exit', (code) => {
setTimeout(() => {
console.log('This will not run');
}, 0);
});
Event: 'message'#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'message'
event is emitted whenever a
message sent by a parent process using childprocess.send()
is received by
the child process.
The listener callback is invoked with the following arguments:
message
<Object> a parsed JSON object or primitive valuesendHandle
<Handle object> anet.Socket
ornet.Server
object, or undefined.
Event: 'rejectionHandled'#
The 'rejectionHandled'
event is emitted whenever a Promise
has been rejected
and an error handler was attached to it (using promise.catch()
, for
example) later than one turn of the Node.js event loop.
The listener callback is invoked with a reference to the rejected Promise
as
the only argument.
The Promise
object would have previously been emitted in an
'unhandledRejection'
event, but during the course of processing gained a
rejection handler.
There is no notion of a top level for a Promise
chain at which rejections can
always be handled. Being inherently asynchronous in nature, a Promise
rejection can be handled at a future point in time — possibly much later than
the event loop turn it takes for the 'unhandledRejection'
event to be emitted.
Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with Promises there can be a growing-and-shrinking list of unhandled rejections.
In synchronous code, the 'uncaughtException'
event is emitted when the list of
unhandled exceptions grows.
In asynchronous code, the 'unhandledRejection'
event is emitted when the list
of unhandled rejections grows, and the 'rejectionHandled'
event is emitted
when the list of unhandled rejections shrinks.
For example:
const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, p) => {
unhandledRejections.set(p, reason);
});
process.on('rejectionHandled', (p) => {
unhandledRejections.delete(p);
});
In this example, the unhandledRejections
Map
will grow and shrink over time,
reflecting rejections that start unhandled and then become handled. It is
possible to record such errors in an error log, either periodically (which is
likely best for long-running application) or upon process exit (which is likely
most convenient for scripts).
Event: 'uncaughtException'#
The 'uncaughtException'
event is emitted when an uncaught JavaScript
exception bubbles all the way back to the event loop. By default, Node.js
handles such exceptions by printing the stack trace to stderr
and exiting.
Adding a handler for the 'uncaughtException'
event overrides this default
behavior.
The listener function is called with the Error
object passed as the only
argument.
For example:
process.on('uncaughtException', (err) => {
fs.writeSync(1, `Caught exception: ${err}\n`);
});
setTimeout(() => {
console.log('This will still run.');
}, 500);
// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');
Warning: Using 'uncaughtException'
correctly#
Note that 'uncaughtException'
is a crude mechanism for exception handling
intended to be used only as a last resort. The event should not be used as
an equivalent to On Error Resume Next
. Unhandled exceptions inherently mean
that an application is in an undefined state. Attempting to resume application
code without properly recovering from the exception can cause additional
unforeseen and unpredictable issues.
Exceptions thrown from within the event handler will not be caught. Instead the process will exit with a non-zero exit code and the stack trace will be printed. This is to avoid infinite recursion.
Attempting to resume normally after an uncaught exception can be similar to pulling out of the power cord when upgrading a computer -- nine out of ten times nothing happens - but the 10th time, the system becomes corrupted.
The correct use of 'uncaughtException'
is to perform synchronous cleanup
of allocated resources (e.g. file descriptors, handles, etc) before shutting
down the process. It is not safe to resume normal operation after
'uncaughtException'
.
To restart a crashed application in a more reliable way, whether uncaughtException
is emitted or not, an external monitor should be employed in a separate process
to detect application failures and recover or restart as needed.
Event: 'unhandledRejection'#
The 'unhandledRejection
' event is emitted whenever a Promise
is rejected and
no error handler is attached to the promise within a turn of the event loop.
When programming with Promises, exceptions are encapsulated as "rejected
promises". Rejections can be caught and handled using promise.catch()
and
are propagated through a Promise
chain. The 'unhandledRejection'
event is
useful for detecting and keeping track of promises that were rejected whose
rejections have not yet been handled.
The listener function is called with the following arguments:
reason
<Error> | <any> The object with which the promise was rejected (typically anError
object).p
thePromise
that was rejected.
For example:
process.on('unhandledRejection', (reason, p) => {
console.log('Unhandled Rejection at:', p, 'reason:', reason);
// application specific logging, throwing an error, or other logic here
});
somePromise.then((res) => {
return reportToUser(JSON.pasre(res)); // note the typo (`pasre`)
}); // no `.catch` or `.then`
The following will also trigger the 'unhandledRejection'
event to be
emitted:
function SomeResource() {
// Initially set the loaded status to a rejected promise
this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}
const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn
In this example case, it is possible to track the rejection as a developer error
as would typically be the case for other 'unhandledRejection'
events. To
address such failures, a non-operational
.catch(() => { })
handler may be attached to
resource.loaded
, which would prevent the 'unhandledRejection'
event from
being emitted. Alternatively, the 'rejectionHandled'
event may be used.
Event: 'warning'#
The 'warning'
event is emitted whenever Node.js emits a process warning.
A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs or security vulnerabilities.
The listener function is called with a single warning
argument whose value is
an Error
object. There are three key properties that describe the warning:
name
<string> The name of the warning (currentlyWarning
by default).message
<string> A system-provided description of the warning.stack
<string> A stack trace to the location in the code where the warning was issued.
process.on('warning', (warning) => {
console.warn(warning.name); // Print the warning name
console.warn(warning.message); // Print the warning message
console.warn(warning.stack); // Print the stack trace
});
By default, Node.js will print process warnings to stderr
. The --no-warnings
command-line option can be used to suppress the default console output but the
'warning'
event will still be emitted by the process
object.
The following example illustrates the warning that is printed to stderr
when
too many listeners have been added to an event
$ node
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) MaxListenersExceededWarning: Possible EventEmitter memory leak
detected. 2 foo listeners added. Use emitter.setMaxListeners() to increase limit
In contrast, the following example turns off the default warning output and
adds a custom handler to the 'warning'
event:
$ node --no-warnings
> const p = process.on('warning', (warning) => console.warn('Do not do that!'));
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!
The --trace-warnings
command-line option can be used to have the default
console output for warnings include the full stack trace of the warning.
Launching Node.js using the --throw-deprecation
command line flag will
cause custom deprecation warnings to be thrown as exceptions.
Using the --trace-deprecation
command line flag will cause the custom
deprecation to be printed to stderr
along with the stack trace.
Using the --no-deprecation
command line flag will suppress all reporting
of the custom deprecation.
The *-deprecation
command line flags only affect warnings that use the name
DeprecationWarning
.
Emitting custom warnings#
See the process.emitWarning()
method for issuing
custom or application-specific warnings.
Signal Events#
Signal events will be emitted when the Node.js process receives a signal. Please
refer to signal(7) for a listing of standard POSIX signal names such as
SIGINT
, SIGHUP
, etc.
The name of each event will be the uppercase common name for the signal (e.g.
'SIGINT'
for SIGINT
signals).
For example:
// Begin reading from stdin so the process does not exit.
process.stdin.resume();
process.on('SIGINT', () => {
console.log('Received SIGINT. Press Control-D to exit.');
});
Note: An easy way to send the SIGINT
signal is with <Ctrl>-C
in most
terminal programs.
It is important to take note of the following:
SIGUSR1
is reserved by Node.js to start the debugger. It's possible to install a listener but doing so will not stop the debugger from starting.SIGTERM
andSIGINT
have default handlers on non-Windows platforms that resets the terminal mode before exiting with code128 + signal number
. If one of these signals has a listener installed, its default behavior will be removed (Node.js will no longer exit).SIGPIPE
is ignored by default. It can have a listener installed.SIGHUP
is generated on Windows when the console window is closed, and on other platforms under various similar conditions, see signal(7). It can have a listener installed, however Node.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior ofSIGHUP
is to terminate Node.js, but once a listener has been installed its default behavior will be removed.SIGTERM
is not supported on Windows, it can be listened on.SIGINT
from the terminal is supported on all platforms, and can usually be generated withCTRL+C
(though this may be configurable). It is not generated when terminal raw mode is enabled.SIGBREAK
is delivered on Windows when<Ctrl>+<Break>
is pressed, on non-Windows platforms it can be listened on, but there is no way to send or generate it.SIGWINCH
is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.SIGKILL
cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.SIGSTOP
cannot have a listener installed.SIGBUS
,SIGFPE
,SIGSEGV
andSIGILL
, when not raised artificially using kill(2), inherently leave the process in a state from which it is not safe to attempt to call JS listeners. Doing so might lead to the process hanging in an endless loop, since listeners attached usingprocess.on()
are called asynchronously and therefore unable to correct the underlying problem.
Note: Windows does not support sending signals, but Node.js offers some
emulation with process.kill()
, and ChildProcess.kill()
. Sending
signal 0
can be used to test for the existence of a process. Sending SIGINT
,
SIGTERM
, and SIGKILL
cause the unconditional termination of the target
process.
process.abort()#
The process.abort()
method causes the Node.js process to exit immediately and
generate a core file.
process.arch#
The process.arch
property returns a String identifying the processor
architecture that the Node.js process is currently running on. For instance
'arm'
, 'ia32'
, or 'x64'
.
console.log(`This processor architecture is ${process.arch}`);
process.argv#
The process.argv
property returns an array containing the command line
arguments passed when the Node.js process was launched. The first element will
be process.execPath
. See process.argv0
if access to the original value of
argv[0]
is needed. The second element will be the path to the JavaScript
file being executed. The remaining elements will be any additional command line
arguments.
For example, assuming the following script for process-args.js
:
// print process.argv
process.argv.forEach((val, index) => {
console.log(`${index}: ${val}`);
});
Launching the Node.js process as:
$ node process-args.js one two=three four
Would generate the output:
0: /usr/local/bin/node
1: /Users/mjr/work/node/process-args.js
2: one
3: two=three
4: four
process.argv0#
The process.argv0
property stores a read-only copy of the original value of
argv[0]
passed when Node.js starts.
$ bash -c 'exec -a customArgv0 ./node'
> process.argv[0]
'/Volumes/code/external/node/out/Release/node'
> process.argv0
'customArgv0'
process.channel#
If the Node.js process was spawned with an IPC channel (see the
Child Process documentation), the process.channel
property is a reference to the IPC channel. If no IPC channel exists, this
property is undefined
.
process.chdir(directory)#
directory
<string>
The process.chdir()
method changes the current working directory of the
Node.js process or throws an exception if doing so fails (for instance, if
the specified directory
does not exist).
console.log(`Starting directory: ${process.cwd()}`);
try {
process.chdir('/tmp');
console.log(`New directory: ${process.cwd()}`);
} catch (err) {
console.error(`chdir: ${err}`);
}
process.config#
The process.config
property returns an Object containing the JavaScript
representation of the configure options used to compile the current Node.js
executable. This is the same as the config.gypi
file that was produced when
running the ./configure
script.
An example of the possible output looks like:
{
target_defaults:
{ cflags: [],
default_configuration: 'Release',
defines: [],
include_dirs: [],
libraries: [] },
variables:
{
host_arch: 'x64',
node_install_npm: 'true',
node_prefix: '',
node_shared_cares: 'false',
node_shared_http_parser: 'false',
node_shared_libuv: 'false',
node_shared_zlib: 'false',
node_use_dtrace: 'false',
node_use_openssl: 'true',
node_shared_openssl: 'false',
strict_aliasing: 'true',
target_arch: 'x64',
v8_use_snapshot: 'true'
}
}
Note: The process.config
property is not read-only and there are
existing modules in the ecosystem that are known to extend, modify, or entirely
replace the value of process.config
.
process.connected#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.connected
property will return
true
so long as the IPC channel is connected and will return false
after
process.disconnect()
is called.
Once process.connected
is false
, it is no longer possible to send messages
over the IPC channel using process.send()
.
process.cpuUsage([previousValue])#
The process.cpuUsage()
method returns the user and system CPU time usage of
the current process, in an object with properties user
and system
, whose
values are microsecond values (millionth of a second). These values measure time
spent in user and system code respectively, and may end up being greater than
actual elapsed time if multiple CPU cores are performing work for this process.
The result of a previous call to process.cpuUsage()
can be passed as the
argument to the function, to get a diff reading.
const startUsage = process.cpuUsage();
// { user: 38579, system: 6986 }
// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);
console.log(process.cpuUsage(startUsage));
// { user: 514883, system: 11226 }
process.cwd()#
- Returns: <string>
The process.cwd()
method returns the current working directory of the Node.js
process.
console.log(`Current directory: ${process.cwd()}`);
process.disconnect()#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.disconnect()
method will close the
IPC channel to the parent process, allowing the child process to exit gracefully
once there are no other connections keeping it alive.
The effect of calling process.disconnect()
is that same as calling the parent
process's ChildProcess.disconnect()
.
If the Node.js process was not spawned with an IPC channel,
process.disconnect()
will be undefined
.
process.emitWarning(warning[, options])#
warning
<string> | <Error> The warning to emit.options
<Object>type
<string> Whenwarning
is a String,type
is the name to use for the type of warning being emitted. Default:Warning
.code
<string> A unique identifier for the warning instance being emitted.ctor
<Function> Whenwarning
is a String,ctor
is an optional function used to limit the generated stack trace. Defaultprocess.emitWarning
detail
<string> Additional text to include with the error.
The process.emitWarning()
method can be used to emit custom or application
specific process warnings. These can be listened for by adding a handler to the
process.on('warning')
event.
// Emit a warning with a code and additional detail.
process.emitWarning('Something happened!', {
code: 'MY_WARNING',
detail: 'This is some additional information'
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional information
In this example, an Error
object is generated internally by
process.emitWarning()
and passed through to the
process.on('warning')
event.
process.on('warning', (warning) => {
console.warn(warning.name); // 'Warning'
console.warn(warning.message); // 'Something happened!'
console.warn(warning.code); // 'MY_WARNING'
console.warn(warning.stack); // Stack trace
console.warn(warning.detail); // 'This is some additional information'
});
If warning
is passed as an Error
object, the options
argument is ignored.
process.emitWarning(warning[, type[, code]][, ctor])#
warning
<string> | <Error> The warning to emit.type
<string> Whenwarning
is a String,type
is the name to use for the type of warning being emitted. Default:Warning
.code
<string> A unique identifier for the warning instance being emitted.ctor
<Function> Whenwarning
is a String,ctor
is an optional function used to limit the generated stack trace. Defaultprocess.emitWarning
The process.emitWarning()
method can be used to emit custom or application
specific process warnings. These can be listened for by adding a handler to the
process.on('warning')
event.
// Emit a warning using a string.
process.emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!
// Emit a warning using a string and a type.
process.emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!
process.emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!
In each of the previous examples, an Error
object is generated internally by
process.emitWarning()
and passed through to the
process.on('warning')
event.
process.on('warning', (warning) => {
console.warn(warning.name);
console.warn(warning.message);
console.warn(warning.code);
console.warn(warning.stack);
});
If warning
is passed as an Error
object, it will be passed through to the
process.on('warning')
event handler unmodified (and the optional type
,
code
and ctor
arguments will be ignored):
// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';
process.emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!
A TypeError
is thrown if warning
is anything other than a string or Error
object.
Note that while process warnings use Error
objects, the process warning
mechanism is not a replacement for normal error handling mechanisms.
The following additional handling is implemented if the warning type
is
DeprecationWarning
:
- If the
--throw-deprecation
command-line flag is used, the deprecation warning is thrown as an exception rather than being emitted as an event. - If the
--no-deprecation
command-line flag is used, the deprecation warning is suppressed. - If the
--trace-deprecation
command-line flag is used, the deprecation warning is printed tostderr
along with the full stack trace.
Avoiding duplicate warnings#
As a best practice, warnings should be emitted only once per process. To do
so, it is recommended to place the emitWarning()
behind a simple boolean
flag as illustrated in the example below:
function emitMyWarning() {
if (!emitMyWarning.warned) {
emitMyWarning.warned = true;
process.emitWarning('Only warn once!');
}
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing
process.env#
The process.env
property returns an object containing the user environment.
See environ(7).
An example of this object looks like:
{
TERM: 'xterm-256color',
SHELL: '/usr/local/bin/bash',
USER: 'maciej',
PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
PWD: '/Users/maciej',
EDITOR: 'vim',
SHLVL: '1',
HOME: '/Users/maciej',
LOGNAME: 'maciej',
_: '/usr/local/bin/node'
}
It is possible to modify this object, but such modifications will not be reflected outside the Node.js process. In other words, the following example would not work:
$ node -e 'process.env.foo = "bar"' && echo $foo
While the following will:
process.env.foo = 'bar';
console.log(process.env.foo);
Assigning a property on process.env
will implicitly convert the value
to a string.
Example:
process.env.test = null;
console.log(process.env.test);
// => 'null'
process.env.test = undefined;
console.log(process.env.test);
// => 'undefined'
Use delete
to delete a property from process.env
.
Example:
process.env.TEST = 1;
delete process.env.TEST;
console.log(process.env.TEST);
// => undefined
On Windows operating systems, environment variables are case-insensitive.
Example:
process.env.TEST = 1;
console.log(process.env.test);
// => 1
process.execArgv#
The process.execArgv
property returns the set of Node.js-specific command-line
options passed when the Node.js process was launched. These options do not
appear in the array returned by the process.argv
property, and do not
include the Node.js executable, the name of the script, or any options following
the script name. These options are useful in order to spawn child processes with
the same execution environment as the parent.
For example:
$ node --harmony script.js --version
Results in process.execArgv
:
['--harmony']
And process.argv
:
['/usr/local/bin/node', 'script.js', '--version']
process.execPath#
The process.execPath
property returns the absolute pathname of the executable
that started the Node.js process.
For example:
'/usr/local/bin/node'
process.exit([code])#
code
<integer> The exit code. Defaults to0
.
The process.exit()
method instructs Node.js to terminate the process
synchronously with an exit status of code
. If code
is omitted, exit uses
either the 'success' code 0
or the value of process.exitCode
if it has been
set. Node.js will not terminate until all the 'exit'
event listeners are
called.
To exit with a 'failure' code:
process.exit(1);
The shell that executed Node.js should see the exit code as 1
.
It is important to note that calling process.exit()
will force the process to
exit as quickly as possible even if there are still asynchronous operations
pending that have not yet completed fully, including I/O operations to
process.stdout
and process.stderr
.
In most situations, it is not actually necessary to call process.exit()
explicitly. The Node.js process will exit on its own if there is no additional
work pending in the event loop. The process.exitCode
property can be set to
tell the process which exit code to use when the process exits gracefully.
For instance, the following example illustrates a misuse of the
process.exit()
method that could lead to data printed to stdout being
truncated and lost:
// This is an example of what *not* to do:
if (someConditionNotMet()) {
printUsageToStdout();
process.exit(1);
}
The reason this is problematic is because writes to process.stdout
in Node.js
are sometimes asynchronous and may occur over multiple ticks of the Node.js
event loop. Calling process.exit()
, however, forces the process to exit
before those additional writes to stdout
can be performed.
Rather than calling process.exit()
directly, the code should set the
process.exitCode
and allow the process to exit naturally by avoiding
scheduling any additional work for the event loop:
// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
printUsageToStdout();
process.exitCode = 1;
}
If it is necessary to terminate the Node.js process due to an error condition,
throwing an uncaught error and allowing the process to terminate accordingly
is safer than calling process.exit()
.
process.exitCode#
A number which will be the process exit code, when the process either
exits gracefully, or is exited via process.exit()
without specifying
a code.
Specifying a code to process.exit(code)
will override any
previous setting of process.exitCode
.
process.getegid()#
The process.getegid()
method returns the numerical effective group identity
of the Node.js process. (See getegid(2).)
if (process.getegid) {
console.log(`Current gid: ${process.getegid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.geteuid()#
- Returns: <Object>
The process.geteuid()
method returns the numerical effective user identity of
the process. (See geteuid(2).)
if (process.geteuid) {
console.log(`Current uid: ${process.geteuid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.getgid()#
- Returns: <Object>
The process.getgid()
method returns the numerical group identity of the
process. (See getgid(2).)
if (process.getgid) {
console.log(`Current gid: ${process.getgid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.getgroups()#
- Returns: <Array>
The process.getgroups()
method returns an array with the supplementary group
IDs. POSIX leaves it unspecified if the effective group ID is included but
Node.js ensures it always is.
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.getuid()#
- Returns: <integer>
The process.getuid()
method returns the numeric user identity of the process.
(See getuid(2).)
if (process.getuid) {
console.log(`Current uid: ${process.getuid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.hrtime([time])#
The process.hrtime()
method returns the current high-resolution real time
in a [seconds, nanoseconds]
tuple Array, where nanoseconds
is the
remaining part of the real time that can't be represented in second precision.
time
is an optional parameter that must be the result of a previous
process.hrtime()
call to diff with the current time. If the parameter
passed in is not a tuple Array, a TypeError
will be thrown. Passing in a
user-defined array instead of the result of a previous call to
process.hrtime()
will lead to undefined behavior.
These times are relative to an arbitrary time in the past, and not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals:
const NS_PER_SEC = 1e9;
const time = process.hrtime();
// [ 1800216, 25 ]
setTimeout(() => {
const diff = process.hrtime(time);
// [ 1, 552 ]
console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
// benchmark took 1000000552 nanoseconds
}, 1000);
process.initgroups(user, extra_group)#
user
<string> | <number> The user name or numeric identifier.extra_group
<string> | <number> A group name or numeric identifier.
The process.initgroups()
method reads the /etc/group
file and initializes
the group access list, using all groups of which the user is a member. This is
a privileged operation that requires that the Node.js process either have root
access or the CAP_SETGID
capability.
Note that care must be taken when dropping privileges. Example:
console.log(process.getgroups()); // [ 0 ]
process.initgroups('bnoordhuis', 1000); // switch user
console.log(process.getgroups()); // [ 27, 30, 46, 1000, 0 ]
process.setgid(1000); // drop root gid
console.log(process.getgroups()); // [ 27, 30, 46, 1000 ]
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.kill(pid[, signal])#
pid
<number> A process IDsignal
<string> | <number> The signal to send, either as a string or number. Defaults to'SIGTERM'
.
The process.kill()
method sends the signal
to the process identified by
pid
.
Signal names are strings such as 'SIGINT'
or 'SIGHUP'
. See Signal Events
and kill(2) for more information.
This method will throw an error if the target pid
does not exist. As a special
case, a signal of 0
can be used to test for the existence of a process.
Windows platforms will throw an error if the pid
is used to kill a process
group.
Note: Even though the name of this function is process.kill()
, it is
really just a signal sender, like the kill
system call. The signal sent may
do something other than kill the target process.
For example:
process.on('SIGHUP', () => {
console.log('Got SIGHUP signal.');
});
setTimeout(() => {
console.log('Exiting.');
process.exit(0);
}, 100);
process.kill(process.pid, 'SIGHUP');
Note: When SIGUSR1
is received by a Node.js process, Node.js will start
the debugger, see Signal Events.
process.mainModule#
The process.mainModule
property provides an alternative way of retrieving
require.main
. The difference is that if the main module changes at
runtime, require.main
may still refer to the original main module in
modules that were required before the change occurred. Generally, it's
safe to assume that the two refer to the same module.
As with require.main
, process.mainModule
will be undefined
if there
is no entry script.
process.memoryUsage()#
The process.memoryUsage()
method returns an object describing the memory usage
of the Node.js process measured in bytes.
For example, the code:
console.log(process.memoryUsage());
Will generate:
{
rss: 4935680,
heapTotal: 1826816,
heapUsed: 650472,
external: 49879
}
heapTotal
and heapUsed
refer to V8's memory usage.
external
refers to the memory usage of C++ objects bound to JavaScript
objects managed by V8.
process.nextTick(callback[, ...args])#
callback
<Function>...args
<any> Additional arguments to pass when invoking thecallback
The process.nextTick()
method adds the callback
to the "next tick queue".
Once the current turn of the event loop turn runs to completion, all callbacks
currently in the next tick queue will be called.
This is not a simple alias to setTimeout(fn, 0)
. It is much more
efficient. It runs before any additional I/O events (including
timers) fire in subsequent ticks of the event loop.
console.log('start');
process.nextTick(() => {
console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback
This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:
function MyThing(options) {
this.setupOptions(options);
process.nextTick(() => {
this.startDoingStuff();
});
}
const thing = new MyThing();
thing.getReadyForStuff();
// thing.startDoingStuff() gets called now, not before.
It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:
// WARNING! DO NOT USE! BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
if (arg) {
cb();
return;
}
fs.stat('file', cb);
}
This API is hazardous because in the following case:
const maybeTrue = Math.random() > 0.5;
maybeSync(maybeTrue, () => {
foo();
});
bar();
It is not clear whether foo()
or bar()
will be called first.
The following approach is much better:
function definitelyAsync(arg, cb) {
if (arg) {
process.nextTick(cb);
return;
}
fs.stat('file', cb);
}
Note: The next tick queue is completely drained on each pass of the
event loop before additional I/O is processed. As a result,
recursively setting nextTick callbacks will block any I/O from
happening, just like a while(true);
loop.
process.pid#
The process.pid
property returns the PID of the process.
console.log(`This process is pid ${process.pid}`);
process.platform#
The process.platform
property returns a string identifying the operating
system platform on which the Node.js process is running. For instance
'darwin'
, 'freebsd'
, 'linux'
, 'sunos'
or 'win32'
console.log(`This platform is ${process.platform}`);
process.release#
The process.release
property returns an Object containing metadata related to
the current release, including URLs for the source tarball and headers-only
tarball.
process.release
contains the following properties:
name
<string> A value that will always be'node'
for Node.js. For legacy io.js releases, this will be'io.js'
.sourceUrl
<string> an absolute URL pointing to a.tar.gz
file containing the source code of the current release.headersUrl
<string> an absolute URL pointing to a.tar.gz
file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.libUrl
<string> an absolute URL pointing to anode.lib
file matching the architecture and version of the current release. This file is used for compiling Node.js native add-ons. This property is only present on Windows builds of Node.js and will be missing on all other platforms.lts
<string> a string label identifying the LTS label for this release. If the Node.js release is not an LTS release, this will beundefined
.
For example:
{
name: 'node',
lts: 'Argon',
sourceUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5.tar.gz',
headersUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5-headers.tar.gz',
libUrl: 'https://nodejs.org/download/release/v4.4.5/win-x64/node.lib'
}
In custom builds from non-release versions of the source tree, only the
name
property may be present. The additional properties should not be
relied upon to exist.
process.send(message[, sendHandle[, options]][, callback])#
message
<Object>sendHandle
<Handle object>options
<Object>callback
<Function>- Returns: <boolean>
If Node.js is spawned with an IPC channel, the process.send()
method can be
used to send messages to the parent process. Messages will be received as a
'message'
event on the parent's ChildProcess
object.
If Node.js was not spawned with an IPC channel, process.send()
will be
undefined
.
Note: This function uses JSON.stringify()
internally to serialize the
message
.
process.setegid(id)#
The process.setegid()
method sets the effective group identity of the process.
(See setegid(2).) The id
can be passed as either a numeric ID or a group
name string. If a group name is specified, this method blocks while resolving
the associated a numeric ID.
if (process.getegid && process.setegid) {
console.log(`Current gid: ${process.getegid()}`);
try {
process.setegid(501);
console.log(`New gid: ${process.getegid()}`);
} catch (err) {
console.log(`Failed to set gid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.seteuid(id)#
The process.seteuid()
method sets the effective user identity of the process.
(See seteuid(2).) The id
can be passed as either a numeric ID or a username
string. If a username is specified, the method blocks while resolving the
associated numeric ID.
if (process.geteuid && process.seteuid) {
console.log(`Current uid: ${process.geteuid()}`);
try {
process.seteuid(501);
console.log(`New uid: ${process.geteuid()}`);
} catch (err) {
console.log(`Failed to set uid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.setgid(id)#
The process.setgid()
method sets the group identity of the process. (See
setgid(2).) The id
can be passed as either a numeric ID or a group name
string. If a group name is specified, this method blocks while resolving the
associated numeric ID.
if (process.getgid && process.setgid) {
console.log(`Current gid: ${process.getgid()}`);
try {
process.setgid(501);
console.log(`New gid: ${process.getgid()}`);
} catch (err) {
console.log(`Failed to set gid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.setgroups(groups)#
groups
<Array>
The process.setgroups()
method sets the supplementary group IDs for the
Node.js process. This is a privileged operation that requires the Node.js process
to have root
or the CAP_SETGID
capability.
The groups
array can contain numeric group IDs, group names or both.
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.setuid(id)#
The process.setuid(id)
method sets the user identity of the process. (See
setuid(2).) The id
can be passed as either a numeric ID or a username string.
If a username is specified, the method blocks while resolving the associated
numeric ID.
if (process.getuid && process.setuid) {
console.log(`Current uid: ${process.getuid()}`);
try {
process.setuid(501);
console.log(`New uid: ${process.getuid()}`);
} catch (err) {
console.log(`Failed to set uid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android).
process.stderr#
The process.stderr
property returns a stream connected to
stderr
(fd 2
). It is a net.Socket
(which is a Duplex
stream) unless fd 2
refers to a file, in which case it is
a Writable stream.
Note: process.stderr
differs from other Node.js streams in important ways,
see note on process I/O for more information.
process.stdin#
The process.stdin
property returns a stream connected to
stdin
(fd 0
). It is a net.Socket
(which is a Duplex
stream) unless fd 0
refers to a file, in which case it is
a Readable stream.
For example:
process.stdin.setEncoding('utf8');
process.stdin.on('readable', () => {
const chunk = process.stdin.read();
if (chunk !== null) {
process.stdout.write(`data: ${chunk}`);
}
});
process.stdin.on('end', () => {
process.stdout.write('end');
});
As a Duplex stream, process.stdin
can also be used in "old" mode that
is compatible with scripts written for Node.js prior to v0.10.
For more information see Stream compatibility.
Note: In "old" streams mode the stdin
stream is paused by default, so one
must call process.stdin.resume()
to read from it. Note also that calling
process.stdin.resume()
itself would switch stream to "old" mode.
process.stdout#
The process.stdout
property returns a stream connected to
stdout
(fd 1
). It is a net.Socket
(which is a Duplex
stream) unless fd 1
refers to a file, in which case it is
a Writable stream.
For example, to copy process.stdin to process.stdout:
process.stdin.pipe(process.stdout);
Note: process.stdout
differs from other Node.js streams in important ways,
see note on process I/O for more information.
A note on process I/O#
process.stdout
and process.stderr
differ from other Node.js streams in
important ways:
- They are used internally by
console.log()
andconsole.error()
, respectively. - They cannot be closed (
end()
will throw). - They will never emit the
'finish'
event. - Writes may be synchronous depending on the what the stream is connected to
and whether the system is Windows or Unix:
- Files: synchronous on Windows and Linux
- TTYs (Terminals): asynchronous on Windows, synchronous on Unix
- Pipes (and sockets): synchronous on Windows, asynchronous on Unix
These behaviors are partly for historical reasons, as changing them would create backwards incompatibility, but they are also expected by some users.
Synchronous writes avoid problems such as output written with console.log()
or
console.error()
being unexpectedly interleaved, or not written at all if
process.exit()
is called before an asynchronous write completes. See
process.exit()
for more information.
Warning: Synchronous writes block the event loop until the write has completed. This can be near instantaneous in the case of output to a file, but under high system load, pipes that are not being read at the receiving end, or with slow terminals or file systems, its possible for the event loop to be blocked often enough and long enough to have severe negative performance impacts. This may not be a problem when writing to an interactive terminal session, but consider this particularly careful when doing production logging to the process output streams.
To check if a stream is connected to a TTY context, check the isTTY
property.
For instance:
$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false
$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false
See the TTY documentation for more information.
process.title#
The process.title
property returns the current process title (i.e. returns
the current value of ps
). Assigning a new value to process.title
modifies
the current value of ps
.
Note: When a new value is assigned, different platforms will impose
different maximum length restrictions on the title. Usually such restrictions
are quite limited. For instance, on Linux and macOS, process.title
is limited
to the size of the binary name plus the length of the command line arguments
because setting the process.title
overwrites the argv
memory of the
process. Node.js v0.8 allowed for longer process title strings by also
overwriting the environ
memory but that was potentially insecure and
confusing in some (rather obscure) cases.
process.umask([mask])#
mask
<number>
The process.umask()
method sets or returns the Node.js process's file mode
creation mask. Child processes inherit the mask from the parent process. Invoked
without an argument, the current mask is returned, otherwise the umask is set to
the argument value and the previous mask is returned.
const newmask = 0o022;
const oldmask = process.umask(newmask);
console.log(
`Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`
);
process.uptime()#
- Returns: <number>
The process.uptime()
method returns the number of seconds the current Node.js
process has been running.
Note: The return value includes fractions of a second. Use Math.floor()
to get whole seconds.
process.version#
The process.version
property returns the Node.js version string.
console.log(`Version: ${process.version}`);
process.versions#
The process.versions
property returns an object listing the version strings of
Node.js and its dependencies. process.versions.modules
indicates the current
ABI version, which is increased whenever a C++ API changes. Node.js will refuse
to load modules that were compiled against a different module ABI version.
console.log(process.versions);
Will generate an object similar to:
{
http_parser: '2.3.0',
node: '1.1.1',
v8: '4.1.0.14',
uv: '1.3.0',
zlib: '1.2.8',
ares: '1.10.0-DEV',
modules: '43',
icu: '55.1',
openssl: '1.0.1k',
unicode: '8.0',
cldr: '29.0',
tz: '2016b' }
Exit Codes#
Node.js will normally exit with a 0
status code when no more async
operations are pending. The following status codes are used in other
cases:
1
Uncaught Fatal Exception - There was an uncaught exception, and it was not handled by a domain or an'uncaughtException'
event handler.2
- Unused (reserved by Bash for builtin misuse)3
Internal JavaScript Parse Error - The JavaScript source code internal in Node.js's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.4
Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node.js's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.5
Fatal Error - There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefixFATAL ERROR
.6
Non-function Internal Exception Handler - There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.7
Internal Exception Handler Run-Time Failure - There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if a'uncaughtException'
ordomain.on('error')
handler throws an error.8
- Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.9
- Invalid Argument - Either an unknown option was specified, or an option requiring a value was provided without a value.10
Internal JavaScript Run-Time Failure - The JavaScript source code internal in Node.js's bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.12
Invalid Debug Argument - The--inspect
and/or--inspect-brk
options were set, but the port number chosen was invalid or unavailable.>128
Signal Exits - If Node.js receives a fatal signal such asSIGKILL
orSIGHUP
, then its exit code will be128
plus the value of the signal code. This is a standard Unix practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code.
Punycode#
Stability: 0 - Deprecated
The version of the punycode module bundled in Node.js is being deprecated.
In a future major version of Node.js this module will be removed. Users
currently depending on the punycode
module should switch to using the
userland-provided Punycode.js module instead.
The punycode
module is a bundled version of the Punycode.js module. It
can be accessed using:
const punycode = require('punycode');
Punycode is a character encoding scheme defined by RFC 3492 that is
primarily intended for use in Internationalized Domain Names. Because host
names in URLs are limited to ASCII characters only, Domain Names that contain
non-ASCII characters must be converted into ASCII using the Punycode scheme.
For instance, the Japanese character that translates into the English word,
'example'
is '例'
. The Internationalized Domain Name, '例.com'
(equivalent
to 'example.com'
) is represented by Punycode as the ASCII string
'xn--fsq.com'
.
The punycode
module provides a simple implementation of the Punycode standard.
Note: The punycode
module is a third-party dependency used by Node.js and
made available to developers as a convenience. Fixes or other modifications to
the module must be directed to the Punycode.js project.
punycode.decode(string)#
string
<string>
The punycode.decode()
method converts a Punycode string of ASCII-only
characters to the equivalent string of Unicode codepoints.
punycode.decode('maana-pta'); // 'mañana'
punycode.decode('--dqo34k'); // '☃-⌘'
punycode.encode(string)#
string
<string>
The punycode.encode()
method converts a string of Unicode codepoints to a
Punycode string of ASCII-only characters.
punycode.encode('mañana'); // 'maana-pta'
punycode.encode('☃-⌘'); // '--dqo34k'
punycode.toASCII(domain)#
domain
<string>
The punycode.toASCII()
method converts a Unicode string representing an
Internationalized Domain Name to Punycode. Only the non-ASCII parts of the
domain name will be converted. Calling punycode.toASCII()
on a string that
already only contains ASCII characters will have no effect.
// encode domain names
punycode.toASCII('mañana.com'); // 'xn--maana-pta.com'
punycode.toASCII('☃-⌘.com'); // 'xn----dqo34k.com'
punycode.toASCII('example.com'); // 'example.com'
punycode.toUnicode(domain)#
domain
<string>
The punycode.toUnicode()
method converts a string representing a domain name
containing Punycode encoded characters into Unicode. Only the Punycode
encoded parts of the domain name are be converted.
// decode domain names
punycode.toUnicode('xn--maana-pta.com'); // 'mañana.com'
punycode.toUnicode('xn----dqo34k.com'); // '☃-⌘.com'
punycode.toUnicode('example.com'); // 'example.com'
punycode.ucs2#
punycode.ucs2.decode(string)#
string
<string>
The punycode.ucs2.decode()
method returns an array containing the numeric
codepoint values of each Unicode symbol in the string.
punycode.ucs2.decode('abc'); // [0x61, 0x62, 0x63]
// surrogate pair for U+1D306 tetragram for centre:
punycode.ucs2.decode('\uD834\uDF06'); // [0x1D306]
punycode.ucs2.encode(codePoints)#
codePoints
<Array>
The punycode.ucs2.encode()
method returns a string based on an array of
numeric code point values.
punycode.ucs2.encode([0x61, 0x62, 0x63]); // 'abc'
punycode.ucs2.encode([0x1D306]); // '\uD834\uDF06'
punycode.version#
Returns a string identifying the current Punycode.js version number.
Query String#
Stability: 2 - Stable
The querystring
module provides utilities for parsing and formatting URL
query strings. It can be accessed using:
const querystring = require('querystring');
querystring.escape(str)#
str
<string>
The querystring.escape()
method performs URL percent-encoding on the given
str
in a manner that is optimized for the specific requirements of URL
query strings.
The querystring.escape()
method is used by querystring.stringify()
and is
generally not expected to be used directly. It is exported primarily to allow
application code to provide a replacement percent-encoding implementation if
necessary by assigning querystring.escape
to an alternative function.
querystring.parse(str[, sep[, eq[, options]]])#
str
<string> The URL query string to parsesep
<string> The substring used to delimit key and value pairs in the query string. Defaults to'&'
.eq
<string>. The substring used to delimit keys and values in the query string. Defaults to'='
.options
<Object>decodeURIComponent
<Function> The function to use when decoding percent-encoded characters in the query string. Defaults toquerystring.unescape()
.maxKeys
<number> Specifies the maximum number of keys to parse. Defaults to1000
. Specify0
to remove key counting limitations.
The querystring.parse()
method parses a URL query string (str
) into a
collection of key and value pairs.
For example, the query string 'foo=bar&abc=xyz&abc=123'
is parsed into:
{
foo: 'bar',
abc: ['xyz', '123']
}
Note: The object returned by the querystring.parse()
method does not
prototypically inherit from the JavaScript Object
. This means that typical
Object
methods such as obj.toString()
, obj.hasOwnProperty()
, and others
are not defined and will not work.
By default, percent-encoded characters within the query string will be assumed
to use UTF-8 encoding. If an alternative character encoding is used, then an
alternative decodeURIComponent
option will need to be specified as illustrated
in the following example:
// Assuming gbkDecodeURIComponent function already exists...
querystring.parse('w=%D6%D0%CE%C4&foo=bar', null, null,
{ decodeURIComponent: gbkDecodeURIComponent });
querystring.stringify(obj[, sep[, eq[, options]]])#
obj
<Object> The object to serialize into a URL query stringsep
<string> The substring used to delimit key and value pairs in the query string. Defaults to'&'
.eq
<string>. The substring used to delimit keys and values in the query string. Defaults to'='
.options
encodeURIComponent
<Function> The function to use when converting URL-unsafe characters to percent-encoding in the query string. Defaults toquerystring.escape()
.
The querystring.stringify()
method produces a URL query string from a
given obj
by iterating through the object's "own properties".
It serializes the following types of values passed in obj
:
<string> | <number> | <boolean> | <string[]> | <number[]> | <boolean[]>
Any other input values will be coerced to empty strings.
For example:
querystring.stringify({ foo: 'bar', baz: ['qux', 'quux'], corge: '' });
// returns 'foo=bar&baz=qux&baz=quux&corge='
querystring.stringify({ foo: 'bar', baz: 'qux' }, ';', ':');
// returns 'foo:bar;baz:qux'
By default, characters requiring percent-encoding within the query string will
be encoded as UTF-8. If an alternative encoding is required, then an alternative
encodeURIComponent
option will need to be specified as illustrated in the
following example:
// Assuming gbkEncodeURIComponent function already exists,
querystring.stringify({ w: '中文', foo: 'bar' }, null, null,
{ encodeURIComponent: gbkEncodeURIComponent });
querystring.unescape(str)#
str
<string>
The querystring.unescape()
method performs decoding of URL percent-encoded
characters on the given str
.
The querystring.unescape()
method is used by querystring.parse()
and is
generally not expected to be used directly. It is exported primarily to allow
application code to provide a replacement decoding implementation if
necessary by assigning querystring.unescape
to an alternative function.
By default, the querystring.unescape()
method will attempt to use the
JavaScript built-in decodeURIComponent()
method to decode. If that fails,
a safer equivalent that does not throw on malformed URLs will be used.
Readline#
Stability: 2 - Stable
The readline
module provides an interface for reading data from a Readable
stream (such as process.stdin
) one line at a time. It can be accessed using:
const readline = require('readline');
The following simple example illustrates the basic use of the readline
module.
const readline = require('readline');
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});
rl.question('What do you think of Node.js? ', (answer) => {
// TODO: Log the answer in a database
console.log(`Thank you for your valuable feedback: ${answer}`);
rl.close();
});
Note: Once this code is invoked, the Node.js application will not
terminate until the readline.Interface
is closed because the interface
waits for data to be received on the input
stream.
Class: Interface#
Instances of the readline.Interface
class are constructed using the
readline.createInterface()
method. Every instance is associated with a
single input
Readable stream and a single output
Writable stream.
The output
stream is used to print prompts for user input that arrives on,
and is read from, the input
stream.
Event: 'close'#
The 'close'
event is emitted when one of the following occur:
- The
rl.close()
method is called and thereadline.Interface
instance has relinquished control over theinput
andoutput
streams; - The
input
stream receives its'end'
event; - The
input
stream receives<ctrl>-D
to signal end-of-transmission (EOT); - The
input
stream receives<ctrl>-C
to signalSIGINT
and there is noSIGINT
event listener registered on thereadline.Interface
instance.
The listener function is called without passing any arguments.
The readline.Interface
instance should be considered to be "finished" once
the 'close'
event is emitted.
Event: 'line'#
The 'line'
event is emitted whenever the input
stream receives an
end-of-line input (\n
, \r
, or \r\n
). This usually occurs when the user
presses the <Enter>
, or <Return>
keys.
The listener function is called with a string containing the single line of received input.
For example:
rl.on('line', (input) => {
console.log(`Received: ${input}`);
});
Event: 'pause'#
The 'pause'
event is emitted when one of the following occur:
- The
input
stream is paused. - The
input
stream is not paused and receives theSIGCONT
event. (See eventsSIGTSTP
andSIGCONT
)
The listener function is called without passing any arguments.
For example:
rl.on('pause', () => {
console.log('Readline paused.');
});
Event: 'resume'#
The 'resume'
event is emitted whenever the input
stream is resumed.
The listener function is called without passing any arguments.
rl.on('resume', () => {
console.log('Readline resumed.');
});
Event: 'SIGCONT'#
The 'SIGCONT'
event is emitted when a Node.js process previously moved into
the background using <ctrl>-Z
(i.e. SIGTSTP
) is then brought back to the
foreground using fg(1p).
If the input
stream was paused before the SIGTSTP
request, this event will
not be emitted.
The listener function is invoked without passing any arguments.
For example:
rl.on('SIGCONT', () => {
// `prompt` will automatically resume the stream
rl.prompt();
});
Note: The 'SIGCONT'
event is not supported on Windows.
Event: 'SIGINT'#
The 'SIGINT'
event is emitted whenever the input
stream receives a
<ctrl>-C
input, known typically as SIGINT
. If there are no 'SIGINT'
event
listeners registered when the input
stream receives a SIGINT
, the 'pause'
event will be emitted.
The listener function is invoked without passing any arguments.
For example:
rl.on('SIGINT', () => {
rl.question('Are you sure you want to exit? ', (answer) => {
if (answer.match(/^y(es)?$/i)) rl.pause();
});
});
Event: 'SIGTSTP'#
The 'SIGTSTP'
event is emitted when the input
stream receives a <ctrl>-Z
input, typically known as SIGTSTP
. If there are no SIGTSTP
event listeners
registered when the input
stream receives a SIGTSTP
, the Node.js process
will be sent to the background.
When the program is resumed using fg(1p), the 'pause'
and SIGCONT
events
will be emitted. These can be used to resume the input
stream.
The 'pause'
and 'SIGCONT'
events will not be emitted if the input
was
paused before the process was sent to the background.
The listener function is invoked without passing any arguments.
For example:
rl.on('SIGTSTP', () => {
// This will override SIGTSTP and prevent the program from going to the
// background.
console.log('Caught SIGTSTP.');
});
Note: The 'SIGTSTP'
event is not supported on Windows.
rl.close()#
The rl.close()
method closes the readline.Interface
instance and
relinquishes control over the input
and output
streams. When called,
the 'close'
event will be emitted.
rl.pause()#
The rl.pause()
method pauses the input
stream, allowing it to be resumed
later if necessary.
Calling rl.pause()
does not immediately pause other events (including
'line'
) from being emitted by the readline.Interface
instance.
rl.prompt([preserveCursor])#
preserveCursor
<boolean> Iftrue
, prevents the cursor placement from being reset to0
.
The rl.prompt()
method writes the readline.Interface
instances configured
prompt
to a new line in output
in order to provide a user with a new
location at which to provide input.
When called, rl.prompt()
will resume the input
stream if it has been
paused.
If the readline.Interface
was created with output
set to null
or
undefined
the prompt is not written.
rl.question(query, callback)#
query
<string> A statement or query to write tooutput
, prepended to the prompt.callback
<Function> A callback function that is invoked with the user's input in response to thequery
.
The rl.question()
method displays the query
by writing it to the output
,
waits for user input to be provided on input
, then invokes the callback
function passing the provided input as the first argument.
When called, rl.question()
will resume the input
stream if it has been
paused.
If the readline.Interface
was created with output
set to null
or
undefined
the query
is not written.
Example usage:
rl.question('What is your favorite food? ', (answer) => {
console.log(`Oh, so your favorite food is ${answer}`);
});
Note: The callback
function passed to rl.question()
does not follow the
typical pattern of accepting an Error
object or null
as the first argument.
The callback
is called with the provided answer as the only argument.
rl.resume()#
The rl.resume()
method resumes the input
stream if it has been paused.
rl.setPrompt(prompt)#
prompt
<string>
The rl.setPrompt()
method sets the prompt that will be written to output
whenever rl.prompt()
is called.
rl.write(data[, key])#
The rl.write()
method will write either data
or a key sequence identified
by key
to the output
. The key
argument is supported only if output
is
a TTY text terminal.
If key
is specified, data
is ignored.
When called, rl.write()
will resume the input
stream if it has been
paused.
If the readline.Interface
was created with output
set to null
or
undefined
the data
and key
are not written.
For example:
rl.write('Delete this!');
// Simulate Ctrl+u to delete the line written previously
rl.write(null, { ctrl: true, name: 'u' });
Note: The rl.write()
method will write the data to the readline
Interface's input
as if it were provided by the user.
readline.clearLine(stream, dir)#
stream
<Writable>dir
<number>-1
- to the left from cursor1
- to the right from cursor0
- the entire line
The readline.clearLine()
method clears current line of given TTY stream
in a specified direction identified by dir
.
readline.clearScreenDown(stream)#
stream
<Writable>
The readline.clearScreenDown()
method clears the given TTY stream from
the current position of the cursor down.
readline.createInterface(options)#
options
<Object>input
<Readable> The Readable stream to listen to. This option is required.output
<Writable> The Writable stream to write readline data to.completer
<Function> An optional function used for Tab autocompletion.terminal
<boolean>true
if theinput
andoutput
streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Defaults to checkingisTTY
on theoutput
stream upon instantiation.historySize
<number> maximum number of history lines retained. To disable the history set this value to0
. Defaults to30
. This option makes sense only ifterminal
is set totrue
by the user or by an internaloutput
check, otherwise the history caching mechanism is not initialized at all.prompt
- the prompt string to use. Default:'> '
crlfDelay
<number> If the delay between\r
and\n
exceedscrlfDelay
milliseconds, both\r
and\n
will be treated as separate end-of-line input. Default to100
milliseconds.crlfDelay
will be coerced to a number no less than100
. It can be set toInfinity
, in which case\r
followed by\n
will always be considered a single newline.removeHistoryDuplicates
<boolean> Iftrue
, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Defaults tofalse
.
The readline.createInterface()
method creates a new readline.Interface
instance.
For example:
const readline = require('readline');
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});
Once the readline.Interface
instance is created, the most common case is to
listen for the 'line'
event:
rl.on('line', (line) => {
console.log(`Received: ${line}`);
});
If terminal
is true
for this instance then the output
stream will get
the best compatibility if it defines an output.columns
property and emits
a 'resize'
event on the output
if or when the columns ever change
(process.stdout
does this automatically when it is a TTY).
Use of the completer
Function#
The completer
function takes the current line entered by the user
as an argument, and returns an Array with 2 entries:
- An Array with matching entries for the completion.
- The substring that was used for the matching.
For instance: [[substr1, substr2, ...], originalsubstring]
.
function completer(line) {
const completions = '.help .error .exit .quit .q'.split(' ');
const hits = completions.filter((c) => c.startsWith(line));
// show all completions if none found
return [hits.length ? hits : completions, line];
}
The completer
function can be called asynchronously if it accepts two
arguments:
function completer(linePartial, callback) {
callback(null, [['123'], linePartial]);
}
readline.cursorTo(stream, x, y)#
The readline.cursorTo()
method moves cursor to the specified position in a
given TTY stream
.
readline.emitKeypressEvents(stream[, interface])#
stream
<Readable>interface
<readline.Interface>
The readline.emitKeypressEvents()
method causes the given Writable
stream
to begin emitting 'keypress'
events corresponding to received input.
Optionally, interface
specifies a readline.Interface
instance for which
autocompletion is disabled when copy-pasted input is detected.
If the stream
is a TTY, then it must be in raw mode.
Note: This is automatically called by any readline instance on its input
if the input
is a terminal. Closing the readline
instance does not stop
the input
from emitting 'keypress'
events.
readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
process.stdin.setRawMode(true);
readline.moveCursor(stream, dx, dy)#
The readline.moveCursor()
method moves the cursor relative to its current
position in a given TTY stream
.
Example: Tiny CLI#
The following example illustrates the use of readline.Interface
class to
implement a small command-line interface:
const readline = require('readline');
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout,
prompt: 'OHAI> '
});
rl.prompt();
rl.on('line', (line) => {
switch (line.trim()) {
case 'hello':
console.log('world!');
break;
default:
console.log(`Say what? I might have heard '${line.trim()}'`);
break;
}
rl.prompt();
}).on('close', () => {
console.log('Have a great day!');
process.exit(0);
});
Example: Read File Stream Line-by-Line#
A common use case for readline
is to consume input from a filesystem
Readable stream one line at a time, as illustrated in the following
example:
const readline = require('readline');
const fs = require('fs');
const rl = readline.createInterface({
input: fs.createReadStream('sample.txt')
});
rl.on('line', (line) => {
console.log(`Line from file: ${line}`);
});
REPL#
Stability: 2 - Stable
The repl
module provides a Read-Eval-Print-Loop (REPL) implementation that
is available both as a standalone program or includible in other applications.
It can be accessed using:
const repl = require('repl');
Design and Features#
The repl
module exports the repl.REPLServer
class. While running, instances
of repl.REPLServer
will accept individual lines of user input, evaluate those
according to a user-defined evaluation function, then output the result. Input
and output may be from stdin
and stdout
, respectively, or may be connected
to any Node.js stream.
Instances of repl.REPLServer
support automatic completion of inputs,
simplistic Emacs-style line editing, multi-line inputs, ANSI-styled output,
saving and restoring current REPL session state, error recovery, and
customizable evaluation functions.
Commands and Special Keys#
The following special commands are supported by all REPL instances:
.break
- When in the process of inputting a multi-line expression, entering the.break
command (or pressing the<ctrl>-C
key combination) will abort further input or processing of that expression..clear
- Resets the REPLcontext
to an empty object and clears any multi-line expression currently being input..exit
- Close the I/O stream, causing the REPL to exit..help
- Show this list of special commands..save
- Save the current REPL session to a file:> .save ./file/to/save.js
.load
- Load a file into the current REPL session.> .load ./file/to/load.js
.editor
- Enter editor mode (<ctrl>-D
to finish,<ctrl>-C
to cancel)
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function welcome(name) {
return `Hello ${name}!`;
}
welcome('Node.js User');
// ^D
'Hello Node.js User!'
>
The following key combinations in the REPL have these special effects:
<ctrl>-C
- When pressed once, has the same effect as the.break
command. When pressed twice on a blank line, has the same effect as the.exit
command.<ctrl>-D
- Has the same effect as the.exit
command.<tab>
- When pressed on a blank line, displays global and local(scope) variables. When pressed while entering other input, displays relevant autocompletion options.
Default Evaluation#
By default, all instances of repl.REPLServer
use an evaluation function that
evaluates JavaScript expressions and provides access to Node.js' built-in
modules. This default behavior can be overridden by passing in an alternative
evaluation function when the repl.REPLServer
instance is created.
JavaScript Expressions#
The default evaluator supports direct evaluation of JavaScript expressions:
> 1 + 1
2
> const m = 2
undefined
> m + 1
3
Unless otherwise scoped within blocks or functions, variables declared
either implicitly or using the const
, let
, or var
keywords
are declared at the global scope.
Global and Local Scope#
The default evaluator provides access to any variables that exist in the global
scope. It is possible to expose a variable to the REPL explicitly by assigning
it to the context
object associated with each REPLServer
. For example:
const repl = require('repl');
const msg = 'message';
repl.start('> ').context.m = msg;
Properties in the context
object appear as local within the REPL:
$ node repl_test.js
> m
'message'
It is important to note that context properties are not read-only by default.
To specify read-only globals, context properties must be defined using
Object.defineProperty()
:
const repl = require('repl');
const msg = 'message';
const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
configurable: false,
enumerable: true,
value: msg
});
Accessing Core Node.js Modules#
The default evaluator will automatically load Node.js core modules into the
REPL environment when used. For instance, unless otherwise declared as a
global or scoped variable, the input fs
will be evaluated on-demand as
global.fs = require('fs')
.
> fs.createReadStream('./some/file');
Assignment of the _
(underscore) variable#
The default evaluator will, by default, assign the result of the most recently
evaluated expression to the special variable _
(underscore).
Explicitly setting _
to a value will disable this behavior.
> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
Expression assignment to _ now disabled.
4
> 1 + 1
2
> _
4
Custom Evaluation Functions#
When a new repl.REPLServer
is created, a custom evaluation function may be
provided. This can be used, for instance, to implement fully customized REPL
applications.
The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:
const repl = require('repl');
const { Translator } = require('translator');
const myTranslator = new Translator('en', 'fr');
function myEval(cmd, context, filename, callback) {
callback(null, myTranslator.translate(cmd));
}
repl.start({ prompt: '> ', eval: myEval });
Recoverable Errors#
As a user is typing input into the REPL prompt, pressing the <enter>
key will
send the current line of input to the eval
function. In order to support
multi-line input, the eval function can return an instance of repl.Recoverable
to the provided callback function:
function myEval(cmd, context, filename, callback) {
let result;
try {
result = vm.runInThisContext(cmd);
} catch (e) {
if (isRecoverableError(e)) {
return callback(new repl.Recoverable(e));
}
}
callback(null, result);
}
function isRecoverableError(error) {
if (error.name === 'SyntaxError') {
return /^(Unexpected end of input|Unexpected token)/.test(error.message);
}
return false;
}
Customizing REPL Output#
By default, repl.REPLServer
instances format output using the
util.inspect()
method before writing the output to the provided Writable
stream (process.stdout
by default). The useColors
boolean option can be
specified at construction to instruct the default writer to use ANSI style
codes to colorize the output from the util.inspect()
method.
It is possible to fully customize the output of a repl.REPLServer
instance
by passing a new function in using the writer
option on construction. The
following example, for instance, simply converts any input text to upper case:
const repl = require('repl');
const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });
function myEval(cmd, context, filename, callback) {
callback(null, cmd);
}
function myWriter(output) {
return output.toUpperCase();
}
Class: REPLServer#
The repl.REPLServer
class inherits from the readline.Interface
class.
Instances of repl.REPLServer
are created using the repl.start()
method and
should not be created directly using the JavaScript new
keyword.
Event: 'exit'#
The 'exit'
event is emitted when the REPL is exited either by receiving the
.exit
command as input, the user pressing <ctrl>-C
twice to signal SIGINT
,
or by pressing <ctrl>-D
to signal 'end'
on the input stream. The listener
callback is invoked without any arguments.
replServer.on('exit', () => {
console.log('Received "exit" event from repl!');
process.exit();
});
Event: 'reset'#
The 'reset'
event is emitted when the REPL's context is reset. This occurs
whenever the .clear
command is received as input unless the REPL is using
the default evaluator and the repl.REPLServer
instance was created with the
useGlobal
option set to true
. The listener callback will be called with a
reference to the context
object as the only argument.
This can be used primarily to re-initialize REPL context to some pre-defined state as illustrated in the following simple example:
const repl = require('repl');
function initializeContext(context) {
context.m = 'test';
}
const r = repl.start({ prompt: '> ' });
initializeContext(r.context);
r.on('reset', initializeContext);
When this code is executed, the global 'm'
variable can be modified but then
reset to its initial value using the .clear
command:
$ ./node example.js
> m
'test'
> m = 1
1
> m
1
> .clear
Clearing context...
> m
'test'
>
replServer.defineCommand(keyword, cmd)#
keyword
<string> The command keyword (without a leading.
character).cmd
<Object> | <Function> The function to invoke when the command is processed.
The replServer.defineCommand()
method is used to add new .
-prefixed commands
to the REPL instance. Such commands are invoked by typing a .
followed by the
keyword
. The cmd
is either a Function or an object with the following
properties:
help
<string> Help text to be displayed when.help
is entered (Optional).action
<Function> The function to execute, optionally accepting a single string argument.
The following example shows two new commands added to the REPL instance:
const repl = require('repl');
const replServer = repl.start({ prompt: '> ' });
replServer.defineCommand('sayhello', {
help: 'Say hello',
action(name) {
this.bufferedCommand = '';
console.log(`Hello, ${name}!`);
this.displayPrompt();
}
});
replServer.defineCommand('saybye', function saybye() {
console.log('Goodbye!');
this.close();
});
The new commands can then be used from within the REPL instance:
> .sayhello Node.js User
Hello, Node.js User!
> .saybye
Goodbye!
replServer.displayPrompt([preserveCursor])#
preserveCursor
<boolean>
The replServer.displayPrompt()
method readies the REPL instance for input
from the user, printing the configured prompt
to a new line in the output
and resuming the input
to accept new input.
When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.
When preserveCursor
is true
, the cursor placement will not be reset to 0
.
The replServer.displayPrompt
method is primarily intended to be called from
within the action function for commands registered using the
replServer.defineCommand()
method.
repl.start(options)#
options
<Object> | <string>prompt
<string> The input prompt to display. Defaults to>
(with a trailing space).input
<Readable> The Readable stream from which REPL input will be read. Defaults toprocess.stdin
.output
<Writable> The Writable stream to which REPL output will be written. Defaults toprocess.stdout
.terminal
<boolean> Iftrue
, specifies that theoutput
should be treated as a a TTY terminal, and have ANSI/VT100 escape codes written to it. Defaults to checking the value of theisTTY
property on theoutput
stream upon instantiation.eval
<Function> The function to be used when evaluating each given line of input. Defaults to an async wrapper for the JavaScripteval()
function. Aneval
function can error withrepl.Recoverable
to indicate the input was incomplete and prompt for additional lines.useColors
<boolean> Iftrue
, specifies that the defaultwriter
function should include ANSI color styling to REPL output. If a customwriter
function is provided then this has no effect. Defaults to the REPL instancesterminal
value.useGlobal
<boolean> Iftrue
, specifies that the default evaluation function will use the JavaScriptglobal
as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value totrue
. Defaults tofalse
.ignoreUndefined
<boolean> Iftrue
, specifies that the default writer will not output the return value of a command if it evaluates toundefined
. Defaults tofalse
.writer
<Function> The function to invoke to format the output of each command before writing tooutput
. Defaults toutil.inspect()
.completer
<Function> An optional function used for custom Tab auto completion. Seereadline.InterfaceCompleter
for an example.replMode
<symbol> A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Acceptable values are:repl.REPL_MODE_SLOPPY
- evaluates expressions in sloppy mode.repl.REPL_MODE_STRICT
- evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with'use strict'
.repl.REPL_MODE_MAGIC
- This value is deprecated, since enhanced spec compliance in V8 has rendered magic mode unnecessary. It is now equivalent torepl.REPL_MODE_SLOPPY
(documented above).
breakEvalOnSigint
- Stop evaluating the current piece of code whenSIGINT
is received, i.e.Ctrl+C
is pressed. This cannot be used together with a customeval
function. Defaults tofalse
.
The repl.start()
method creates and starts a repl.REPLServer
instance.
If options
is a string, then it specifies the input prompt:
const repl = require('repl');
// a Unix style prompt
repl.start('$ ');
The Node.js REPL#
Node.js itself uses the repl
module to provide its own interactive interface
for executing JavaScript. This can be used by executing the Node.js binary
without passing any arguments (or by passing the -i
argument):
$ node
> const a = [1, 2, 3];
undefined
> a
[ 1, 2, 3 ]
> a.forEach((v) => {
... console.log(v);
... });
1
2
3
Environment Variable Options#
Various behaviors of the Node.js REPL can be customized using the following environment variables:
NODE_REPL_HISTORY
- When a valid path is given, persistent REPL history will be saved to the specified file rather than.node_repl_history
in the user's home directory. Setting this value to""
will disable persistent REPL history. Whitespace will be trimmed from the value.NODE_REPL_HISTORY_SIZE
- Defaults to1000
. Controls how many lines of history will be persisted if history is available. Must be a positive number.NODE_REPL_MODE
- May be any ofsloppy
,strict
, ormagic
. Defaults tosloppy
, which will allow non-strict mode code to be run.magic
is deprecated and treated as an alias ofsloppy
.
Persistent History#
By default, the Node.js REPL will persist history between node
REPL sessions
by saving inputs to a .node_repl_history
file located in the user's home
directory. This can be disabled by setting the environment variable
NODE_REPL_HISTORY=""
.
NODE_REPL_HISTORY_FILE#
Stability: 0 - Deprecated: Use NODE_REPL_HISTORY
instead.
Previously in Node.js/io.js v2.x, REPL history was controlled by using a
NODE_REPL_HISTORY_FILE
environment variable, and the history was saved in JSON
format. This variable has now been deprecated, and the old JSON REPL history
file will be automatically converted to a simplified plain text format. This new
file will be saved to either the user's home directory, or a directory defined
by the NODE_REPL_HISTORY
variable, as documented in the
Environment Variable Options.
Using the Node.js REPL with advanced line-editors#
For advanced line-editors, start Node.js with the environmental variable
NODE_NO_READLINE=1
. This will start the main and debugger REPL in canonical
terminal settings, which will allow use with rlwrap
.
For example, the following can be added to a .bashrc
file:
alias node="env NODE_NO_READLINE=1 rlwrap node"
Starting multiple REPL instances against a single running instance#
It is possible to create and run multiple REPL instances against a single
running instance of Node.js that share a single global
object but have
separate I/O interfaces.
The following example, for instance, provides separate REPLs on stdin
, a Unix
socket, and a TCP socket:
const net = require('net');
const repl = require('repl');
let connections = 0;
repl.start({
prompt: 'Node.js via stdin> ',
input: process.stdin,
output: process.stdout
});
net.createServer((socket) => {
connections += 1;
repl.start({
prompt: 'Node.js via Unix socket> ',
input: socket,
output: socket
}).on('exit', () => {
socket.end();
});
}).listen('/tmp/node-repl-sock');
net.createServer((socket) => {
connections += 1;
repl.start({
prompt: 'Node.js via TCP socket> ',
input: socket,
output: socket
}).on('exit', () => {
socket.end();
});
}).listen(5001);
Running this application from the command line will start a REPL on stdin.
Other REPL clients may connect through the Unix socket or TCP socket. telnet
,
for instance, is useful for connecting to TCP sockets, while socat
can be used
to connect to both Unix and TCP sockets.
By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.
For an example of running a "full-featured" (terminal
) REPL over
a net.Server
and net.Socket
instance, see: https://gist.github.com/2209310
For an example of running a REPL instance over curl(1), see: https://gist.github.com/2053342
Stream#
Stability: 2 - Stable
A stream is an abstract interface for working with streaming data in Node.js.
The stream
module provides a base API that makes it easy to build objects
that implement the stream interface.
There are many stream objects provided by Node.js. For instance, a
request to an HTTP server and process.stdout
are both stream instances.
Streams can be readable, writable, or both. All streams are instances of
EventEmitter
.
The stream
module can be accessed using:
const stream = require('stream');
While it is important for all Node.js users to understand how streams work,
the stream
module itself is most useful for developers that are creating new
types of stream instances. Developers who are primarily consuming stream
objects will rarely (if ever) have need to use the stream
module directly.
Organization of this Document#
This document is divided into two primary sections with a third section for additional notes. The first section explains the elements of the stream API that are required to use streams within an application. The second section explains the elements of the API that are required to implement new types of streams.
Types of Streams#
There are four fundamental stream types within Node.js:
- Readable - streams from which data can be read (for example
fs.createReadStream()
). - Writable - streams to which data can be written (for example
fs.createWriteStream()
). - Duplex - streams that are both Readable and Writable (for example
net.Socket
). - Transform - Duplex streams that can modify or transform the data as it
is written and read (for example
zlib.createDeflate()
).
Object Mode#
All streams created by Node.js APIs operate exclusively on strings and Buffer
(or Uint8Array
) objects. It is possible, however, for stream implementations
to work with other types of JavaScript values (with the exception of null
,
which serves a special purpose within streams). Such streams are considered to
operate in "object mode".
Stream instances are switched into object mode using the objectMode
option
when the stream is created. Attempting to switch an existing stream into
object mode is not safe.
Buffering#
Both Writable and Readable streams will store data in an internal
buffer that can be retrieved using writable._writableState.getBuffer()
or
readable._readableState.buffer
, respectively.
The amount of data potentially buffered depends on the highWaterMark
option
passed into the streams constructor. For normal streams, the highWaterMark
option specifies a total number of bytes. For streams operating in object mode,
the highWaterMark
specifies a total number of objects.
Data is buffered in Readable streams when the implementation calls
stream.push(chunk)
. If the consumer of the Stream does not
call stream.read()
, the data will sit in the internal
queue until it is consumed.
Once the total size of the internal read buffer reaches the threshold specified
by highWaterMark
, the stream will temporarily stop reading data from the
underlying resource until the data currently buffered can be consumed (that is,
the stream will stop calling the internal readable._read()
method that is
used to fill the read buffer).
Data is buffered in Writable streams when the
writable.write(chunk)
method is called repeatedly. While the
total size of the internal write buffer is below the threshold set by
highWaterMark
, calls to writable.write()
will return true
. Once
the size of the internal buffer reaches or exceeds the highWaterMark
, false
will be returned.
A key goal of the stream
API, particularly the stream.pipe()
method,
is to limit the buffering of data to acceptable levels such that sources and
destinations of differing speeds will not overwhelm the available memory.
Because Duplex and Transform streams are both Readable and Writable,
each maintain two separate internal buffers used for reading and writing,
allowing each side to operate independently of the other while maintaining an
appropriate and efficient flow of data. For example, net.Socket
instances
are Duplex streams whose Readable side allows consumption of data received
from the socket and whose Writable side allows writing data to the socket.
Because data may be written to the socket at a faster or slower rate than data
is received, it is important for each side to operate (and buffer) independently
of the other.
API for Stream Consumers#
Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:
const http = require('http');
const server = http.createServer((req, res) => {
// req is an http.IncomingMessage, which is a Readable Stream
// res is an http.ServerResponse, which is a Writable Stream
let body = '';
// Get the data as utf8 strings.
// If an encoding is not set, Buffer objects will be received.
req.setEncoding('utf8');
// Readable streams emit 'data' events once a listener is added
req.on('data', (chunk) => {
body += chunk;
});
// the end event indicates that the entire body has been received
req.on('end', () => {
try {
const data = JSON.parse(body);
// write back something interesting to the user:
res.write(typeof data);
res.end();
} catch (er) {
// uh oh! bad json!
res.statusCode = 400;
return res.end(`error: ${er.message}`);
}
});
});
server.listen(1337);
// $ curl localhost:1337 -d "{}"
// object
// $ curl localhost:1337 -d "\"foo\""
// string
// $ curl localhost:1337 -d "not json"
// error: Unexpected token o in JSON at position 1
Writable streams (such as res
in the example) expose methods such as
write()
and end()
that are used to write data onto the stream.
Readable streams use the EventEmitter
API for notifying application
code when data is available to be read off the stream. That available data can
be read from the stream in multiple ways.
Both Writable and Readable streams use the EventEmitter
API in
various ways to communicate the current state of the stream.
Duplex and Transform streams are both Writable and Readable.
Applications that are either writing data to or consuming data from a stream
are not required to implement the stream interfaces directly and will generally
have no reason to call require('stream')
.
Developers wishing to implement new types of streams should refer to the section API for Stream Implementers.
Writable Streams#
Writable streams are an abstraction for a destination to which data is written.
Examples of Writable streams include:
- HTTP requests, on the client
- HTTP responses, on the server
- fs write streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdin
process.stdout
,process.stderr
Note: Some of these examples are actually Duplex streams that implement the Writable interface.
All Writable streams implement the interface defined by the
stream.Writable
class.
While specific instances of Writable streams may differ in various ways, all Writable streams follow the same fundamental usage pattern as illustrated in the example below:
const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');
Class: stream.Writable#
Event: 'close'#
The 'close'
event is emitted when the stream and any of its underlying
resources (a file descriptor, for example) have been closed. The event indicates
that no more events will be emitted, and no further computation will occur.
Not all Writable streams will emit the 'close'
event.
Event: 'drain'#
If a call to stream.write(chunk)
returns false
, the
'drain'
event will be emitted when it is appropriate to resume writing data
to the stream.
// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
let i = 1000000;
write();
function write() {
let ok = true;
do {
i--;
if (i === 0) {
// last time!
writer.write(data, encoding, callback);
} else {
// see if we should continue, or wait
// don't pass the callback, because we're not done yet.
ok = writer.write(data, encoding);
}
} while (i > 0 && ok);
if (i > 0) {
// had to stop early!
// write some more once it drains
writer.once('drain', write);
}
}
}
Event: 'error'#
The 'error'
event is emitted if an error occurred while writing or piping
data. The listener callback is passed a single Error
argument when called.
Note: The stream is not closed when the 'error'
event is emitted.
Event: 'finish'#
The 'finish'
event is emitted after the stream.end()
method
has been called, and all data has been flushed to the underlying system.
const writer = getWritableStreamSomehow();
for (let i = 0; i < 100; i++) {
writer.write(`hello, #${i}!\n`);
}
writer.end('This is the end\n');
writer.on('finish', () => {
console.error('All writes are now complete.');
});
Event: 'pipe'#
src
<stream.Readable> source stream that is piping to this writable
The 'pipe'
event is emitted when the stream.pipe()
method is called on
a readable stream, adding this writable to its set of destinations.
const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
console.error('something is piping into the writer');
assert.equal(src, reader);
});
reader.pipe(writer);
Event: 'unpipe'#
The 'unpipe'
event is emitted when the stream.unpipe()
method is called
on a Readable stream, removing this Writable from its set of
destinations.
const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
console.error('Something has stopped piping into the writer.');
assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);
writable.cork()#
The writable.cork()
method forces all written data to be buffered in memory.
The buffered data will be flushed when either the stream.uncork()
or
stream.end()
methods are called.
The primary intent of writable.cork()
is to avoid a situation where writing
many small chunks of data to a stream do not cause a backup in the internal
buffer that would have an adverse impact on performance. In such situations,
implementations that implement the writable._writev()
method can perform
buffered writes in a more optimized manner.
See also: writable.uncork()
.
writable.end([chunk][, encoding][, callback])#
chunk
<string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
<string> The encoding, ifchunk
is a stringcallback
<Function> Optional callback for when the stream is finished
Calling the writable.end()
method signals that no more data will be written
to the Writable. The optional chunk
and encoding
arguments allow one
final additional chunk of data to be written immediately before closing the
stream. If provided, the optional callback
function is attached as a listener
for the 'finish'
event.
Calling the stream.write()
method after calling
stream.end()
will raise an error.
// write 'hello, ' and then end with 'world!'
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// writing more now is not allowed!
writable.setDefaultEncoding(encoding)#
encoding
<string> The new default encoding- Returns:
this
The writable.setDefaultEncoding()
method sets the default encoding
for a
Writable stream.
writable.uncork()#
The writable.uncork()
method flushes all data buffered since
stream.cork()
was called.
When using writable.cork()
and writable.uncork()
to manage the buffering
of writes to a stream, it is recommended that calls to writable.uncork()
be
deferred using process.nextTick()
. Doing so allows batching of all
writable.write()
calls that occur within a given Node.js event loop phase.
stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());
If the writable.cork()
method is called multiple times on a stream, the same
number of calls to writable.uncork()
must be called to flush the buffered
data.
stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
stream.uncork();
// The data will not be flushed until uncork() is called a second time.
stream.uncork();
});
See also: writable.cork()
.
writable.write(chunk[, encoding][, callback])#
chunk
<string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
<string> The encoding, ifchunk
is a stringcallback
<Function> Callback for when this chunk of data is flushed- Returns: <boolean>
false
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The writable.write()
method writes some data to the stream, and calls the
supplied callback
once the data has been fully handled. If an error
occurs, the callback
may or may not be called with the error as its
first argument. To reliably detect write errors, add a listener for the
'error'
event.
The return value is true
if the internal buffer is less than the
highWaterMark
configured when the stream was created after admitting chunk
.
If false
is returned, further attempts to write data to the stream should
stop until the 'drain'
event is emitted.
While a stream is not draining, calls to write()
will buffer chunk
, and
return false. Once all currently buffered chunks are drained (accepted for
delivery by the operating system), the 'drain'
event will be emitted.
It is recommended that once write() returns false, no more chunks be written
until the 'drain'
event is emitted. While calling write()
on a stream that
is not draining is allowed, Node.js will buffer all written chunks until
maximum memory usage occurs, at which point it will abort unconditionally.
Even before it aborts, high memory usage will cause poor garbage collector
performance and high RSS (which is not typically released back to the system,
even after the memory is no longer required). Since TCP sockets may never
drain if the remote peer does not read the data, writing a socket that is
not draining may lead to a remotely exploitable vulnerability.
Writing data while the stream is not draining is particularly
problematic for a Transform, because the Transform
streams are paused
by default until they are piped or an 'data'
or 'readable'
event handler
is added.
If the data to be written can be generated or fetched on demand, it is
recommended to encapsulate the logic into a Readable and use
stream.pipe()
. However, if calling write()
is preferred, it is
possible to respect backpressure and avoid memory issues using the
'drain'
event:
function write(data, cb) {
if (!stream.write(data)) {
stream.once('drain', cb);
} else {
process.nextTick(cb);
}
}
// Wait for cb to be called before doing any other write.
write('hello', () => {
console.log('write completed, do more writes now');
});
A Writable stream in object mode will always ignore the encoding
argument.
writable.destroy([error])#
- Returns:
this
Destroy the stream, and emit the passed error. After this call, the
writable stream has ended. Implementors should not override this method,
but instead implement writable._destroy
.
Readable Streams#
Readable streams are an abstraction for a source from which data is consumed.
Examples of Readable streams include:
- HTTP responses, on the client
- HTTP requests, on the server
- fs read streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdout and stderr
process.stdin
All Readable streams implement the interface defined by the
stream.Readable
class.
Two Modes#
Readable streams effectively operate in one of two modes: flowing and paused.
When in flowing mode, data is read from the underlying system automatically
and provided to an application as quickly as possible using events via the
EventEmitter
interface.
In paused mode, the stream.read()
method must be called
explicitly to read chunks of data from the stream.
All Readable streams begin in paused mode but can be switched to flowing mode in one of the following ways:
- Adding a
'data'
event handler. - Calling the
stream.resume()
method. - Calling the
stream.pipe()
method to send the data to a Writable.
The Readable can switch back to paused mode using one of the following:
- If there are no pipe destinations, by calling the
stream.pause()
method. - If there are pipe destinations, by removing any
'data'
event handlers, and removing all pipe destinations by calling thestream.unpipe()
method.
The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.
Note: For backwards compatibility reasons, removing 'data'
event
handlers will not automatically pause the stream. Also, if there are piped
destinations, then calling stream.pause()
will not guarantee
that the stream will remain paused once those destinations drain and ask for
more data.
Note: If a Readable is switched into flowing mode and there are no
consumers available to handle the data, that data will be lost. This can occur,
for instance, when the readable.resume()
method is called without a listener
attached to the 'data'
event, or when a 'data'
event handler is removed
from the stream.
Three States#
The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.
Specifically, at any given point in time, every Readable is in one of three possible states:
readable._readableState.flowing = null
readable._readableState.flowing = false
readable._readableState.flowing = true
When readable._readableState.flowing
is null
, no mechanism for consuming the
streams data is provided so the stream will not generate its data. While in this
state, attaching a listener for the 'data'
event, calling the readable.pipe()
method, or calling the readable.resume()
method will switch
readable._readableState.flowing
to true
, causing the Readable to begin
actively emitting events as data is generated.
Calling readable.pause()
, readable.unpipe()
, or receiving "back pressure"
will cause the readable._readableState.flowing
to be set as false
,
temporarily halting the flowing of events but not halting the generation of
data. While in this state, attaching a listener for the 'data'
event
would not cause readable._readableState.flowing
to switch to true
.
const { PassThrough, Writable } = require('stream');
const pass = new PassThrough();
const writable = new Writable();
pass.pipe(writable);
pass.unpipe(writable);
// flowing is now false
pass.on('data', (chunk) => { console.log(chunk.toString()); });
pass.write('ok'); // will not emit 'data'
pass.resume(); // must be called to make 'data' being emitted
While readable._readableState.flowing
is false
, data may be accumulating
within the streams internal buffer.
Choose One#
The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream.
Use of the readable.pipe()
method is recommended for most users as it has been
implemented to provide the easiest way of consuming stream data. Developers that
require more fine-grained control over the transfer and generation of data can
use the EventEmitter
and readable.pause()
/readable.resume()
APIs.
Class: stream.Readable#
Event: 'close'#
The 'close'
event is emitted when the stream and any of its underlying
resources (a file descriptor, for example) have been closed. The event indicates
that no more events will be emitted, and no further computation will occur.
Not all Readable streams will emit the 'close'
event.
Event: 'data'#
chunk
<Buffer> | <string> | <any> The chunk of data. For streams that are not operating in object mode, the chunk will be either a string orBuffer
. For streams that are in object mode, the chunk can be any JavaScript value other thannull
.
The 'data'
event is emitted whenever the stream is relinquishing ownership of
a chunk of data to a consumer. This may occur whenever the stream is switched
in flowing mode by calling readable.pipe()
, readable.resume()
, or by
attaching a listener callback to the 'data'
event. The 'data'
event will
also be emitted whenever the readable.read()
method is called and a chunk of
data is available to be returned.
Attaching a 'data'
event listener to a stream that has not been explicitly
paused will switch the stream into flowing mode. Data will then be passed as
soon as it is available.
The listener callback will be passed the chunk of data as a string if a default
encoding has been specified for the stream using the
readable.setEncoding()
method; otherwise the data will be passed as a
Buffer
.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
});
Event: 'end'#
The 'end'
event is emitted when there is no more data to be consumed from
the stream.
Note: The 'end'
event will not be emitted unless the data is
completely consumed. This can be accomplished by switching the stream into
flowing mode, or by calling stream.read()
repeatedly until
all data has been consumed.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
console.log('There will be no more data.');
});
Event: 'error'#
The 'error'
event may be emitted by a Readable implementation at any time.
Typically, this may occur if the underlying stream is unable to generate data
due to an underlying internal failure, or when a stream implementation attempts
to push an invalid chunk of data.
The listener callback will be passed a single Error
object.
Event: 'readable'#
The 'readable'
event is emitted when there is data available to be read from
the stream. In some cases, attaching a listener for the 'readable'
event will
cause some amount of data to be read into an internal buffer.
const readable = getReadableStreamSomehow();
readable.on('readable', () => {
// there is some data to read now
});
The 'readable'
event will also be emitted once the end of the stream data
has been reached but before the 'end'
event is emitted.
Effectively, the 'readable'
event indicates that the stream has new
information: either new data is available or the end of the stream has been
reached. In the former case, stream.read()
will return the
available data. In the latter case, stream.read()
will return
null
. For instance, in the following example, foo.txt
is an empty file:
const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
console.log('readable:', rr.read());
});
rr.on('end', () => {
console.log('end');
});
The output of running this script is:
$ node test.js
readable: null
end
Note: In general, the readable.pipe()
and 'data'
event mechanisms are
easier to understand than the 'readable'
event.
However, handling 'readable'
might result in increased throughput.
readable.isPaused()#
- Returns: <boolean>
The readable.isPaused()
method returns the current operating state of the
Readable. This is used primarily by the mechanism that underlies the
readable.pipe()
method. In most typical cases, there will be no reason to
use this method directly.
const readable = new stream.Readable();
readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false
readable.pause()#
- Returns:
this
The readable.pause()
method will cause a stream in flowing mode to stop
emitting 'data'
events, switching out of flowing mode. Any data that
becomes available will remain in the internal buffer.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
readable.pause();
console.log('There will be no additional data for 1 second.');
setTimeout(() => {
console.log('Now data will start flowing again.');
readable.resume();
}, 1000);
});
readable.pipe(destination[, options])#
destination
<stream.Writable> The destination for writing dataoptions
<Object> Pipe optionsend
<boolean> End the writer when the reader ends. Defaults totrue
.
The readable.pipe()
method attaches a Writable stream to the readable
,
causing it to switch automatically into flowing mode and push all of its data
to the attached Writable. The flow of data will be automatically managed so
that the destination Writable stream is not overwhelmed by a faster Readable
stream.
The following example pipes all of the data from the readable
into a file
named file.txt
:
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'
readable.pipe(writable);
It is possible to attach multiple Writable streams to a single Readable stream.
The readable.pipe()
method returns a reference to the destination stream
making it possible to set up chains of piped streams:
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);
By default, stream.end()
is called on the destination Writable
stream when the source Readable stream emits 'end'
, so that the
destination is no longer writable. To disable this default behavior, the end
option can be passed as false
, causing the destination stream to remain open,
as illustrated in the following example:
reader.pipe(writer, { end: false });
reader.on('end', () => {
writer.end('Goodbye\n');
});
One important caveat is that if the Readable stream emits an error during processing, the Writable destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.
Note: The process.stderr
and process.stdout
Writable streams are
never closed until the Node.js process exits, regardless of the specified
options.
readable.read([size])#
size
<number> Optional argument to specify how much data to read.- Return <string> | <Buffer> | <null>
The readable.read()
method pulls some data out of the internal buffer and
returns it. If no data available to be read, null
is returned. By default,
the data will be returned as a Buffer
object unless an encoding has been
specified using the readable.setEncoding()
method or the stream is operating
in object mode.
The optional size
argument specifies a specific number of bytes to read. If
size
bytes are not available to be read, null
will be returned unless
the stream has ended, in which case all of the data remaining in the internal
buffer will be returned (even if it exceeds size
bytes).
If the size
argument is not specified, all of the data contained in the
internal buffer will be returned.
The readable.read()
method should only be called on Readable streams operating
in paused mode. In flowing mode, readable.read()
is called automatically until
the internal buffer is fully drained.
const readable = getReadableStreamSomehow();
readable.on('readable', () => {
let chunk;
while (null !== (chunk = readable.read())) {
console.log(`Received ${chunk.length} bytes of data.`);
}
});
In general, it is recommended that developers avoid the use of the 'readable'
event and the readable.read()
method in favor of using either
readable.pipe()
or the 'data'
event.
A Readable stream in object mode will always return a single item from
a call to readable.read(size)
, regardless of the value of the
size
argument.
Note: If the readable.read()
method returns a chunk of data, a 'data'
event will also be emitted.
Note: Calling stream.read([size])
after the 'end'
event has been emitted will return null
. No runtime error will be raised.
readable.resume()#
- Returns:
this
The readable.resume()
method causes an explicitly paused Readable stream to
resume emitting 'data'
events, switching the stream into flowing mode.
The readable.resume()
method can be used to fully consume the data from a
stream without actually processing any of that data as illustrated in the
following example:
getReadableStreamSomehow()
.resume()
.on('end', () => {
console.log('Reached the end, but did not read anything.');
});
readable.setEncoding(encoding)#
encoding
<string> The encoding to use.- Returns:
this
The readable.setEncoding()
method sets the character encoding for
data read from the Readable stream.
By default, no encoding is assigned and stream data will be returned as
Buffer
objects. Setting an encoding causes the stream data
to be returned as strings of the specified encoding rather than as Buffer
objects. For instance, calling readable.setEncoding('utf8')
will cause the
output data to be interpreted as UTF-8 data, and passed as strings. Calling
readable.setEncoding('hex')
will cause the data to be encoded in hexadecimal
string format.
The Readable stream will properly handle multi-byte characters delivered through
the stream that would otherwise become improperly decoded if simply pulled from
the stream as Buffer
objects.
const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
assert.equal(typeof chunk, 'string');
console.log('got %d characters of string data', chunk.length);
});
readable.unpipe([destination])#
destination
<stream.Writable> Optional specific stream to unpipe
The readable.unpipe()
method detaches a Writable stream previously attached
using the stream.pipe()
method.
If the destination
is not specified, then all pipes are detached.
If the destination
is specified, but no pipe is set up for it, then
the method does nothing.
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second
readable.pipe(writable);
setTimeout(() => {
console.log('Stop writing to file.txt');
readable.unpipe(writable);
console.log('Manually close the file stream');
writable.end();
}, 1000);
readable.unshift(chunk)#
chunk
<Buffer> | <Uint8Array> | <string> | <any> Chunk of data to unshift onto the read queue. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.
The readable.unshift()
method pushes a chunk of data back into the internal
buffer. This is useful in certain situations where a stream is being consumed by
code that needs to "un-consume" some amount of data that it has optimistically
pulled out of the source, so that the data can be passed on to some other party.
Note: The stream.unshift(chunk)
method cannot be called after the
'end'
event has been emitted or a runtime error will be thrown.
Developers using stream.unshift()
often should consider switching to
use of a Transform stream instead. See the API for Stream Implementers
section for more information.
// Pull off a header delimited by \n\n
// use unshift() if we get too much
// Call the callback with (error, header, stream)
const { StringDecoder } = require('string_decoder');
function parseHeader(stream, callback) {
stream.on('error', callback);
stream.on('readable', onReadable);
const decoder = new StringDecoder('utf8');
let header = '';
function onReadable() {
let chunk;
while (null !== (chunk = stream.read())) {
const str = decoder.write(chunk);
if (str.match(/\n\n/)) {
// found the header boundary
const split = str.split(/\n\n/);
header += split.shift();
const remaining = split.join('\n\n');
const buf = Buffer.from(remaining, 'utf8');
stream.removeListener('error', callback);
// remove the readable listener before unshifting
stream.removeListener('readable', onReadable);
if (buf.length)
stream.unshift(buf);
// now the body of the message can be read from the stream.
callback(null, header, stream);
} else {
// still reading the header.
header += str;
}
}
}
}
Note: Unlike stream.push(chunk)
, stream.unshift(chunk)
will not end the reading process by resetting the internal reading state of the
stream. This can cause unexpected results if readable.unshift()
is called
during a read (i.e. from within a stream._read()
implementation on a custom stream). Following the call to readable.unshift()
with an immediate stream.push('')
will reset the reading state
appropriately, however it is best to simply avoid calling readable.unshift()
while in the process of performing a read.
readable.wrap(stream)#
stream
<Stream> An "old style" readable stream
Versions of Node.js prior to v0.10 had streams that did not implement the
entire stream
module API as it is currently defined. (See Compatibility
for more information.)
When using an older Node.js library that emits 'data'
events and has a
stream.pause()
method that is advisory only, the
readable.wrap()
method can be used to create a Readable stream that uses
the old stream as its data source.
It will rarely be necessary to use readable.wrap()
but the method has been
provided as a convenience for interacting with older Node.js applications and
libraries.
For example:
const { OldReader } = require('./old-api-module.js');
const { Readable } = require('stream');
const oreader = new OldReader();
const myReader = new Readable().wrap(oreader);
myReader.on('readable', () => {
myReader.read(); // etc.
});
readable.destroy([error])#
Destroy the stream, and emit 'error'
. After this call, the
readable stream will release any internal resources.
Implementors should not override this method, but instead implement
readable._destroy
.
Duplex and Transform Streams#
Class: stream.Duplex#
Duplex streams are streams that implement both the Readable and Writable interfaces.
Examples of Duplex streams include:
Class: stream.Transform#
Transform streams are Duplex streams where the output is in some way related to the input. Like all Duplex streams, Transform streams implement both the Readable and Writable interfaces.
Examples of Transform streams include:
transform.destroy([error])#
Destroy the stream, and emit 'error'
. After this call, the
transform stream would release any internal resources.
implementors should not override this method, but instead implement
readable._destroy
.
The default implementation of _destroy
for Transform
also emit 'close'
.
API for Stream Implementers#
The stream
module API has been designed to make it possible to easily
implement streams using JavaScript's prototypal inheritance model.
First, a stream developer would declare a new JavaScript class that extends one
of the four basic stream classes (stream.Writable
, stream.Readable
,
stream.Duplex
, or stream.Transform
), making sure they call the appropriate
parent class constructor:
const { Writable } = require('stream');
class MyWritable extends Writable {
constructor(options) {
super(options);
// ...
}
}
The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:
Use-case |
Class |
Method(s) to implement |
---|---|---|
Reading only |
||
Writing only |
||
Reading and writing |
||
Operate on written data, then read the result |
Note: The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for Stream Consumers section). Doing so may lead to adverse side effects in application code consuming the stream.
Simplified Construction#
For many simple cases, it is possible to construct a stream without relying on
inheritance. This can be accomplished by directly creating instances of the
stream.Writable
, stream.Readable
, stream.Duplex
or stream.Transform
objects and passing appropriate methods as constructor options.
For example:
const { Writable } = require('stream');
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
}
});
Implementing a Writable Stream#
The stream.Writable
class is extended to implement a Writable stream.
Custom Writable streams must call the new stream.Writable([options])
constructor and implement the writable._write()
method. The
writable._writev()
method may also be implemented.
Constructor: new stream.Writable(options)#
options
<Object>highWaterMark
<number> Buffer level whenstream.write()
starts returningfalse
. Defaults to16384
(16kb), or16
forobjectMode
streams.decodeStrings
<boolean> Whether or not to decode strings into Buffers before passing them tostream._write()
. Defaults totrue
objectMode
<boolean> Whether or not thestream.write(anyObj)
is a valid operation. When set, it becomes possible to write JavaScript values other than string,Buffer
orUint8Array
if supported by the stream implementation. Defaults tofalse
write
<Function> Implementation for thestream._write()
method.writev
<Function> Implementation for thestream._writev()
method.destroy
<Function> Implementation for thestream._destroy()
method.final
<Function> Implementation for thestream._final()
method.
For example:
const { Writable } = require('stream');
class MyWritable extends Writable {
constructor(options) {
// Calls the stream.Writable() constructor
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Writable } = require('stream');
const util = require('util');
function MyWritable(options) {
if (!(this instanceof MyWritable))
return new MyWritable(options);
Writable.call(this, options);
}
util.inherits(MyWritable, Writable);
Or, using the Simplified Constructor approach:
const { Writable } = require('stream');
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
},
writev(chunks, callback) {
// ...
}
});
writable._write(chunk, encoding, callback)#
chunk
<Buffer> | <string> | <any> The chunk to be written. Will always be a buffer unless thedecodeStrings
option was set tofalse
or the stream is operating in object mode.encoding
<string> If the chunk is a string, thenencoding
is the character encoding of that string. If chunk is aBuffer
, or if the stream is operating in object mode,encoding
may be ignored.callback
<Function> Call this function (optionally with an error argument) when processing is complete for the supplied chunk.
All Writable stream implementations must provide a
writable._write()
method to send data to the underlying
resource.
Note: Transform streams provide their own implementation of the
writable._write()
.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Writable class methods only.
The callback
method must be called to signal either that the write completed
successfully or failed with an error. The first argument passed to the
callback
must be the Error
object if the call failed or null
if the
write succeeded.
It is important to note that all calls to writable.write()
that occur between
the time writable._write()
is called and the callback
is called will cause
the written data to be buffered. Once the callback
is invoked, the stream will
emit a 'drain'
event. If a stream implementation is capable of processing
multiple chunks of data at once, the writable._writev()
method should be
implemented.
If the decodeStrings
property is set in the constructor options, then
chunk
may be a string rather than a Buffer, and encoding
will
indicate the character encoding of the string. This is to support
implementations that have an optimized handling for certain string
data encodings. If the decodeStrings
property is explicitly set to false
,
the encoding
argument can be safely ignored, and chunk
will remain the same
object that is passed to .write()
.
The writable._write()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
writable._writev(chunks, callback)#
chunks
<Array> The chunks to be written. Each chunk has following format:{ chunk: ..., encoding: ... }
.callback
<Function> A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Writable class methods only.
The writable._writev()
method may be implemented in addition to
writable._write()
in stream implementations that are capable of processing
multiple chunks of data at once. If implemented, the method will be called with
all chunks of data currently buffered in the write queue.
The writable._writev()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
writable._destroy(err, callback)#
err
<Error> An error.callback
<Function> A callback function that takes an optional error argument which is invoked when the writable is destroyed.
writable._final(callback)#
callback
<Function> Call this function (optionally with an error argument) when you are done writing any remaining data.
Note: _final()
must not be called directly. It MAY be implemented
by child classes, and if so, will be called by the internal Writable
class methods only.
This optional function will be called before the stream closes, delaying the
finish
event until callback
is called. This is useful to close resources
or write buffered data before a stream ends.
Errors While Writing#
It is recommended that errors occurring during the processing of the
writable._write()
and writable._writev()
methods are reported by invoking
the callback and passing the error as the first argument. This will cause an
'error'
event to be emitted by the Writable. Throwing an Error from within
writable._write()
can result in unexpected and inconsistent behavior depending
on how the stream is being used. Using the callback ensures consistent and
predictable handling of errors.
const { Writable } = require('stream');
const myWritable = new Writable({
write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
}
});
An Example Writable Stream#
The following illustrates a rather simplistic (and somewhat pointless) custom Writable stream implementation. While this specific Writable stream instance is not of any real particular usefulness, the example illustrates each of the required elements of a custom Writable stream instance:
const { Writable } = require('stream');
class MyWritable extends Writable {
constructor(options) {
super(options);
// ...
}
_write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
}
}
Implementing a Readable Stream#
The stream.Readable
class is extended to implement a Readable stream.
Custom Readable streams must call the new stream.Readable([options])
constructor and implement the readable._read()
method.
new stream.Readable(options)#
options
<Object>highWaterMark
<number> The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Defaults to16384
(16kb), or16
forobjectMode
streamsencoding
<string> If specified, then buffers will be decoded to strings using the specified encoding. Defaults tonull
objectMode
<boolean> Whether this stream should behave as a stream of objects. Meaning thatstream.read(n)
returns a single value instead of a Buffer of size n. Defaults tofalse
read
<Function> Implementation for thestream._read()
method.destroy
<Function> Implementation for thestream._destroy()
method.
For example:
const { Readable } = require('stream');
class MyReadable extends Readable {
constructor(options) {
// Calls the stream.Readable(options) constructor
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Readable } = require('stream');
const util = require('util');
function MyReadable(options) {
if (!(this instanceof MyReadable))
return new MyReadable(options);
Readable.call(this, options);
}
util.inherits(MyReadable, Readable);
Or, using the Simplified Constructor approach:
const { Readable } = require('stream');
const myReadable = new Readable({
read(size) {
// ...
}
});
readable._read(size)#
size
<number> Number of bytes to read asynchronously
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.
All Readable stream implementations must provide an implementation of the
readable._read()
method to fetch data from the underlying resource.
When readable._read()
is called, if data is available from the resource, the
implementation should begin pushing that data into the read queue using the
this.push(dataChunk)
method. _read()
should continue reading
from the resource and pushing data until readable.push()
returns false
. Only
when _read()
is called again after it has stopped should it resume pushing
additional data onto the queue.
Note: Once the readable._read()
method has been called, it will not be
called again until the readable.push()
method is called.
The size
argument is advisory. For implementations where a "read" is a
single operation that returns data can use the size
argument to determine how
much data to fetch. Other implementations may ignore this argument and simply
provide data whenever it becomes available. There is no need to "wait" until
size
bytes are available before calling stream.push(chunk)
.
The readable._read()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
readable.push(chunk[, encoding])#
chunk
<Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to push into the read queue. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value.encoding
<string> Encoding of string chunks. Must be a valid Buffer encoding, such as'utf8'
or'ascii'
- Returns <boolean>
true
if additional chunks of data may continued to be pushed;false
otherwise.
When chunk
is a Buffer
, Uint8Array
or string
, the chunk
of data will
be added to the internal queue for users of the stream to consume.
Passing chunk
as null
signals the end of the stream (EOF), after which no
more data can be written.
When the Readable is operating in paused mode, the data added with
readable.push()
can be read out by calling the
readable.read()
method when the 'readable'
event is
emitted.
When the Readable is operating in flowing mode, the data added with
readable.push()
will be delivered by emitting a 'data'
event.
The readable.push()
method is designed to be as flexible as possible. For
example, when wrapping a lower-level source that provides some form of
pause/resume mechanism, and a data callback, the low-level source can be wrapped
by the custom Readable instance as illustrated in the following example:
// source is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.
class SourceWrapper extends Readable {
constructor(options) {
super(options);
this._source = getLowlevelSourceObject();
// Every time there's data, push it into the internal buffer.
this._source.ondata = (chunk) => {
// if push() returns false, then stop reading from source
if (!this.push(chunk))
this._source.readStop();
};
// When the source ends, push the EOF-signaling `null` chunk
this._source.onend = () => {
this.push(null);
};
}
// _read will be called when the stream wants to pull more data in
// the advisory size argument is ignored in this case.
_read(size) {
this._source.readStart();
}
}
Note: The readable.push()
method is intended be called only by Readable
Implementers, and only from within the readable._read()
method.
Errors While Reading#
It is recommended that errors occurring during the processing of the
readable._read()
method are emitted using the 'error'
event rather than
being thrown. Throwing an Error from within readable._read()
can result in
unexpected and inconsistent behavior depending on whether the stream is
operating in flowing or paused mode. Using the 'error'
event ensures
consistent and predictable handling of errors.
const { Readable } = require('stream');
const myReadable = new Readable({
read(size) {
if (checkSomeErrorCondition()) {
process.nextTick(() => this.emit('error', err));
return;
}
// do some work
}
});
An Example Counting Stream#
The following is a basic example of a Readable stream that emits the numerals from 1 to 1,000,000 in ascending order, and then ends.
const { Readable } = require('stream');
class Counter extends Readable {
constructor(opt) {
super(opt);
this._max = 1000000;
this._index = 1;
}
_read() {
const i = this._index++;
if (i > this._max)
this.push(null);
else {
const str = '' + i;
const buf = Buffer.from(str, 'ascii');
this.push(buf);
}
}
}
Implementing a Duplex Stream#
A Duplex stream is one that implements both Readable and Writable, such as a TCP socket connection.
Because JavaScript does not have support for multiple inheritance, the
stream.Duplex
class is extended to implement a Duplex stream (as opposed
to extending the stream.Readable
and stream.Writable
classes).
Note: The stream.Duplex
class prototypically inherits from
stream.Readable
and parasitically from stream.Writable
, but instanceof
will work properly for both base classes due to overriding
Symbol.hasInstance
on stream.Writable
.
Custom Duplex streams must call the new stream.Duplex([options])
constructor and implement both the readable._read()
and
writable._write()
methods.
new stream.Duplex(options)#
options
<Object> Passed to both Writable and Readable constructors. Also has the following fields:allowHalfOpen
<boolean> Defaults totrue
. If set tofalse
, then the stream will automatically end the writable side when the readable side ends.readableObjectMode
<boolean> Defaults tofalse
. SetsobjectMode
for readable side of the stream. Has no effect ifobjectMode
istrue
.writableObjectMode
<boolean> Defaults tofalse
. SetsobjectMode
for writable side of the stream. Has no effect ifobjectMode
istrue
.
For example:
const { Duplex } = require('stream');
class MyDuplex extends Duplex {
constructor(options) {
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Duplex } = require('stream');
const util = require('util');
function MyDuplex(options) {
if (!(this instanceof MyDuplex))
return new MyDuplex(options);
Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);
Or, using the Simplified Constructor approach:
const { Duplex } = require('stream');
const myDuplex = new Duplex({
read(size) {
// ...
},
write(chunk, encoding, callback) {
// ...
}
});
An Example Duplex Stream#
The following illustrates a simple example of a Duplex stream that wraps a hypothetical lower-level source object to which data can be written, and from which data can be read, albeit using an API that is not compatible with Node.js streams. The following illustrates a simple example of a Duplex stream that buffers incoming written data via the Writable interface that is read back out via the Readable interface.
const { Duplex } = require('stream');
const kSource = Symbol('source');
class MyDuplex extends Duplex {
constructor(source, options) {
super(options);
this[kSource] = source;
}
_write(chunk, encoding, callback) {
// The underlying source only deals with strings
if (Buffer.isBuffer(chunk))
chunk = chunk.toString();
this[kSource].writeSomeData(chunk);
callback();
}
_read(size) {
this[kSource].fetchSomeData(size, (data, encoding) => {
this.push(Buffer.from(data, encoding));
});
}
}
The most important aspect of a Duplex stream is that the Readable and Writable sides operate independently of one another despite co-existing within a single object instance.
Object Mode Duplex Streams#
For Duplex streams, objectMode
can be set exclusively for either the Readable
or Writable side using the readableObjectMode
and writableObjectMode
options
respectively.
In the following example, for instance, a new Transform stream (which is a type of Duplex stream) is created that has an object mode Writable side that accepts JavaScript numbers that are converted to hexadecimal strings on the Readable side.
const { Transform } = require('stream');
// All Transform streams are also Duplex Streams
const myTransform = new Transform({
writableObjectMode: true,
transform(chunk, encoding, callback) {
// Coerce the chunk to a number if necessary
chunk |= 0;
// Transform the chunk into something else.
const data = chunk.toString(16);
// Push the data onto the readable queue.
callback(null, '0'.repeat(data.length % 2) + data);
}
});
myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));
myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64
Implementing a Transform Stream#
A Transform stream is a Duplex stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data.
Note: There is no requirement that the output be the same size as the input,
the same number of chunks, or arrive at the same time. For example, a
Hash stream will only ever have a single chunk of output which is
provided when the input is ended. A zlib
stream will produce output
that is either much smaller or much larger than its input.
The stream.Transform
class is extended to implement a Transform stream.
The stream.Transform
class prototypically inherits from stream.Duplex
and
implements its own versions of the writable._write()
and readable._read()
methods. Custom Transform implementations must implement the
transform._transform()
method and may also implement
the transform._flush()
method.
Note: Care must be taken when using Transform streams in that data written to the stream can cause the Writable side of the stream to become paused if the output on the Readable side is not consumed.
new stream.Transform(options)#
options
<Object> Passed to both Writable and Readable constructors. Also has the following fields:transform
<Function> Implementation for thestream._transform()
method.flush
<Function> Implementation for thestream._flush()
method.
For example:
const { Transform } = require('stream');
class MyTransform extends Transform {
constructor(options) {
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Transform } = require('stream');
const util = require('util');
function MyTransform(options) {
if (!(this instanceof MyTransform))
return new MyTransform(options);
Transform.call(this, options);
}
util.inherits(MyTransform, Transform);
Or, using the Simplified Constructor approach:
const { Transform } = require('stream');
const myTransform = new Transform({
transform(chunk, encoding, callback) {
// ...
}
});
Events: 'finish' and 'end'#
The 'finish'
and 'end'
events are from the stream.Writable
and stream.Readable
classes, respectively. The 'finish'
event is emitted
after stream.end()
is called and all chunks have been processed
by stream._transform()
. The 'end'
event is emitted
after all data has been output, which occurs after the callback in
transform._flush()
has been called.
transform._flush(callback)#
callback
<Function> A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.
In some cases, a transform operation may need to emit an additional bit of
data at the end of the stream. For example, a zlib
compression stream will
store an amount of internal state used to optimally compress the output. When
the stream ends, however, that additional data needs to be flushed so that the
compressed data will be complete.
Custom Transform implementations may implement the transform._flush()
method. This will be called when there is no more written data to be consumed,
but before the 'end'
event is emitted signaling the end of the
Readable stream.
Within the transform._flush()
implementation, the readable.push()
method
may be called zero or more times, as appropriate. The callback
function must
be called when the flush operation is complete.
The transform._flush()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
transform._transform(chunk, encoding, callback)#
chunk
<Buffer> | <string> | <any> The chunk to be transformed. Will always be a buffer unless thedecodeStrings
option was set tofalse
or the stream is operating in object mode.encoding
<string> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value - 'buffer', ignore it in this case.callback
<Function> A callback function (optionally with an error argument and data) to be called after the suppliedchunk
has been processed.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.
All Transform stream implementations must provide a _transform()
method to accept input and produce output. The transform._transform()
implementation handles the bytes being written, computes an output, then passes
that output off to the readable portion using the readable.push()
method.
The transform.push()
method may be called zero or more times to generate
output from a single input chunk, depending on how much is to be output
as a result of the chunk.
It is possible that no output is generated from any given chunk of input data.
The callback
function must be called only when the current chunk is completely
consumed. The first argument passed to the callback
must be an Error
object
if an error occurred while processing the input or null
otherwise. If a second
argument is passed to the callback
, it will be forwarded on to the
readable.push()
method. In other words the following are equivalent:
transform.prototype._transform = function(data, encoding, callback) {
this.push(data);
callback();
};
transform.prototype._transform = function(data, encoding, callback) {
callback(null, data);
};
The transform._transform()
method is prefixed with an underscore because it
is internal to the class that defines it, and should never be called directly by
user programs.
transform._transform()
is never called in parallel; streams implement a
queue mechanism, and to receive the next chunk, callback
must be
called, either synchronously or asynchronously.
Class: stream.PassThrough#
The stream.PassThrough
class is a trivial implementation of a Transform
stream that simply passes the input bytes across to the output. Its purpose is
primarily for examples and testing, but there are some use cases where
stream.PassThrough
is useful as a building block for novel sorts of streams.
Additional Notes#
Compatibility with Older Node.js Versions#
In versions of Node.js prior to v0.10, the Readable stream interface was simpler, but also less powerful and less useful.
- Rather than waiting for calls the
stream.read()
method,'data'
events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost. - The
stream.pause()
method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive'data'
events even when the stream was in a paused state.
In Node.js v0.10, the Readable class was added. For backwards compatibility
with older Node.js programs, Readable streams switch into "flowing mode" when a
'data'
event handler is added, or when the
stream.resume()
method is called. The effect is that, even
when not using the new stream.read()
method and
'readable'
event, it is no longer necessary to worry about losing
'data'
chunks.
While most applications will continue to function normally, this introduces an edge case in the following conditions:
- No
'data'
event listener is added. - The
stream.resume()
method is never called. - The stream is not piped to any writable destination.
For example, consider the following code:
// WARNING! BROKEN!
net.createServer((socket) => {
// we add an 'end' method, but never consume the data
socket.on('end', () => {
// It will never get here.
socket.end('The message was received but was not processed.\n');
});
}).listen(1337);
In versions of Node.js prior to v0.10, the incoming message data would be simply discarded. However, in Node.js v0.10 and beyond, the socket remains paused forever.
The workaround in this situation is to call the
stream.resume()
method to begin the flow of data:
// Workaround
net.createServer((socket) => {
socket.on('end', () => {
socket.end('The message was received but was not processed.\n');
});
// start the flow of data, discarding it.
socket.resume();
}).listen(1337);
In addition to new Readable streams switching into flowing mode,
pre-v0.10 style streams can be wrapped in a Readable class using the
readable.wrap()
method.
readable.read(0)
#
There are some cases where it is necessary to trigger a refresh of the
underlying readable stream mechanisms, without actually consuming any
data. In such cases, it is possible to call readable.read(0)
, which will
always return null
.
If the internal read buffer is below the highWaterMark
, and the
stream is not currently reading, then calling stream.read(0)
will trigger
a low-level stream._read()
call.
While most applications will almost never need to do this, there are situations within Node.js where this is done, particularly in the Readable stream class internals.
readable.push('')
#
Use of readable.push('')
is not recommended.
Pushing a zero-byte string, Buffer
or Uint8Array
to a stream that is not in
object mode has an interesting side effect. Because it is a call to
readable.push()
, the call will end the reading process.
However, because the argument is an empty string, no data is added to the
readable buffer so there is nothing for a user to consume.
String Decoder#
Stability: 2 - Stable
The string_decoder
module provides an API for decoding Buffer
objects into
strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16
characters. It can be accessed using:
const { StringDecoder } = require('string_decoder');
The following example shows the basic use of the StringDecoder
class.
const { StringDecoder } = require('string_decoder');
const decoder = new StringDecoder('utf8');
const cent = Buffer.from([0xC2, 0xA2]);
console.log(decoder.write(cent));
const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(decoder.write(euro));
When a Buffer
instance is written to the StringDecoder
instance, an
internal buffer is used to ensure that the decoded string does not contain
any incomplete multibyte characters. These are held in the buffer until the
next call to stringDecoder.write()
or until stringDecoder.end()
is called.
In the following example, the three UTF-8 encoded bytes of the European Euro
symbol (€
) are written over three separate operations:
const { StringDecoder } = require('string_decoder');
const decoder = new StringDecoder('utf8');
decoder.write(Buffer.from([0xE2]));
decoder.write(Buffer.from([0x82]));
console.log(decoder.end(Buffer.from([0xAC])));
Class: new StringDecoder([encoding])#
encoding
<string> The character encoding theStringDecoder
will use. Defaults to'utf8'
.
Creates a new StringDecoder
instance.
stringDecoder.end(buffer)#
buffer
<Buffer> ABuffer
containing the bytes to decode.
Returns any remaining input stored in the internal buffer as a string. Bytes representing incomplete UTF-8 and UTF-16 characters will be replaced with substitution characters appropriate for the character encoding.
If the buffer
argument is provided, one final call to stringDecoder.write()
is performed before returning the remaining input.
stringDecoder.write(buffer)#
buffer
<Buffer> ABuffer
containing the bytes to decode.
Returns a decoded string, ensuring that any incomplete multibyte characters at
the end of the Buffer
are omitted from the returned string and stored in an
internal buffer for the next call to stringDecoder.write()
or
stringDecoder.end()
.
Timers#
Stability: 2 - Stable
The timer
module exposes a global API for scheduling functions to
be called at some future period of time. Because the timer functions are
globals, there is no need to call require('timers')
to use the API.
The timer functions within Node.js implement a similar API as the timers API provided by Web Browsers but use a different internal implementation that is built around the Node.js Event Loop.
Class: Immediate#
This object is created internally and is returned from setImmediate()
. It
can be passed to clearImmediate()
in order to cancel the scheduled
actions.
Class: Timeout#
This object is created internally and is returned from setTimeout()
and
setInterval()
. It can be passed to clearTimeout()
or
clearInterval()
(respectively) in order to cancel the scheduled actions.
By default, when a timer is scheduled using either setTimeout()
or
setInterval()
, the Node.js event loop will continue running as long as the
timer is active. Each of the Timeout
objects returned by these functions
export both timeout.ref()
and timeout.unref()
functions that can be used to
control this default behavior.
timeout.ref()#
When called, requests that the Node.js event loop not exit so long as the
Timeout
is active. Calling timeout.ref()
multiple times will have no effect.
Note: By default, all Timeout
objects are "ref'd", making it normally
unnecessary to call timeout.ref()
unless timeout.unref()
had been called
previously.
Returns a reference to the Timeout
.
timeout.unref()#
When called, the active Timeout
object will not require the Node.js event loop
to remain active. If there is no other activity keeping the event loop running,
the process may exit before the Timeout
object's callback is invoked. Calling
timeout.unref()
multiple times will have no effect.
Note: Calling timeout.unref()
creates an internal timer that will wake the
Node.js event loop. Creating too many of these can adversely impact performance
of the Node.js application.
Returns a reference to the Timeout
.
Scheduling Timers#
A timer in Node.js is an internal construct that calls a given function after a certain period of time. When a timer's function is called varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.
setImmediate(callback[, ...args])#
callback
<Function> The function to call at the end of this turn of the Node.js Event Loop...args
<any> Optional arguments to pass when thecallback
is called.
Schedules the "immediate" execution of the callback
after I/O events'
callbacks. Returns an Immediate
for use with clearImmediate()
.
When multiple calls to setImmediate()
are made, the callback
functions are
queued for execution in the order in which they are created. The entire callback
queue is processed every event loop iteration. If an immediate timer is queued
from inside an executing callback, that timer will not be triggered until the
next event loop iteration.
If callback
is not a function, a TypeError
will be thrown.
Note: This method has a custom variant for promises that is available using
util.promisify()
:
const util = require('util');
const setImmediatePromise = util.promisify(setImmediate);
setImmediatePromise('foobar').then((value) => {
// value === 'foobar' (passing values is optional)
// This is executed after all I/O callbacks.
});
// or with async function
async function timerExample() {
console.log('Before I/O callbacks');
await setImmediatePromise();
console.log('After I/O callbacks');
}
timerExample();
setInterval(callback, delay[, ...args])#
callback
<Function> The function to call when the timer elapses.delay
<number> The number of milliseconds to wait before calling thecallback
....args
<any> Optional arguments to pass when thecallback
is called.
Schedules repeated execution of callback
every delay
milliseconds.
Returns a Timeout
for use with clearInterval()
.
When delay
is larger than 2147483647
or less than 1
, the delay
will be
set to 1
.
If callback
is not a function, a TypeError
will be thrown.
setTimeout(callback, delay[, ...args])#
callback
<Function> The function to call when the timer elapses.delay
<number> The number of milliseconds to wait before calling thecallback
....args
<any> Optional arguments to pass when thecallback
is called.
Schedules execution of a one-time callback
after delay
milliseconds.
Returns a Timeout
for use with clearTimeout()
.
The callback
will likely not be invoked in precisely delay
milliseconds.
Node.js makes no guarantees about the exact timing of when callbacks will fire,
nor of their ordering. The callback will be called as close as possible to the
time specified.
Note: When delay
is larger than 2147483647
or less than 1
, the delay
will be set to 1
.
If callback
is not a function, a TypeError
will be thrown.
Note: This method has a custom variant for promises that is available using
util.promisify()
:
const util = require('util');
const setTimeoutPromise = util.promisify(setTimeout);
setTimeoutPromise(40, 'foobar').then((value) => {
// value === 'foobar' (passing values is optional)
// This is executed after about 40 milliseconds.
});
Cancelling Timers#
The setImmediate()
, setInterval()
, and setTimeout()
methods
each return objects that represent the scheduled timers. These can be used to
cancel the timer and prevent it from triggering.
It is not possible to cancel timers that were created using the promisified
variants of setImmediate()
, setTimeout()
.
clearImmediate(immediate)#
immediate
<Immediate> AnImmediate
object as returned bysetImmediate()
.
Cancels an Immediate
object created by setImmediate()
.
clearInterval(timeout)#
timeout
<Timeout> ATimeout
object as returned bysetInterval()
.
Cancels a Timeout
object created by setInterval()
.
clearTimeout(timeout)#
timeout
<Timeout> ATimeout
object as returned bysetTimeout()
.
Cancels a Timeout
object created by setTimeout()
.
TLS (SSL)#
Stability: 2 - Stable
The tls
module provides an implementation of the Transport Layer Security
(TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL.
The module can be accessed using:
const tls = require('tls');
TLS/SSL Concepts#
The TLS/SSL is a public/private key infrastructure (PKI). For most common cases, each client and server must have a private key.
Private keys can be generated in multiple ways. The example below illustrates use of the OpenSSL command-line interface to generate a 2048-bit RSA private key:
openssl genrsa -out ryans-key.pem 2048
With TLS/SSL, all servers (and some clients) must have a certificate. Certificates are public keys that correspond to a private key, and that are digitally signed either by a Certificate Authority or by the owner of the private key (such certificates are referred to as "self-signed"). The first step to obtaining a certificate is to create a Certificate Signing Request (CSR) file.
The OpenSSL command-line interface can be used to generate a CSR for a private key:
openssl req -new -sha256 -key ryans-key.pem -out ryans-csr.pem
Once the CSR file is generated, it can either be sent to a Certificate Authority for signing or used to generate a self-signed certificate.
Creating a self-signed certificate using the OpenSSL command-line interface is illustrated in the example below:
openssl x509 -req -in ryans-csr.pem -signkey ryans-key.pem -out ryans-cert.pem
Once the certificate is generated, it can be used to generate a .pfx
or
.p12
file:
openssl pkcs12 -export -in ryans-cert.pem -inkey ryans-key.pem \
-certfile ca-cert.pem -out ryans.pfx
Where:
in
: is the signed certificateinkey
: is the associated private keycertfile
: is a concatenation of all Certificate Authority (CA) certs into a single file, e.g.cat ca1-cert.pem ca2-cert.pem > ca-cert.pem
Perfect Forward Secrecy#
The term "Forward Secrecy" or "Perfect Forward Secrecy" describes a feature of key-agreement (i.e., key-exchange) methods. That is, the server and client keys are used to negotiate new temporary keys that are used specifically and only for the current communication session. Practically, this means that even if the server's private key is compromised, communication can only be decrypted by eavesdroppers if the attacker manages to obtain the key-pair specifically generated for the session.
Perfect Forward Secrecy is achieved by randomly generating a key pair for key-agreement on every TLS/SSL handshake (in contrast to using the same key for all sessions). Methods implementing this technique are called "ephemeral".
Currently two methods are commonly used to achieve Perfect Forward Secrecy (note the character "E" appended to the traditional abbreviations):
- DHE - An ephemeral version of the Diffie Hellman key-agreement protocol.
- ECDHE - An ephemeral version of the Elliptic Curve Diffie Hellman key-agreement protocol.
Ephemeral methods may have some performance drawbacks, because key generation is expensive.
To use Perfect Forward Secrecy using DHE
with the tls
module, it is required
to generate Diffie-Hellman parameters and specify them with the dhparam
option to tls.createSecureContext()
. The following illustrates the use of
the OpenSSL command-line interface to generate such parameters:
openssl dhparam -outform PEM -out dhparam.pem 2048
If using Perfect Forward Secrecy using ECDHE
, Diffie-Hellman parameters are
not required and a default ECDHE curve will be used. The ecdhCurve
property
can be used when creating a TLS Server to specify the name of an alternative
curve to use, see tls.createServer()
for more info.
ALPN, NPN and SNI#
ALPN (Application-Layer Protocol Negotiation Extension), NPN (Next Protocol Negotiation) and, SNI (Server Name Indication) are TLS handshake extensions:
- ALPN/NPN - Allows the use of one TLS server for multiple protocols (HTTP, SPDY, HTTP/2)
- SNI - Allows the use of one TLS server for multiple hostnames with different SSL certificates.
Note: Use of ALPN is recommended over NPN. The NPN extension has never been formally defined or documented and generally not recommended for use.
Client-initiated renegotiation attack mitigation#
The TLS protocol allows clients to renegotiate certain aspects of the TLS session. Unfortunately, session renegotiation requires a disproportionate amount of server-side resources, making it a potential vector for denial-of-service attacks.
To mitigate the risk, renegotiation is limited to three times every ten minutes.
An 'error'
event is emitted on the tls.TLSSocket
instance when this
threshold is exceeded. The limits are configurable:
tls.CLIENT_RENEG_LIMIT
<number> Specifies the number of renegotiation requests. Defaults to3
.tls.CLIENT_RENEG_WINDOW
<number> Specifies the time renegotiation window in seconds. Defaults to600
(10 minutes).
Note: The default renegotiation limits should not be modified without a full understanding of the implications and risks.
To test the renegotiation limits on a server, connect to it using the OpenSSL
command-line client (openssl s_client -connect address:port
) then input
R<CR>
(i.e., the letter R
followed by a carriage return) multiple times.
Modifying the Default TLS Cipher suite#
Node.js is built with a default suite of enabled and disabled TLS ciphers. Currently, the default cipher suite is:
ECDHE-RSA-AES128-GCM-SHA256:
ECDHE-ECDSA-AES128-GCM-SHA256:
ECDHE-RSA-AES256-GCM-SHA384:
ECDHE-ECDSA-AES256-GCM-SHA384:
DHE-RSA-AES128-GCM-SHA256:
ECDHE-RSA-AES128-SHA256:
DHE-RSA-AES128-SHA256:
ECDHE-RSA-AES256-SHA384:
DHE-RSA-AES256-SHA384:
ECDHE-RSA-AES256-SHA256:
DHE-RSA-AES256-SHA256:
HIGH:
!aNULL:
!eNULL:
!EXPORT:
!DES:
!RC4:
!MD5:
!PSK:
!SRP:
!CAMELLIA
This default can be replaced entirely using the --tls-cipher-list
command
line switch. For instance, the following makes
ECDHE-RSA-AES128-GCM-SHA256:!RC4
the default TLS cipher suite:
node --tls-cipher-list="ECDHE-RSA-AES128-GCM-SHA256:!RC4"
The default can also be replaced on a per client or server basis using the
ciphers
option from tls.createSecureContext()
, which is also available
in tls.createServer()
, tls.connect()
, and when creating new
tls.TLSSocket
s.
Consult OpenSSL cipher list format documentation for details on the format.
Note: The default cipher suite included within Node.js has been carefully
selected to reflect current security best practices and risk mitigation.
Changing the default cipher suite can have a significant impact on the security
of an application. The --tls-cipher-list
switch and ciphers
option should by
used only if absolutely necessary.
The default cipher suite prefers GCM ciphers for Chrome's 'modern cryptography' setting and also prefers ECDHE and DHE ciphers for Perfect Forward Secrecy, while offering some backward compatibility.
128 bit AES is preferred over 192 and 256 bit AES in light of specific attacks affecting larger AES key sizes.
Old clients that rely on insecure and deprecated RC4 or DES-based ciphers (like Internet Explorer 6) cannot complete the handshaking process with the default configuration. If these clients must be supported, the TLS recommendations may offer a compatible cipher suite. For more details on the format, see the OpenSSL cipher list format documentation.
Class: tls.Server#
The tls.Server
class is a subclass of net.Server
that accepts encrypted
connections using TLS or SSL.
Event: 'newSession'#
The 'newSession'
event is emitted upon creation of a new TLS session. This may
be used to store sessions in external storage. The listener callback is passed
three arguments when called:
sessionId
- The TLS session identifiersessionData
- The TLS session datacallback
<Function> A callback function taking no arguments that must be invoked in order for data to be sent or received over the secure connection.
Note: Listening for this event will have an effect only on connections established after the addition of the event listener.
Event: 'OCSPRequest'#
The 'OCSPRequest'
event is emitted when the client sends a certificate status
request. The listener callback is passed three arguments when called:
certificate
<Buffer> The server certificateissuer
<Buffer> The issuer's certificatecallback
<Function> A callback function that must be invoked to provide the results of the OCSP request.
The server's current certificate can be parsed to obtain the OCSP URL
and certificate ID; after obtaining an OCSP response, callback(null, resp)
is
then invoked, where resp
is a Buffer
instance containing the OCSP response.
Both certificate
and issuer
are Buffer
DER-representations of the
primary and issuer's certificates. These can be used to obtain the OCSP
certificate ID and OCSP endpoint URL.
Alternatively, callback(null, null)
may be called, indicating that there was
no OCSP response.
Calling callback(err)
will result in a socket.destroy(err)
call.
The typical flow of an OCSP Request is as follows:
- Client connects to the server and sends an
'OCSPRequest'
(via the status info extension in ClientHello). - Server receives the request and emits the
'OCSPRequest'
event, calling the listener if registered. - Server extracts the OCSP URL from either the
certificate
orissuer
and performs an OCSP request to the CA. - Server receives
OCSPResponse
from the CA and sends it back to the client via thecallback
argument - Client validates the response and either destroys the socket or performs a handshake.
Note: The issuer
can be null
if the certificate is either self-signed or
the issuer is not in the root certificates list. (An issuer may be provided
via the ca
option when establishing the TLS connection.)
Note: Listening for this event will have an effect only on connections established after the addition of the event listener.
Note: An npm module like asn1.js may be used to parse the certificates.
Event: 'resumeSession'#
The 'resumeSession'
event is emitted when the client requests to resume a
previous TLS session. The listener callback is passed two arguments when
called:
sessionId
- The TLS/SSL session identifiercallback
<Function> A callback function to be called when the prior session has been recovered.
When called, the event listener may perform a lookup in external storage using
the given sessionId
and invoke callback(null, sessionData)
once finished. If
the session cannot be resumed (i.e., doesn't exist in storage) the callback may
be invoked as callback(null, null)
. Calling callback(err)
will terminate the
incoming connection and destroy the socket.
Note: Listening for this event will have an effect only on connections established after the addition of the event listener.
The following illustrates resuming a TLS session:
const tlsSessionStore = {};
server.on('newSession', (id, data, cb) => {
tlsSessionStore[id.toString('hex')] = data;
cb();
});
server.on('resumeSession', (id, cb) => {
cb(null, tlsSessionStore[id.toString('hex')] || null);
});
Event: 'secureConnection'#
The 'secureConnection'
event is emitted after the handshaking process for a
new connection has successfully completed. The listener callback is passed a
single argument when called:
tlsSocket
<tls.TLSSocket> The established TLS socket.
The tlsSocket.authorized
property is a boolean
indicating whether the
client has been verified by one of the supplied Certificate Authorities for the
server. If tlsSocket.authorized
is false
, then socket.authorizationError
is set to describe how authorization failed. Note that depending on the settings
of the TLS server, unauthorized connections may still be accepted.
The tlsSocket.npnProtocol
and tlsSocket.alpnProtocol
properties are strings
that contain the selected NPN and ALPN protocols, respectively. When both NPN
and ALPN extensions are received, ALPN takes precedence over NPN and the next
protocol is selected by ALPN.
When ALPN has no selected protocol, tlsSocket.alpnProtocol
returns false
.
The tlsSocket.servername
property is a string containing the server name
requested via SNI.
Event: 'tlsClientError'#
The 'tlsClientError'
event is emitted when an error occurs before a secure
connection is established. The listener callback is passed two arguments when
called:
exception
<Error> TheError
object describing the errortlsSocket
<tls.TLSSocket> Thetls.TLSSocket
instance from which the error originated.
server.addContext(hostname, context)#
hostname
<string> A SNI hostname or wildcard (e.g.'*'
)context
<Object> An object containing any of the possible properties from thetls.createSecureContext()
options
arguments (e.g.key
,cert
,ca
, etc).
The server.addContext()
method adds a secure context that will be used if
the client request's SNI hostname matches the supplied hostname
(or wildcard).
server.address()#
Returns the bound address, the address family name, and port of the
server as reported by the operating system. See net.Server.address()
for
more information.
server.close([callback])#
callback
<Function> An optional listener callback that will be registered to listen for the server instance's'close'
event.
The server.close()
method stops the server from accepting new connections.
This function operates asynchronously. The 'close'
event will be emitted
when the server has no more open connections.
server.connections#
Stability: 0 - Deprecated: Use server.getConnections()
instead.
Returns the current number of concurrent connections on the server.
server.getTicketKeys()#
Returns a Buffer
instance holding the keys currently used for
encryption/decryption of the TLS Session Tickets
server.listen(port[, hostname][, callback])#
port
<number> The TCP/IP port on which to begin listening for connections. A value of0
(zero) will assign a random port.hostname
<string> The hostname, IPv4, or IPv6 address on which to begin listening for connections. Ifundefined
, the server will accept connections on any IPv6 address (::
) when IPv6 is available, or any IPv4 address (0.0.0.0
) otherwise.callback
<Function> A callback function to be invoked when the server has begun listening on theport
andhostname
.
The server.listen()
methods instructs the server to begin accepting
connections on the specified port
and hostname
.
This function operates asynchronously. If the callback
is given, it will be
called when the server has started listening.
See net.Server
for more information.
server.setTicketKeys(keys)#
keys
<Buffer> The keys used for encryption/decryption of the TLS Session Tickets.
Updates the keys for encryption/decryption of the TLS Session Tickets.
Note: The key's Buffer
should be 48 bytes long. See ticketKeys
option in
tls.createServer for
more information on how it is used.
Note: Changes to the ticket keys are effective only for future server connections. Existing or currently pending server connections will use the previous keys.
Class: tls.TLSSocket#
The tls.TLSSocket
is a subclass of net.Socket
that performs transparent
encryption of written data and all required TLS negotiation.
Instances of tls.TLSSocket
implement the duplex Stream interface.
Note: Methods that return TLS connection metadata (e.g.
tls.TLSSocket.getPeerCertificate()
will only return data while the
connection is open.
new tls.TLSSocket(socket[, options])#
socket
<net.Socket> An instance ofnet.Socket
options
<Object>isServer
: The SSL/TLS protocol is asymetrical, TLSSockets must know if they are to behave as a server or a client. Iftrue
the TLS socket will be instantiated as a server. Defaults tofalse
.server
<net.Server> An optionalnet.Server
instance.requestCert
: Whether to authenticate the remote peer by requesting a certificate. Clients always request a server certificate. Servers (isServer
is true) may optionally setrequestCert
to true to request a client certificate.rejectUnauthorized
: Optional, seetls.createServer()
NPNProtocols
: Optional, seetls.createServer()
ALPNProtocols
: Optional, seetls.createServer()
SNICallback
: Optional, seetls.createServer()
session
<Buffer> An optionalBuffer
instance containing a TLS session.requestOCSP
<boolean> Iftrue
, specifies that the OCSP status request extension will be added to the client hello and an'OCSPResponse'
event will be emitted on the socket before establishing a secure communicationsecureContext
: Optional TLS context object created withtls.createSecureContext()
. If asecureContext
is not provided, one will be created by passing the entireoptions
object totls.createSecureContext()
.- ...: Optional
tls.createSecureContext()
options that are used if thesecureContext
option is missing, otherwise they are ignored.
Construct a new tls.TLSSocket
object from an existing TCP socket.
Event: 'OCSPResponse'#
The 'OCSPResponse'
event is emitted if the requestOCSP
option was set
when the tls.TLSSocket
was created and an OCSP response has been received.
The listener callback is passed a single argument when called:
response
<Buffer> The server's OCSP response
Typically, the response
is a digitally signed object from the server's CA that
contains information about server's certificate revocation status.
Event: 'secureConnect'#
The 'secureConnect'
event is emitted after the handshaking process for a new
connection has successfully completed. The listener callback will be called
regardless of whether or not the server's certificate has been authorized. It
is the client's responsibility to check the tlsSocket.authorized
property to
determine if the server certificate was signed by one of the specified CAs. If
tlsSocket.authorized === false
, then the error can be found by examining the
tlsSocket.authorizationError
property. If either ALPN or NPN was used,
the tlsSocket.alpnProtocol
or tlsSocket.npnProtocol
properties can be
checked to determine the negotiated protocol.
tlsSocket.address()#
Returns the bound address, the address family name, and port of the
underlying socket as reported by the operating system. Returns an
object with three properties, e.g.,
{ port: 12346, family: 'IPv4', address: '127.0.0.1' }
tlsSocket.authorizationError#
Returns the reason why the peer's certificate was not been verified. This
property is set only when tlsSocket.authorized === false
.
tlsSocket.authorized#
Returns true
if the peer certificate was signed by one of the CAs specified
when creating the tls.TLSSocket
instance, otherwise false
.
tlsSocket.encrypted#
Always returns true
. This may be used to distinguish TLS sockets from regular
net.Socket
instances.
tlsSocket.getCipher()#
Returns an object representing the cipher name and the SSL/TLS protocol version that first defined the cipher.
For example: { name: 'AES256-SHA', version: 'TLSv1/SSLv3' }
See SSL_CIPHER_get_name()
and SSL_CIPHER_get_version()
in
https://www.openssl.org/docs/man1.0.2/ssl/SSL_CIPHER_get_name.html for more
information.
tlsSocket.getEphemeralKeyInfo()#
Returns an object representing the type, name, and size of parameter of
an ephemeral key exchange in Perfect Forward Secrecy on a client
connection. It returns an empty object when the key exchange is not
ephemeral. As this is only supported on a client socket; null
is returned
if called on a server socket. The supported types are 'DH'
and 'ECDH'
. The
name
property is available only when type is 'ECDH'.
For Example: { type: 'ECDH', name: 'prime256v1', size: 256 }
tlsSocket.getPeerCertificate([detailed])#
detailed
<boolean> Include the full certificate chain iftrue
, otherwise include just the peer's certificate.
Returns an object representing the peer's certificate. The returned object has some properties corresponding to the fields of the certificate.
If the full certificate chain was requested, each certificate will include a
issuerCertificate
property containing an object representing its issuer's
certificate.
For example:
{ subject:
{ C: 'UK',
ST: 'Acknack Ltd',
L: 'Rhys Jones',
O: 'node.js',
OU: 'Test TLS Certificate',
CN: 'localhost' },
issuer:
{ C: 'UK',
ST: 'Acknack Ltd',
L: 'Rhys Jones',
O: 'node.js',
OU: 'Test TLS Certificate',
CN: 'localhost' },
issuerCertificate:
{ ... another certificate, possibly with a .issuerCertificate ... },
raw: < RAW DER buffer >,
valid_from: 'Nov 11 09:52:22 2009 GMT',
valid_to: 'Nov 6 09:52:22 2029 GMT',
fingerprint: '2A:7A:C2:DD:E5:F9:CC:53:72:35:99:7A:02:5A:71:38:52:EC:8A:DF',
serialNumber: 'B9B0D332A1AA5635' }
If the peer does not provide a certificate, an empty object will be returned.
tlsSocket.getProtocol()#
Returns a string containing the negotiated SSL/TLS protocol version of the
current connection. The value 'unknown'
will be returned for connected
sockets that have not completed the handshaking process. The value null
will
be returned for server sockets or disconnected client sockets.
Example responses include:
SSLv3
TLSv1
TLSv1.1
TLSv1.2
unknown
See https://www.openssl.org/docs/man1.0.2/ssl/SSL_get_version.html for more information.
tlsSocket.getSession()#
Returns the ASN.1 encoded TLS session or undefined
if no session was
negotiated. Can be used to speed up handshake establishment when reconnecting
to the server.
tlsSocket.getTLSTicket()#
Returns the TLS session ticket or undefined
if no session was negotiated.
Note: This only works with client TLS sockets. Useful only for debugging,
for session reuse provide session
option to tls.connect()
.
tlsSocket.localAddress#
Returns the string representation of the local IP address.
tlsSocket.localPort#
Returns the numeric representation of the local port.
tlsSocket.remoteAddress#
Returns the string representation of the remote IP address. For example,
'74.125.127.100'
or '2001:4860:a005::68'
.
tlsSocket.remoteFamily#
Returns the string representation of the remote IP family. 'IPv4'
or 'IPv6'
.
tlsSocket.remotePort#
Returns the numeric representation of the remote port. For example, 443
.
tlsSocket.renegotiate(options, callback)#
options
<Object>rejectUnauthorized
<boolean> If notfalse
, the server certificate is verified against the list of supplied CAs. An'error'
event is emitted if verification fails;err.code
contains the OpenSSL error code. Defaults totrue
.requestCert
callback
<Function> A function that will be called when the renegotiation request has been completed.
The tlsSocket.renegotiate()
method initiates a TLS renegotiation process.
Upon completion, the callback
function will be passed a single argument
that is either an Error
(if the request failed) or null
.
Note: This method can be used to request a peer's certificate after the secure connection has been established.
Note: When running as the server, the socket will be destroyed with an error
after handshakeTimeout
timeout.
tlsSocket.setMaxSendFragment(size)#
size
<number> The maximum TLS fragment size. Defaults to16384
. The maximum value is16384
.
The tlsSocket.setMaxSendFragment()
method sets the maximum TLS fragment size.
Returns true
if setting the limit succeeded; false
otherwise.
Smaller fragment sizes decrease the buffering latency on the client: larger fragments are buffered by the TLS layer until the entire fragment is received and its integrity is verified; large fragments can span multiple roundtrips and their processing can be delayed due to packet loss or reordering. However, smaller fragments add extra TLS framing bytes and CPU overhead, which may decrease overall server throughput.
tls.connect(options[, callback])#
options
<Object>host
<string> Host the client should connect to, defaults to 'localhost'.port
<number> Port the client should connect to.path
<string> Creates unix socket connection to path. If this option is specified,host
andport
are ignored.socket
<net.Socket> Establish secure connection on a given socket rather than creating a new socket. If this option is specified,path
,host
andport
are ignored. Usually, a socket is already connected when passed totls.connect()
, but it can be connected later. Note that connection/disconnection/destruction ofsocket
is the user's responsibility, callingtls.connect()
will not causenet.connect()
to be called.rejectUnauthorized
<boolean> If notfalse
, the server certificate is verified against the list of supplied CAs. An'error'
event is emitted if verification fails;err.code
contains the OpenSSL error code. Defaults totrue
.NPNProtocols
<string[]> | <Buffer[]> | <Uint8Array[]> | <Buffer> | <Uint8Array> An array of strings,Buffer
s orUint8Array
s, or a singleBuffer
orUint8Array
containing supported NPN protocols.Buffer
s should have the format[len][name][len][name]...
e.g.0x05hello0x05world
, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g.['hello', 'world']
.ALPNProtocols
: <string[]> | <Buffer[]> | <Uint8Array[]> | <Buffer> | <Uint8Array> An array of strings,Buffer
s orUint8Array
s, or a singleBuffer
orUint8Array
containing the supported ALPN protocols.Buffer
s should have the format[len][name][len][name]...
e.g.0x05hello0x05world
, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g.['hello', 'world']
.servername
: <string> Server name for the SNI (Server Name Indication) TLS extension.checkServerIdentity(servername, cert)
<Function> A callback function to be used when checking the server's hostname against the certificate. This should throw an error if verification fails. The method should returnundefined
if theservername
andcert
are verified.session
<Buffer> ABuffer
instance, containing TLS session.minDHSize
<number> Minimum size of the DH parameter in bits to accept a TLS connection. When a server offers a DH parameter with a size less thanminDHSize
, the TLS connection is destroyed and an error is thrown. Defaults to1024
.secureContext
: Optional TLS context object created withtls.createSecureContext()
. If asecureContext
is not provided, one will be created by passing the entireoptions
object totls.createSecureContext()
.lookup
: <Function> Custom lookup function. Defaults todns.lookup()
.- ...: Optional
tls.createSecureContext()
options that are used if thesecureContext
option is missing, otherwise they are ignored.
callback
<Function>
The callback
function, if specified, will be added as a listener for the
'secureConnect'
event.
tls.connect()
returns a tls.TLSSocket
object.
The following implements a simple "echo server" example:
const tls = require('tls');
const fs = require('fs');
const options = {
// Necessary only if using the client certificate authentication
key: fs.readFileSync('client-key.pem'),
cert: fs.readFileSync('client-cert.pem'),
// Necessary only if the server uses the self-signed certificate
ca: [ fs.readFileSync('server-cert.pem') ]
};
const socket = tls.connect(8000, options, () => {
console.log('client connected',
socket.authorized ? 'authorized' : 'unauthorized');
process.stdin.pipe(socket);
process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
console.log(data);
});
socket.on('end', () => {
server.close();
});
Or
const tls = require('tls');
const fs = require('fs');
const options = {
pfx: fs.readFileSync('client.pfx')
};
const socket = tls.connect(8000, options, () => {
console.log('client connected',
socket.authorized ? 'authorized' : 'unauthorized');
process.stdin.pipe(socket);
process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
console.log(data);
});
socket.on('end', () => {
server.close();
});
tls.connect(path[, options][, callback])#
path
<string> Default value foroptions.path
.options
<Object> Seetls.connect()
.callback
<Function> Seetls.connect()
.
Same as tls.connect()
except that path
can be provided
as an argument instead of an option.
Note: A path option, if specified, will take precedence over the path argument.
tls.connect(port[, host][, options][, callback])#
port
<number> Default value foroptions.port
.host
<string> Optional default value foroptions.host
.options
<Object> Seetls.connect()
.callback
<Function> Seetls.connect()
.
Same as tls.connect()
except that port
and host
can be provided
as arguments instead of options.
Note: A port or host option, if specified, will take precedence over any port or host argument.
tls.createSecureContext(options)#
options
<Object>pfx
<string> | <Buffer> Optional PFX or PKCS12 encoded private key and certificate chain.pfx
is an alternative to providingkey
andcert
individually. PFX is usually encrypted, if it is,passphrase
will be used to decrypt it.key
<string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> Optional private keys in PEM format. PEM allows the option of private keys being encrypted. Encrypted keys will be decrypted withoptions.passphrase
. Multiple keys using different algorithms can be provided either as an array of unencrypted key strings or buffers, or an array of objects in the form{pem: <string|buffer>[, passphrase: <string>]}
. The object form can only occur in an array.object.passphrase
is optional. Encrypted keys will be decrypted withobject.passphrase
if provided, oroptions.passphrase
if it is not.passphrase
<string> Optional shared passphrase used for a single private key and/or a PFX.cert
<string> | <string[]> | <Buffer> | <Buffer[]> Optional cert chains in PEM format. One cert chain should be provided per private key. Each cert chain should consist of the PEM formatted certificate for a provided privatekey
, followed by the PEM formatted intermediate certificates (if any), in order, and not including the root CA (the root CA must be pre-known to the peer, seeca
). When providing multiple cert chains, they do not have to be in the same order as their private keys inkey
. If the intermediate certificates are not provided, the peer will not be able to validate the certificate, and the handshake will fail.ca
<string> | <string[]> | <Buffer> | <Buffer[]> Optionally override the trusted CA certificates. Default is to trust the well-known CAs curated by Mozilla. Mozilla's CAs are completely replaced when CAs are explicitly specified using this option. The value can be a string or Buffer, or an Array of strings and/or Buffers. Any string or Buffer can contain multiple PEM CAs concatenated together. The peer's certificate must be chainable to a CA trusted by the server for the connection to be authenticated. When using certificates that are not chainable to a well-known CA, the certificate's CA must be explicitly specified as a trusted or the connection will fail to authenticate. If the peer uses a certificate that doesn't match or chain to one of the default CAs, use theca
option to provide a CA certificate that the peer's certificate can match or chain to. For self-signed certificates, the certificate is its own CA, and must be provided.crl
<string> | <string[]> | <Buffer> | <Buffer[]> Optional PEM formatted CRLs (Certificate Revocation Lists).ciphers
<string> Optional cipher suite specification, replacing the default. For more information, see modifying the default cipher suite.honorCipherOrder
<boolean> Attempt to use the server's cipher suite preferences instead of the client's. Whentrue
, causesSSL_OP_CIPHER_SERVER_PREFERENCE
to be set insecureOptions
, see OpenSSL Options for more information.ecdhCurve
<string> A string describing a named curve to use for ECDH key agreement orfalse
to disable ECDH. Defaults totls.DEFAULT_ECDH_CURVE
. Usecrypto.getCurves()
to obtain a list of available curve names. On recent releases,openssl ecparam -list_curves
will also display the name and description of each available elliptic curve.dhparam
<string> | <Buffer> Diffie Hellman parameters, required for Perfect Forward Secrecy. Useopenssl dhparam
to create the parameters. The key length must be greater than or equal to 1024 bits, otherwise an error will be thrown. It is strongly recommended to use 2048 bits or larger for stronger security. If omitted or invalid, the parameters are silently discarded and DHE ciphers will not be available.secureProtocol
<string> Optional SSL method to use, default is"SSLv23_method"
. The possible values are listed as SSL_METHODS, use the function names as strings. For example,"SSLv3_method"
to force SSL version 3.secureOptions
<number> Optionally affect the OpenSSL protocol behavior, which is not usually necessary. This should be used carefully if at all! Value is a numeric bitmask of theSSL_OP_*
options from OpenSSL Options.sessionIdContext
<string> Optional opaque identifier used by servers to ensure session state is not shared between applications. Unused by clients.
Note:
tls.createServer()
sets the default value of thehonorCipherOrder
option totrue
, other APIs that create secure contexts leave it unset.tls.createServer()
uses a 128 bit truncated SHA1 hash value generated fromprocess.argv
as the default value of thesessionIdContext
option, other APIs that create secure contexts have no default value.
The tls.createSecureContext()
method creates a credentials object.
A key is required for ciphers that make use of certificates. Either key
or
pfx
can be used to provide it.
If the 'ca' option is not given, then Node.js will use the default publicly trusted list of CAs as given in http://mxr.mozilla.org/mozilla/source/security/nss/lib/ckfw/builtins/certdata.txt.
tls.createServer([options][, secureConnectionListener])#
options
<Object>handshakeTimeout
<number> Abort the connection if the SSL/TLS handshake does not finish in the specified number of milliseconds. Defaults to120
seconds. A'tlsClientError'
is emitted on thetls.Server
object whenever a handshake times out.requestCert
<boolean> Iftrue
the server will request a certificate from clients that connect and attempt to verify that certificate. Defaults tofalse
.rejectUnauthorized
<boolean> If notfalse
the server will reject any connection which is not authorized with the list of supplied CAs. This option only has an effect ifrequestCert
istrue
. Defaults totrue
.NPNProtocols
<string[]> | <Buffer[]> | <Uint8Array[]> | <Buffer> | <Uint8Array> An array of strings,Buffer
s orUint8Array
s, or a singleBuffer
orUint8Array
containing supported NPN protocols.Buffer
s should have the format[len][name][len][name]...
e.g.0x05hello0x05world
, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g.['hello', 'world']
. (Protocols should be ordered by their priority.)ALPNProtocols
: <string[]> | <Buffer[]> | <Uint8Array[]> | <Buffer> | <Uint8Array> An array of strings,Buffer
s orUint8Array
s, or a singleBuffer
orUint8Array
containing the supported ALPN protocols.Buffer
s should have the format[len][name][len][name]...
e.g.0x05hello0x05world
, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g.['hello', 'world']
. (Protocols should be ordered by their priority.) When the server receives both NPN and ALPN extensions from the client, ALPN takes precedence over NPN and the server does not send an NPN extension to the client.SNICallback(servername, cb)
<Function> A function that will be called if the client supports SNI TLS extension. Two arguments will be passed when called:servername
andcb
.SNICallback
should invokecb(null, ctx)
, wherectx
is a SecureContext instance. (tls.createSecureContext(...)
can be used to get a proper SecureContext.) IfSNICallback
wasn't provided the default callback with high-level API will be used (see below).sessionTimeout
<number> An integer specifying the number of seconds after which the TLS session identifiers and TLS session tickets created by the server will time out. See SSL_CTX_set_timeout for more details.ticketKeys
: A 48-byteBuffer
instance consisting of a 16-byte prefix, a 16-byte HMAC key, and a 16-byte AES key. This can be used to accept TLS session tickets on multiple instances of the TLS server.- ...: Any
tls.createSecureContext()
options can be provided. For servers, the identity options (pfx
orkey
/cert
) are usually required.
secureConnectionListener
<Function>
Creates a new tls.Server. The secureConnectionListener
, if provided, is
automatically set as a listener for the 'secureConnection'
event.
Note: The ticketKeys
options is automatically shared between cluster
module workers.
The following illustrates a simple echo server:
const tls = require('tls');
const fs = require('fs');
const options = {
key: fs.readFileSync('server-key.pem'),
cert: fs.readFileSync('server-cert.pem'),
// This is necessary only if using the client certificate authentication.
requestCert: true,
// This is necessary only if the client uses the self-signed certificate.
ca: [ fs.readFileSync('client-cert.pem') ]
};
const server = tls.createServer(options, (socket) => {
console.log('server connected',
socket.authorized ? 'authorized' : 'unauthorized');
socket.write('welcome!\n');
socket.setEncoding('utf8');
socket.pipe(socket);
});
server.listen(8000, () => {
console.log('server bound');
});
Or
const tls = require('tls');
const fs = require('fs');
const options = {
pfx: fs.readFileSync('server.pfx'),
// This is necessary only if using the client certificate authentication.
requestCert: true,
};
const server = tls.createServer(options, (socket) => {
console.log('server connected',
socket.authorized ? 'authorized' : 'unauthorized');
socket.write('welcome!\n');
socket.setEncoding('utf8');
socket.pipe(socket);
});
server.listen(8000, () => {
console.log('server bound');
});
This server can be tested by connecting to it using openssl s_client
:
openssl s_client -connect 127.0.0.1:8000
tls.getCiphers()#
Returns an array with the names of the supported SSL ciphers.
For example:
console.log(tls.getCiphers()); // ['AES128-SHA', 'AES256-SHA', ...]
tls.DEFAULT_ECDH_CURVE#
The default curve name to use for ECDH key agreement in a tls server. The
default value is 'prime256v1'
(NIST P-256). Consult RFC 4492 and
FIPS.186-4 for more details.
Deprecated APIs#
Class: CryptoStream#
Stability: 0 - Deprecated: Use tls.TLSSocket
instead.
The tls.CryptoStream
class represents a stream of encrypted data. This class
has been deprecated and should no longer be used.
cryptoStream.bytesWritten#
The cryptoStream.bytesWritten
property returns the total number of bytes
written to the underlying socket including the bytes required for the
implementation of the TLS protocol.
Class: SecurePair#
Stability: 0 - Deprecated: Use tls.TLSSocket
instead.
Returned by tls.createSecurePair()
.
Event: 'secure'#
The 'secure'
event is emitted by the SecurePair
object once a secure
connection has been established.
As with checking for the server secureConnection
event, pair.cleartext.authorized
should be inspected to confirm whether the
certificate used is properly authorized.
tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])#
Stability: 0 - Deprecated: Use tls.TLSSocket
instead.
context
<Object> A secure context object as returned bytls.createSecureContext()
isServer
<boolean>true
to specify that this TLS connection should be opened as a server.requestCert
<boolean>true
to specify whether a server should request a certificate from a connecting client. Only applies whenisServer
istrue
.rejectUnauthorized
<boolean> If notfalse
a server automatically reject clients with invalid certificates. Only applies whenisServer
istrue
.options
secureContext
: An optional TLS context object fromtls.createSecureContext()
isServer
: Iftrue
the TLS socket will be instantiated in server-mode. Defaults tofalse
.server
<net.Server> An optionalnet.Server
instancerequestCert
: Optional, seetls.createServer()
rejectUnauthorized
: Optional, seetls.createServer()
NPNProtocols
: Optional, seetls.createServer()
ALPNProtocols
: Optional, seetls.createServer()
SNICallback
: Optional, seetls.createServer()
session
<Buffer> An optionalBuffer
instance containing a TLS session.requestOCSP
<boolean> Iftrue
, specifies that the OCSP status request extension will be added to the client hello and an'OCSPResponse'
event will be emitted on the socket before establishing a secure communication
Creates a new secure pair object with two streams, one of which reads and writes the encrypted data and the other of which reads and writes the cleartext data. Generally, the encrypted stream is piped to/from an incoming encrypted data stream and the cleartext one is used as a replacement for the initial encrypted stream.
tls.createSecurePair()
returns a tls.SecurePair
object with cleartext
and
encrypted
stream properties.
Note: cleartext
has the same API as tls.TLSSocket
.
Note: The tls.createSecurePair()
method is now deprecated in favor of
tls.TLSSocket()
. For example, the code:
pair = tls.createSecurePair(/* ... */);
pair.encrypted.pipe(socket);
socket.pipe(pair.encrypted);
can be replaced by:
secure_socket = tls.TLSSocket(socket, options);
where secure_socket
has the same API as pair.cleartext
.
Tracing#
Trace Event provides a mechanism to centralize tracing information generated by V8, Node core, and userspace code.
Tracing can be enabled by passing the --trace-events-enabled
flag when starting a
Node.js application.
The set of categories for which traces are recorded can be specified using the
--trace-event-categories
flag followed by a list of comma separated category names.
By default the node
and v8
categories are enabled.
node --trace-events-enabled --trace-event-categories v8,node server.js
Running Node.js with tracing enabled will produce log files that can be opened
in the chrome://tracing
tab of Chrome.
TTY#
Stability: 2 - Stable
The tty
module provides the tty.ReadStream
and tty.WriteStream
classes.
In most cases, it will not be necessary or possible to use this module directly.
However, it can be accessed using:
const tty = require('tty');
When Node.js detects that it is being run inside a text terminal ("TTY")
context, the process.stdin
will, by default, be initialized as an instance of
tty.ReadStream
and both process.stdout
and process.stderr
will, by
default be instances of tty.WriteStream
. The preferred method of determining
whether Node.js is being run within a TTY context is to check that the value of
the process.stdout.isTTY
property is true
:
$ node -p -e "Boolean(process.stdout.isTTY)"
true
$ node -p -e "Boolean(process.stdout.isTTY)" | cat
false
In most cases, there should be little to no reason for an application to
create instances of the tty.ReadStream
and tty.WriteStream
classes.
Class: tty.ReadStream#
The tty.ReadStream
class is a subclass of net.Socket
that represents the
readable side of a TTY. In normal circumstances process.stdin
will be the
only tty.ReadStream
instance in a Node.js process and there should be no
reason to create additional instances.
readStream.isRaw#
A boolean
that is true
if the TTY is currently configured to operate as a
raw device. Defaults to false
.
readStream.setRawMode(mode)#
Allows configuration of tty.ReadStream
so that it operates as a raw device.
When in raw mode, input is always available character-by-character, not
including modifiers. Additionally, all special processing of characters by the
terminal is disabled, including echoing input characters.
Note that CTRL
+C
will no longer cause a SIGINT
when in this mode.
mode
<boolean> Iftrue
, configures thetty.ReadStream
to operate as a raw device. Iffalse
, configures thetty.ReadStream
to operate in its default mode. ThereadStream.isRaw
property will be set to the resulting mode.
Class: tty.WriteStream#
The tty.WriteStream
class is a subclass of net.Socket
that represents the
writable side of a TTY. In normal circumstances, process.stdout
and
process.stderr
will be the only tty.WriteStream
instances created for a
Node.js process and there should be no reason to create additional instances.
Event: 'resize'#
The 'resize'
event is emitted whenever either of the writeStream.columns
or writeStream.rows
properties have changed. No arguments are passed to the
listener callback when called.
process.stdout.on('resize', () => {
console.log('screen size has changed!');
console.log(`${process.stdout.columns}x${process.stdout.rows}`);
});
Note: On Windows resize events will be emitted only if stdin is unpaused
(by a call to resume()
or by adding a data listener) and in raw mode. It can
also be triggered if a terminal control sequence that moves the cursor is
written to the screen. Also, the resize event will only be signaled if the
console screen buffer height was also changed. For example shrinking the
console window height will not cause the resize event to be emitted. Increasing
the console window height will only be registered when the new console window
height is greater than the current console buffer size.
writeStream.columns#
A number
specifying the number of columns the TTY currently has. This property
is updated whenever the 'resize'
event is emitted.
writeStream.rows#
A number
specifying the number of rows the TTY currently has. This property
is updated whenever the 'resize'
event is emitted.
tty.isatty(fd)#
fd
<number> A numeric file descriptor
The tty.isatty()
method returns true
if the given fd
is associated with
a TTY and false
if is not.
UDP / Datagram Sockets#
Stability: 2 - Stable
The dgram
module provides an implementation of UDP Datagram sockets.
const dgram = require('dgram');
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.log(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
const address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// server listening 0.0.0.0:41234
Class: dgram.Socket#
The dgram.Socket
object is an EventEmitter
that encapsulates the
datagram functionality.
New instances of dgram.Socket
are created using dgram.createSocket()
.
The new
keyword is not to be used to create dgram.Socket
instances.
Event: 'close'#
The 'close'
event is emitted after a socket is closed with close()
.
Once triggered, no new 'message'
events will be emitted on this socket.
Event: 'error'#
exception
<Error>
The 'error'
event is emitted whenever any error occurs. The event handler
function is passed a single Error object.
Event: 'listening'#
The 'listening'
event is emitted whenever a socket begins listening for
datagram messages. This occurs as soon as UDP sockets are created.
Event: 'message'#
The 'message'
event is emitted when a new datagram is available on a socket.
The event handler function is passed two arguments: msg
and rinfo
.
socket.addMembership(multicastAddress[, multicastInterface])#
Tells the kernel to join a multicast group at the given multicastAddress
and
multicastInterface
using the IP_ADD_MEMBERSHIP
socket option. If the
multicastInterface
argument is not specified, the operating system will choose
one interface and will add membership to it. To add membership to every
available interface, call addMembership
multiple times, once per interface.
socket.address()#
Returns an object containing the address information for a socket.
For UDP sockets, this object will contain address
, family
and port
properties.
socket.bind([port][, address][, callback])#
port
<number> - Integer, Optionaladdress
<string>, Optionalcallback
<Function> with no parameters, Optional. Called when binding is complete.
For UDP sockets, causes the dgram.Socket
to listen for datagram
messages on a named port
and optional address
. If port
is not
specified or is 0
, the operating system will attempt to bind to a
random port. If address
is not specified, the operating system will
attempt to listen on all addresses. Once binding is complete, a
'listening'
event is emitted and the optional callback
function is
called.
Note that specifying both a 'listening'
event listener and passing a
callback
to the socket.bind()
method is not harmful but not very
useful.
A bound datagram socket keeps the Node.js process running to receive datagram messages.
If binding fails, an 'error'
event is generated. In rare case (e.g.
attempting to bind with a closed socket), an Error
may be thrown.
Example of a UDP server listening on port 41234:
const dgram = require('dgram');
const server = dgram.createSocket('udp4');
server.on('error', (err) => {
console.log(`server error:\n${err.stack}`);
server.close();
});
server.on('message', (msg, rinfo) => {
console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});
server.on('listening', () => {
const address = server.address();
console.log(`server listening ${address.address}:${address.port}`);
});
server.bind(41234);
// server listening 0.0.0.0:41234
socket.bind(options[, callback])#
options
<Object> - Required. Supports the following properties:callback
<Function> - Optional.
For UDP sockets, causes the dgram.Socket
to listen for datagram
messages on a named port
and optional address
that are passed as
properties of an options
object passed as the first argument. If
port
is not specified or is 0
, the operating system will attempt
to bind to a random port. If address
is not specified, the operating
system will attempt to listen on all addresses. Once binding is
complete, a 'listening'
event is emitted and the optional callback
function is called.
Note that specifying both a 'listening'
event listener and passing a
callback
to the socket.bind()
method is not harmful but not very
useful.
The options
object may contain an additional exclusive
property that is
use when using dgram.Socket
objects with the cluster
module. When
exclusive
is set to false
(the default), cluster workers will use the same
underlying socket handle allowing connection handling duties to be shared.
When exclusive
is true
, however, the handle is not shared and attempted
port sharing results in an error.
A bound datagram socket keeps the Node.js process running to receive datagram messages.
If binding fails, an 'error'
event is generated. In rare case (e.g.
attempting to bind with a closed socket), an Error
may be thrown.
An example socket listening on an exclusive port is shown below.
socket.bind({
address: 'localhost',
port: 8000,
exclusive: true
});
socket.close([callback])#
Close the underlying socket and stop listening for data on it. If a callback is
provided, it is added as a listener for the 'close'
event.
socket.dropMembership(multicastAddress[, multicastInterface])#
Instructs the kernel to leave a multicast group at multicastAddress
using the
IP_DROP_MEMBERSHIP
socket option. This method is automatically called by the
kernel when the socket is closed or the process terminates, so most apps will
never have reason to call this.
If multicastInterface
is not specified, the operating system will attempt to
drop membership on all valid interfaces.
socket.ref()#
By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The socket.unref()
method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active. The socket.ref()
method adds the socket back to the reference
counting and restores the default behavior.
Calling socket.ref()
multiples times will have no additional effect.
The socket.ref()
method returns a reference to the socket so calls can be
chained.
socket.send(msg, [offset, length,] port [, address] [, callback])#
msg
<Buffer> | <Uint8Array> | <string> | <array> Message to be sentoffset
<number> Integer. Optional. Offset in the buffer where the message starts.length
<number> Integer. Optional. Number of bytes in the message.port
<number> Integer. Destination port.address
<string> Destination hostname or IP address. Optional.callback
<Function> Called when the message has been sent. Optional.
Broadcasts a datagram on the socket. The destination port
and address
must
be specified.
The msg
argument contains the message to be sent.
Depending on its type, different behavior can apply. If msg
is a Buffer
or Uint8Array
,
the offset
and length
specify the offset within the Buffer
where the
message begins and the number of bytes in the message, respectively.
If msg
is a String
, then it is automatically converted to a Buffer
with 'utf8'
encoding. With messages that
contain multi-byte characters, offset
and length
will be calculated with
respect to byte length and not the character position.
If msg
is an array, offset
and length
must not be specified.
The address
argument is a string. If the value of address
is a host name,
DNS will be used to resolve the address of the host. If address
is not
provided or otherwise falsy, '127.0.0.1'
(for udp4
sockets) or '::1'
(for udp6
sockets) will be used by default.
If the socket has not been previously bound with a call to bind
, the socket
is assigned a random port number and is bound to the "all interfaces" address
('0.0.0.0'
for udp4
sockets, '::0'
for udp6
sockets.)
An optional callback
function may be specified to as a way of reporting
DNS errors or for determining when it is safe to reuse the buf
object.
Note that DNS lookups delay the time to send for at least one tick of the
Node.js event loop.
The only way to know for sure that the datagram has been sent is by using a
callback
. If an error occurs and a callback
is given, the error will be
passed as the first argument to the callback
. If a callback
is not given,
the error is emitted as an 'error'
event on the socket
object.
Offset and length are optional but both must be set if either are used.
They are supported only when the first argument is a Buffer
or Uint8Array
.
Example of sending a UDP packet to a random port on localhost
;
const dgram = require('dgram');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
client.close();
});
Example of sending a UDP packet composed of multiple buffers to a random port
on 127.0.0.1
;
const dgram = require('dgram');
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, (err) => {
client.close();
});
Sending multiple buffers might be faster or slower depending on the application and operating system. It is important to run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers is faster.
A Note about UDP datagram size
The maximum size of an IPv4/v6
datagram depends on the MTU
(Maximum Transmission Unit) and on the Payload Length
field size.
The
Payload Length
field is16 bits
wide, which means that a normal payload exceed 64K octets including the internet header and data (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header); this is generally true for loopback interfaces, but such long datagram messages are impractical for most hosts and networks.The
MTU
is the largest size a given link layer technology can support for datagram messages. For any link,IPv4
mandates a minimumMTU
of68
octets, while the recommendedMTU
for IPv4 is576
(typically recommended as theMTU
for dial-up type applications), whether they arrive whole or in fragments.For
IPv6
, the minimumMTU
is1280
octets, however, the mandatory minimum fragment reassembly buffer size is1500
octets. The value of68
octets is very small, since most current link layer technologies, like Ethernet, have a minimumMTU
of1500
.
It is impossible to know in advance the MTU of each link through which
a packet might travel. Sending a datagram greater than the receiver MTU
will
not work because the packet will get silently dropped without informing the
source that the data did not reach its intended recipient.
socket.setBroadcast(flag)#
flag
<boolean>
Sets or clears the SO_BROADCAST
socket option. When set to true
, UDP
packets may be sent to a local interface's broadcast address.
socket.setMulticastLoopback(flag)#
flag
<boolean>
Sets or clears the IP_MULTICAST_LOOP
socket option. When set to true
,
multicast packets will also be received on the local interface.
socket.setMulticastTTL(ttl)#
ttl
<number> Integer
Sets the IP_MULTICAST_TTL
socket option. While TTL generally stands for
"Time to Live", in this context it specifies the number of IP hops that a
packet is allowed to travel through, specifically for multicast traffic. Each
router or gateway that forwards a packet decrements the TTL. If the TTL is
decremented to 0 by a router, it will not be forwarded.
The argument passed to to socket.setMulticastTTL()
is a number of hops
between 0 and 255. The default on most systems is 1
but can vary.
socket.setTTL(ttl)#
ttl
<number> Integer
Sets the IP_TTL
socket option. While TTL generally stands for "Time to Live",
in this context it specifies the number of IP hops that a packet is allowed to
travel through. Each router or gateway that forwards a packet decrements the
TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.
Changing TTL values is typically done for network probes or when multicasting.
The argument to socket.setTTL()
is a number of hops between 1 and 255.
The default on most systems is 64 but can vary.
socket.unref()#
By default, binding a socket will cause it to block the Node.js process from
exiting as long as the socket is open. The socket.unref()
method can be used
to exclude the socket from the reference counting that keeps the Node.js
process active, allowing the process to exit even if the socket is still
listening.
Calling socket.unref()
multiple times will have no addition effect.
The socket.unref()
method returns a reference to the socket so calls can be
chained.
Change to asynchronous socket.bind()
behavior#
As of Node.js v0.10, dgram.Socket#bind()
changed to an asynchronous
execution model. Legacy code that assumes synchronous behavior, as in the
following example:
const s = dgram.createSocket('udp4');
s.bind(1234);
s.addMembership('224.0.0.114');
Must be changed to pass a callback function to the dgram.Socket#bind()
function:
const s = dgram.createSocket('udp4');
s.bind(1234, () => {
s.addMembership('224.0.0.114');
});
dgram
module functions#
dgram.createSocket(options[, callback])#
options
<Object>callback
<Function> Attached as a listener to'message'
events.- Returns: <dgram.Socket>
Creates a dgram.Socket
object. The options
argument is an object that
should contain a type
field of either udp4
or udp6
and an optional
boolean reuseAddr
field.
When reuseAddr
is true
socket.bind()
will reuse the address, even if
another process has already bound a socket on it. reuseAddr
defaults to
false
. The optional callback
function is added as a listener for 'message'
events.
Once the socket is created, calling socket.bind()
will instruct the
socket to begin listening for datagram messages. When address
and port
are
not passed to socket.bind()
the method will bind the socket to the "all
interfaces" address on a random port (it does the right thing for both udp4
and udp6
sockets). The bound address and port can be retrieved using
socket.address().address
and socket.address().port
.
dgram.createSocket(type[, callback])#
type
<string> - Either 'udp4' or 'udp6'callback
<Function> - Attached as a listener to'message'
events. Optional- Returns: <dgram.Socket>
Creates a dgram.Socket
object of the specified type
. The type
argument
can be either udp4
or udp6
. An optional callback
function can be passed
which is added as a listener for 'message'
events.
Once the socket is created, calling socket.bind()
will instruct the
socket to begin listening for datagram messages. When address
and port
are
not passed to socket.bind()
the method will bind the socket to the "all
interfaces" address on a random port (it does the right thing for both udp4
and udp6
sockets). The bound address and port can be retrieved using
socket.address().address
and socket.address().port
.
URL#
Stability: 2 - Stable
The url
module provides utilities for URL resolution and parsing. It can be
accessed using:
const url = require('url');
URL Strings and URL Objects#
A URL string is a structured string containing multiple meaningful components. When parsed, a URL object is returned containing properties for each of these components.
The url
module provides two APIs for working with URLs: a legacy API that is
Node.js specific, and a newer API that implements the same
WHATWG URL Standard used by web browsers.
Note: While the Legacy API has not been deprecated, it is maintained solely for backwards compatibility with existing applications. New application code should use the WHATWG API.
A comparison between the WHATWG and Legacy APIs is provided below. Above the URL
'http://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash'
, properties of
an object returned by the legacy url.parse()
are shown. Below it are
properties of a WHATWG URL
object.
Note: WHATWG URL's origin
property includes protocol
and host
, but not
username
or password
.
┌─────────────────────────────────────────────────────────────────────────────────────────────┐
│ href │
├──────────┬──┬─────────────────────┬─────────────────────┬───────────────────────────┬───────┤
│ protocol │ │ auth │ host │ path │ hash │
│ │ │ ├──────────────┬──────┼──────────┬────────────────┤ │
│ │ │ │ hostname │ port │ pathname │ search │ │
│ │ │ │ │ │ ├─┬──────────────┤ │
│ │ │ │ │ │ │ │ query │ │
" https: // user : pass @ sub.host.com : 8080 /p/a/t/h ? query=string #hash "
│ │ │ │ │ hostname │ port │ │ │ │
│ │ │ │ ├──────────────┴──────┤ │ │ │
│ protocol │ │ username │ password │ host │ │ │ │
├──────────┴──┼──────────┴──────────┼─────────────────────┤ │ │ │
│ origin │ │ origin │ pathname │ search │ hash │
├─────────────┴─────────────────────┴─────────────────────┴──────────┴────────────────┴───────┤
│ href │
└─────────────────────────────────────────────────────────────────────────────────────────────┘
(all spaces in the "" line should be ignored -- they are purely for formatting)
Parsing the URL string using the WHATWG API:
const { URL } = require('url');
const myURL =
new URL('https://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash');
Note: In Web Browsers, the WHATWG URL
class is a global that is always
available. In Node.js, however, the URL
class must be accessed via
require('url').URL
.
Parsing the URL string using the Legacy API:
const url = require('url');
const myURL =
url.parse('https://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash');
The WHATWG URL API#
Class: URL#
Browser-compatible URL
class, implemented by following the WHATWG URL
Standard. Examples of parsed URLs may be found in the Standard itself.
Note: In accordance with browser conventions, all properties of URL
objects
are implemented as getters and setters on the class prototype, rather than as
data properties on the object itself. Thus, unlike legacy urlObjects, using
the delete
keyword on any properties of URL
objects (e.g. delete
myURL.protocol
, delete myURL.pathname
, etc) has no effect but will still
return true
.
Constructor: new URL(input[, base])#
Creates a new URL
object by parsing the input
relative to the base
. If
base
is passed as a string, it will be parsed equivalent to new URL(base)
.
const { URL } = require('url');
const myURL = new URL('/foo', 'https://example.org/');
// https://example.org/foo
A TypeError
will be thrown if the input
or base
are not valid URLs. Note
that an effort will be made to coerce the given values into strings. For
instance:
const { URL } = require('url');
const myURL = new URL({ toString: () => 'https://example.org/' });
// https://example.org/
Unicode characters appearing within the hostname of input
will be
automatically converted to ASCII using the Punycode algorithm.
const { URL } = require('url');
const myURL = new URL('https://你好你好');
// https://xn--6qqa088eba/
Note: This feature is only available if the node
executable was compiled
with ICU enabled. If not, the domain names are passed through unchanged.
url.hash#
Gets and sets the fragment portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://example.org/foo#bar');
console.log(myURL.hash);
// Prints #bar
myURL.hash = 'baz';
console.log(myURL.href);
// Prints https://example.org/foo#baz
Invalid URL characters included in the value assigned to the hash
property
are percent-encoded. Note that the selection of which characters to
percent-encode may vary somewhat from what the url.parse()
and
url.format()
methods would produce.
url.host#
Gets and sets the host portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://example.org:81/foo');
console.log(myURL.host);
// Prints example.org:81
myURL.host = 'example.com:82';
console.log(myURL.href);
// Prints https://example.com:82/foo
Invalid host values assigned to the host
property are ignored.
url.hostname#
Gets and sets the hostname portion of the URL. The key difference between
url.host
and url.hostname
is that url.hostname
does not include the
port.
const { URL } = require('url');
const myURL = new URL('https://example.org:81/foo');
console.log(myURL.hostname);
// Prints example.org
myURL.hostname = 'example.com:82';
console.log(myURL.href);
// Prints https://example.com:81/foo
Invalid hostname values assigned to the hostname
property are ignored.
url.href#
Gets and sets the serialized URL.
const { URL } = require('url');
const myURL = new URL('https://example.org/foo');
console.log(myURL.href);
// Prints https://example.org/foo
myURL.href = 'https://example.com/bar';
console.log(myURL.href);
// Prints https://example.com/bar
Getting the value of the href
property is equivalent to calling
url.toString()
.
Setting the value of this property to a new value is equivalent to creating a
new URL
object using new URL(value)
. Each of the URL
object's properties will be modified.
If the value assigned to the href
property is not a valid URL, a TypeError
will be thrown.
url.origin#
Gets the read-only serialization of the URL's origin.
const { URL } = require('url');
const myURL = new URL('https://example.org/foo/bar?baz');
console.log(myURL.origin);
// Prints https://example.org
const { URL } = require('url');
const idnURL = new URL('https://你好你好');
console.log(idnURL.origin);
// Prints https://xn--6qqa088eba
console.log(idnURL.hostname);
// Prints xn--6qqa088eba
url.password#
Gets and sets the password portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.password);
// Prints xyz
myURL.password = '123';
console.log(myURL.href);
// Prints https://abc:123@example.com
Invalid URL characters included in the value assigned to the password
property
are percent-encoded. Note that the selection of which characters to
percent-encode may vary somewhat from what the url.parse()
and
url.format()
methods would produce.
url.pathname#
Gets and sets the path portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://example.org/abc/xyz?123');
console.log(myURL.pathname);
// Prints /abc/xyz
myURL.pathname = '/abcdef';
console.log(myURL.href);
// Prints https://example.org/abcdef?123
Invalid URL characters included in the value assigned to the pathname
property are percent-encoded. Note that the selection of which characters
to percent-encode may vary somewhat from what the url.parse()
and
url.format()
methods would produce.
url.port#
Gets and sets the port portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://example.org:8888');
console.log(myURL.port);
// Prints 8888
// Default ports are automatically transformed to the empty string
// (HTTPS protocol's default port is 443)
myURL.port = '443';
console.log(myURL.port);
// Prints the empty string
console.log(myURL.href);
// Prints https://example.org/
myURL.port = 1234;
console.log(myURL.port);
// Prints 1234
console.log(myURL.href);
// Prints https://example.org:1234/
// Completely invalid port strings are ignored
myURL.port = 'abcd';
console.log(myURL.port);
// Prints 1234
// Leading numbers are treated as a port number
myURL.port = '5678abcd';
console.log(myURL.port);
// Prints 5678
// Non-integers are truncated
myURL.port = 1234.5678;
console.log(myURL.port);
// Prints 1234
// Out-of-range numbers are ignored
myURL.port = 1e10;
console.log(myURL.port);
// Prints 1234
The port value may be set as either a number or as a String containing a number
in the range 0
to 65535
(inclusive). Setting the value to the default port
of the URL
objects given protocol
will result in the port
value becoming
the empty string (''
).
If an invalid string is assigned to the port
property, but it begins with a
number, the leading number is assigned to port
. Otherwise, or if the number
lies outside the range denoted above, it is ignored.
url.protocol#
Gets and sets the protocol portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://example.org');
console.log(myURL.protocol);
// Prints https:
myURL.protocol = 'ftp';
console.log(myURL.href);
// Prints ftp://example.org/
Invalid URL protocol values assigned to the protocol
property are ignored.
url.search#
Gets and sets the serialized query portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://example.org/abc?123');
console.log(myURL.search);
// Prints ?123
myURL.search = 'abc=xyz';
console.log(myURL.href);
// Prints https://example.org/abc?abc=xyz
Any invalid URL characters appearing in the value assigned the search
property will be percent-encoded. Note that the selection of which
characters to percent-encode may vary somewhat from what the url.parse()
and url.format()
methods would produce.
url.searchParams#
Gets the URLSearchParams
object representing the query parameters of the
URL. This property is read-only; to replace the entirety of query parameters of
the URL, use the url.search
setter. See URLSearchParams
documentation for details.
url.username#
Gets and sets the username portion of the URL.
const { URL } = require('url');
const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.username);
// Prints abc
myURL.username = '123';
console.log(myURL.href);
// Prints https://123:xyz@example.com/
Any invalid URL characters appearing in the value assigned the username
property will be percent-encoded. Note that the selection of which
characters to percent-encode may vary somewhat from what the url.parse()
and url.format()
methods would produce.
url.toString()#
- Returns: <string>
The toString()
method on the URL
object returns the serialized URL. The
value returned is equivalent to that of url.href
and url.toJSON()
.
Because of the need for standard compliance, this method does not allow users
to customize the serialization process of the URL. For more flexibility,
require('url').format()
method might be of interest.
url.toJSON()#
- Returns: <string>
The toJSON()
method on the URL
object returns the serialized URL. The
value returned is equivalent to that of url.href
and
url.toString()
.
This method is automatically called when an URL
object is serialized
with JSON.stringify()
.
const { URL } = require('url');
const myURLs = [
new URL('https://www.example.com'),
new URL('https://test.example.org')
];
console.log(JSON.stringify(myURLs));
// Prints ["https://www.example.com/","https://test.example.org/"]
Class: URLSearchParams#
The URLSearchParams
API provides read and write access to the query of a
URL
. The URLSearchParams
class can also be used standalone with one of the
four following constructors.
The WHATWG URLSearchParams
interface and the querystring
module have
similar purpose, but the purpose of the querystring
module is more
general, as it allows the customization of delimiter characters (&
and =
).
On the other hand, this API is designed purely for URL query strings.
const { URL, URLSearchParams } = require('url');
const myURL = new URL('https://example.org/?abc=123');
console.log(myURL.searchParams.get('abc'));
// Prints 123
myURL.searchParams.append('abc', 'xyz');
console.log(myURL.href);
// Prints https://example.org/?abc=123&abc=xyz
myURL.searchParams.delete('abc');
myURL.searchParams.set('a', 'b');
console.log(myURL.href);
// Prints https://example.org/?a=b
const newSearchParams = new URLSearchParams(myURL.searchParams);
// The above is equivalent to
// const newSearchParams = new URLSearchParams(myURL.search);
newSearchParams.append('a', 'c');
console.log(myURL.href);
// Prints https://example.org/?a=b
console.log(newSearchParams.toString());
// Prints a=b&a=c
// newSearchParams.toString() is implicitly called
myURL.search = newSearchParams;
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c
newSearchParams.delete('a');
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c
Constructor: new URLSearchParams()#
Instantiate a new empty URLSearchParams
object.
Constructor: new URLSearchParams(string)#
string
<string> A query string
Parse the string
as a query string, and use it to instantiate a new
URLSearchParams
object. A leading '?'
, if present, is ignored.
const { URLSearchParams } = require('url');
let params;
params = new URLSearchParams('user=abc&query=xyz');
console.log(params.get('user'));
// Prints 'abc'
console.log(params.toString());
// Prints 'user=abc&query=xyz'
params = new URLSearchParams('?user=abc&query=xyz');
console.log(params.toString());
// Prints 'user=abc&query=xyz'
Constructor: new URLSearchParams(obj)#
obj
<Object> An object representing a collection of key-value pairs
Instantiate a new URLSearchParams
object with a query hash map. The key and
value of each property of obj
are always coerced to strings.
Note: Unlike querystring
module, duplicate keys in the form of array
values are not allowed. Arrays are stringified using array.toString()
,
which simply joins all array elements with commas.
const { URLSearchParams } = require('url');
const params = new URLSearchParams({
user: 'abc',
query: ['first', 'second']
});
console.log(params.getAll('query'));
// Prints [ 'first,second' ]
console.log(params.toString());
// Prints 'user=abc&query=first%2Csecond'
Constructor: new URLSearchParams(iterable)#
iterable
<Iterable> An iterable object whose elements are key-value pairs
Instantiate a new URLSearchParams
object with an iterable map in a way that
is similar to Map
's constructor. iterable
can be an Array or any
iterable object. That means iterable
can be another URLSearchParams
, in
which case the constructor will simply create a clone of the provided
URLSearchParams
. Elements of iterable
are key-value pairs, and can
themselves be any iterable object.
Duplicate keys are allowed.
const { URLSearchParams } = require('url');
let params;
// Using an array
params = new URLSearchParams([
['user', 'abc'],
['query', 'first'],
['query', 'second']
]);
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'
// Using a Map object
const map = new Map();
map.set('user', 'abc');
map.set('query', 'xyz');
params = new URLSearchParams(map);
console.log(params.toString());
// Prints 'user=abc&query=xyz'
// Using a generator function
function* getQueryPairs() {
yield ['user', 'abc'];
yield ['query', 'first'];
yield ['query', 'second'];
}
params = new URLSearchParams(getQueryPairs());
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'
// Each key-value pair must have exactly two elements
new URLSearchParams([
['user', 'abc', 'error']
]);
// Throws TypeError [ERR_INVALID_TUPLE]:
// Each query pair must be an iterable [name, value] tuple
urlSearchParams.append(name, value)#
Append a new name-value pair to the query string.
urlSearchParams.delete(name)#
name
<string>
Remove all name-value pairs whose name is name
.
urlSearchParams.entries()#
- Returns: <Iterator>
Returns an ES6 Iterator over each of the name-value pairs in the query.
Each item of the iterator is a JavaScript Array. The first item of the Array
is the name
, the second item of the Array is the value
.
Alias for urlSearchParams[@@iterator]()
.
urlSearchParams.forEach(fn[, thisArg])#
fn
<Function> Function invoked for each name-value pair in the query.thisArg
<Object> Object to be used asthis
value for whenfn
is called
Iterates over each name-value pair in the query and invokes the given function.
const { URL } = require('url');
const myURL = new URL('https://example.org/?a=b&c=d');
myURL.searchParams.forEach((value, name, searchParams) => {
console.log(name, value, myURL.searchParams === searchParams);
});
// Prints:
// a b true
// c d true
urlSearchParams.get(name)#
Returns the value of the first name-value pair whose name is name
. If there
are no such pairs, null
is returned.
urlSearchParams.getAll(name)#
Returns the values of all name-value pairs whose name is name
. If there are
no such pairs, an empty array is returned.
urlSearchParams.has(name)#
Returns true
if there is at least one name-value pair whose name is name
.
urlSearchParams.keys()#
- Returns: <Iterator>
Returns an ES6 Iterator over the names of each name-value pair.
const { URLSearchParams } = require('url');
const params = new URLSearchParams('foo=bar&foo=baz');
for (const name of params.keys()) {
console.log(name);
}
// Prints:
// foo
// foo
urlSearchParams.set(name, value)#
Sets the value in the URLSearchParams
object associated with name
to
value
. If there are any pre-existing name-value pairs whose names are name
,
set the first such pair's value to value
and remove all others. If not,
append the name-value pair to the query string.
const { URLSearchParams } = require('url');
const params = new URLSearchParams();
params.append('foo', 'bar');
params.append('foo', 'baz');
params.append('abc', 'def');
console.log(params.toString());
// Prints foo=bar&foo=baz&abc=def
params.set('foo', 'def');
params.set('xyz', 'opq');
console.log(params.toString());
// Prints foo=def&abc=def&xyz=opq
urlSearchParams.sort()#
Sort all existing name-value pairs in-place by their names. Sorting is done with a stable sorting algorithm, so relative order between name-value pairs with the same name is preserved.
This method can be used, in particular, to increase cache hits.
const { URLSearchParams } = require('url');
const params = new URLSearchParams('query[]=abc&type=search&query[]=123');
params.sort();
console.log(params.toString());
// Prints query%5B%5D=abc&query%5B%5D=123&type=search
urlSearchParams.toString()#
- Returns: <string>
Returns the search parameters serialized as a string, with characters percent-encoded where necessary.
urlSearchParams.values()#
- Returns: <Iterator>
Returns an ES6 Iterator over the values of each name-value pair.
urlSearchParams[@@iterator]()#
- Returns: <Iterator>
Returns an ES6 Iterator over each of the name-value pairs in the query string.
Each item of the iterator is a JavaScript Array. The first item of the Array
is the name
, the second item of the Array is the value
.
Alias for urlSearchParams.entries()
.
const { URLSearchParams } = require('url');
const params = new URLSearchParams('foo=bar&xyz=baz');
for (const [name, value] of params) {
console.log(name, value);
}
// Prints:
// foo bar
// xyz baz
url.domainToASCII(domain)#
Returns the Punycode ASCII serialization of the domain
. If domain
is an
invalid domain, the empty string is returned.
It performs the inverse operation to url.domainToUnicode()
.
const url = require('url');
console.log(url.domainToASCII('español.com'));
// Prints xn--espaol-zwa.com
console.log(url.domainToASCII('中文.com'));
// Prints xn--fiq228c.com
console.log(url.domainToASCII('xn--iñvalid.com'));
// Prints an empty string
url.domainToUnicode(domain)#
Returns the Unicode serialization of the domain
. If domain
is an invalid
domain, the empty string is returned.
It performs the inverse operation to url.domainToASCII()
.
const url = require('url');
console.log(url.domainToUnicode('xn--espaol-zwa.com'));
// Prints español.com
console.log(url.domainToUnicode('xn--fiq228c.com'));
// Prints 中文.com
console.log(url.domainToUnicode('xn--iñvalid.com'));
// Prints an empty string
url.format(URL[, options])#
URL
<URL> A WHATWG URL objectoptions
<Object>auth
<boolean>true
if the serialized URL string should include the username and password,false
otherwise. Defaults totrue
.fragment
<boolean>true
if the serialized URL string should include the fragment,false
otherwise. Defaults totrue
.search
<boolean>true
if the serialized URL string should include the search query,false
otherwise. Defaults totrue
.unicode
<boolean>true
if Unicode characters appearing in the host component of the URL string should be encoded directly as opposed to being Punycode encoded. Defaults tofalse
.
Returns a customizable serialization of a URL String representation of a WHATWG URL object.
The URL object has both a toString()
method and href
property that return
string serializations of the URL. These are not, however, customizable in
any way. The url.format(URL[, options])
method allows for basic customization
of the output.
For example:
const { URL } = require('url');
const myURL = new URL('https://a:b@你好你好?abc#foo');
console.log(myURL.href);
// Prints https://a:b@xn--6qqa088eba/?abc#foo
console.log(myURL.toString());
// Prints https://a:b@xn--6qqa088eba/?abc#foo
console.log(url.format(myURL, { fragment: false, unicode: true, auth: false }));
// Prints 'https://你好你好/?abc'
Legacy URL API#
Legacy urlObject#
The legacy urlObject (require('url').Url
) is created and returned by the
url.parse()
function.
urlObject.auth#
The auth
property is the username and password portion of the URL, also
referred to as "userinfo". This string subset follows the protocol
and
double slashes (if present) and precedes the host
component, delimited by an
ASCII "at sign" (@
). The format of the string is {username}[:{password}]
,
with the [:{password}]
portion being optional.
For example: 'user:pass'
urlObject.hash#
The hash
property consists of the "fragment" portion of the URL including
the leading ASCII hash (#
) character.
For example: '#hash'
urlObject.host#
The host
property is the full lower-cased host portion of the URL, including
the port
if specified.
For example: 'sub.host.com:8080'
urlObject.hostname#
The hostname
property is the lower-cased host name portion of the host
component without the port
included.
For example: 'sub.host.com'
urlObject.href#
The href
property is the full URL string that was parsed with both the
protocol
and host
components converted to lower-case.
For example: 'http://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash'
urlObject.path#
The path
property is a concatenation of the pathname
and search
components.
For example: '/p/a/t/h?query=string'
No decoding of the path
is performed.
urlObject.pathname#
The pathname
property consists of the entire path section of the URL. This
is everything following the host
(including the port
) and before the start
of the query
or hash
components, delimited by either the ASCII question
mark (?
) or hash (#
) characters.
For example '/p/a/t/h'
No decoding of the path string is performed.
urlObject.port#
The port
property is the numeric port portion of the host
component.
For example: '8080'
urlObject.protocol#
The protocol
property identifies the URL's lower-cased protocol scheme.
For example: 'http:'
urlObject.query#
The query
property is either the query string without the leading ASCII
question mark (?
), or an object returned by the querystring
module's
parse()
method. Whether the query
property is a string or object is
determined by the parseQueryString
argument passed to url.parse()
.
For example: 'query=string'
or {'query': 'string'}
If returned as a string, no decoding of the query string is performed. If returned as an object, both keys and values are decoded.
urlObject.search#
The search
property consists of the entire "query string" portion of the
URL, including the leading ASCII question mark (?
) character.
For example: '?query=string'
No decoding of the query string is performed.
urlObject.slashes#
The slashes
property is a boolean
with a value of true
if two ASCII
forward-slash characters (/
) are required following the colon in the
protocol
.
url.format(urlObject)#
urlObject
<Object> | <string> A URL object (as returned byurl.parse()
or constructed otherwise). If a string, it is converted to an object by passing it tourl.parse()
.
The url.format()
method returns a formatted URL string derived from
urlObject
.
If urlObject
is not an object or a string, url.parse()
will throw a
TypeError
.
The formatting process operates as follows:
- A new empty string
result
is created. - If
urlObject.protocol
is a string, it is appended as-is toresult
. - Otherwise, if
urlObject.protocol
is notundefined
and is not a string, anError
is thrown. - For all string values of
urlObject.protocol
that do not end with an ASCII colon (:
) character, the literal string:
will be appended toresult
. - If either of the following conditions is true, then the literal string
//
will be appended toresult
:urlObject.slashes
property is true;urlObject.protocol
begins withhttp
,https
,ftp
,gopher
, orfile
;
- If the value of the
urlObject.auth
property is truthy, and eitherurlObject.host
orurlObject.hostname
are notundefined
, the value ofurlObject.auth
will be coerced into a string and appended toresult
followed by the literal string@
. - If the
urlObject.host
property isundefined
then:- If the
urlObject.hostname
is a string, it is appended toresult
. - Otherwise, if
urlObject.hostname
is notundefined
and is not a string, anError
is thrown. - If the
urlObject.port
property value is truthy, andurlObject.hostname
is notundefined
:- The literal string
:
is appended toresult
, and - The value of
urlObject.port
is coerced to a string and appended toresult
.
- The literal string
- If the
- Otherwise, if the
urlObject.host
property value is truthy, the value ofurlObject.host
is coerced to a string and appended toresult
. - If the
urlObject.pathname
property is a string that is not an empty string:- If the
urlObject.pathname
does not start with an ASCII forward slash (/
), then the literal string '/' is appended toresult
. - The value of
urlObject.pathname
is appended toresult
.
- If the
- Otherwise, if
urlObject.pathname
is notundefined
and is not a string, anError
is thrown. - If the
urlObject.search
property isundefined
and if theurlObject.query
property is anObject
, the literal string?
is appended toresult
followed by the output of calling thequerystring
module'sstringify()
method passing the value ofurlObject.query
. - Otherwise, if
urlObject.search
is a string:- If the value of
urlObject.search
does not start with the ASCII question mark (?
) character, the literal string?
is appended toresult
. - The value of
urlObject.search
is appended toresult
.
- If the value of
- Otherwise, if
urlObject.search
is notundefined
and is not a string, anError
is thrown. - If the
urlObject.hash
property is a string:- If the value of
urlObject.hash
does not start with the ASCII hash (#
) character, the literal string#
is appended toresult
. - The value of
urlObject.hash
is appended toresult
.
- If the value of
- Otherwise, if the
urlObject.hash
property is notundefined
and is not a string, anError
is thrown. result
is returned.
url.parse(urlString[, parseQueryString[, slashesDenoteHost]])#
urlString
<string> The URL string to parse.parseQueryString
<boolean> Iftrue
, thequery
property will always be set to an object returned by thequerystring
module'sparse()
method. Iffalse
, thequery
property on the returned URL object will be an unparsed, undecoded string. Defaults tofalse
.slashesDenoteHost
<boolean> Iftrue
, the first token after the literal string//
and preceding the next/
will be interpreted as thehost
. For instance, given//foo/bar
, the result would be{host: 'foo', pathname: '/bar'}
rather than{pathname: '//foo/bar'}
. Defaults tofalse
.
The url.parse()
method takes a URL string, parses it, and returns a URL
object.
A TypeError
is thrown if urlString
is not a string.
A URIError
is thrown if the auth
property is present but cannot be decoded.
url.resolve(from, to)#
The url.resolve()
method resolves a target URL relative to a base URL in a
manner similar to that of a Web browser resolving an anchor tag HREF.
For example:
const url = require('url');
url.resolve('/one/two/three', 'four'); // '/one/two/four'
url.resolve('http://example.com/', '/one'); // 'http://example.com/one'
url.resolve('http://example.com/one', '/two'); // 'http://example.com/two'
Percent-Encoding in URLs#
URLs are permitted to only contain a certain range of characters. Any character falling outside of that range must be encoded. How such characters are encoded, and which characters to encode depends entirely on where the character is located within the structure of the URL.
Legacy API#
Within the Legacy API, spaces (' '
) and the following characters will be
automatically escaped in the properties of URL objects:
< > " ` \r \n \t { } | \ ^ '
For example, the ASCII space character (' '
) is encoded as %20
. The ASCII
forward slash (/
) character is encoded as %3C
.
WHATWG API#
The WHATWG URL Standard uses a more selective and fine grained approach to selecting encoded characters than that used by the Legacy API.
The WHATWG algorithm defines three "percent-encode sets" that describe ranges of characters that must be percent-encoded:
The C0 control percent-encode set includes code points in range U+0000 to U+001F (inclusive) and all code points greater than U+007E.
The path percent-encode set includes the C0 control percent-encode set and code points U+0020, U+0022, U+0023, U+003C, U+003E, U+003F, U+0060, U+007B, and U+007D.
The userinfo encode set includes the path percent-encode set and code points U+002F, U+003A, U+003B, U+003D, U+0040, U+005B, U+005C, U+005D, U+005E, and U+007C.
The userinfo percent-encode set is used exclusively for username and passwords encoded within the URL. The path percent-encode set is used for the path of most URLs. The C0 control percent-encode set is used for all other cases, including URL fragments in particular, but also host and path under certain specific conditions.
When non-ASCII characters appear within a hostname, the hostname is encoded using the Punycode algorithm. Note, however, that a hostname may contain both Punycode encoded and percent-encoded characters. For example:
const { URL } = require('url');
const myURL = new URL('https://%CF%80.com/foo');
console.log(myURL.href);
// Prints https://xn--1xa.com/foo
console.log(myURL.origin);
// Prints https://π.com
Util#
Stability: 2 - Stable
The util
module is primarily designed to support the needs of Node.js' own
internal APIs. However, many of the utilities are useful for application and
module developers as well. It can be accessed using:
const util = require('util');
util.callbackify(original)#
original
<Function> Anasync
function- Returns: <Function> a callback style function
Takes an async
function (or a function that returns a Promise) and returns a
function following the Node.js error first callback style. In the callback, the
first argument will be the rejection reason (or null
if the Promise resolved),
and the second argument will be the resolved value.
For example:
const util = require('util');
async function fn() {
return await Promise.resolve('hello world');
}
const callbackFunction = util.callbackify(fn);
callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});
Will print:
hello world
Note:
The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an
'uncaughtException'
event, and if not handled will exit.Since
null
has a special meaning as the first argument to a callback, if a wrapped function rejects aPromise
with a falsy value as a reason, the value is wrapped in anError
with the original value stored in a field namedreason
.function fn() { return Promise.reject(null); } const callbackFunction = util.callbackify(fn); callbackFunction((err, ret) => { // When the Promise was rejected with `null` it is wrapped with an Error and // the original value is stored in `reason`. err && err.hasOwnProperty('reason') && err.reason === null; // true });
util.debuglog(section)#
section
<string> A string identifying the portion of the application for which thedebuglog
function is being created.- Returns: <Function> The logging function
The util.debuglog()
method is used to create a function that conditionally
writes debug messages to stderr
based on the existence of the NODE_DEBUG
environment variable. If the section
name appears within the value of that
environment variable, then the returned function operates similar to
console.error()
. If not, then the returned function is a no-op.
For example:
const util = require('util');
const debuglog = util.debuglog('foo');
debuglog('hello from foo [%d]', 123);
If this program is run with NODE_DEBUG=foo
in the environment, then
it will output something like:
FOO 3245: hello from foo [123]
where 3245
is the process id. If it is not run with that
environment variable set, then it will not print anything.
Multiple comma-separated section
names may be specified in the NODE_DEBUG
environment variable. For example: NODE_DEBUG=fs,net,tls
.
util.deprecate(function, string)#
The util.deprecate()
method wraps the given function
or class in such a way that
it is marked as deprecated.
const util = require('util');
exports.puts = util.deprecate(function() {
for (let i = 0, len = arguments.length; i < len; ++i) {
process.stdout.write(arguments[i] + '\n');
}
}, 'util.puts: Use console.log instead');
When called, util.deprecate()
will return a function that will emit a
DeprecationWarning
using the process.on('warning')
event. By default,
this warning will be emitted and printed to stderr
exactly once, the first
time it is called. After the warning is emitted, the wrapped function
is called.
If either the --no-deprecation
or --no-warnings
command line flags are
used, or if the process.noDeprecation
property is set to true
prior to
the first deprecation warning, the util.deprecate()
method does nothing.
If the --trace-deprecation
or --trace-warnings
command line flags are set,
or the process.traceDeprecation
property is set to true
, a warning and a
stack trace are printed to stderr
the first time the deprecated function is
called.
If the --throw-deprecation
command line flag is set, or the
process.throwDeprecation
property is set to true
, then an exception will be
thrown when the deprecated function is called.
The --throw-deprecation
command line flag and process.throwDeprecation
property take precedence over --trace-deprecation
and
process.traceDeprecation
.
util.format(format[, ...args])#
format
<string> Aprintf
-like format string.
The util.format()
method returns a formatted string using the first argument
as a printf
-like format.
The first argument is a string containing zero or more placeholder tokens. Each placeholder token is replaced with the converted value from the corresponding argument. Supported placeholders are:
%s
- String.%d
- Number (integer or floating point value).%i
- Integer.%f
- Floating point value.%j
- JSON. Replaced with the string'[Circular]'
if the argument contains circular references.%%
- single percent sign ('%'
). This does not consume an argument.
If the placeholder does not have a corresponding argument, the placeholder is not replaced.
util.format('%s:%s', 'foo');
// Returns: 'foo:%s'
If there are more arguments passed to the util.format()
method than the number
of placeholders, the extra arguments are coerced into strings then concatenated
to the returned string, each delimited by a space. Excessive arguments whose
typeof
is 'object'
or 'symbol'
(except null
) will be transformed by
util.inspect()
.
util.format('%s:%s', 'foo', 'bar', 'baz'); // 'foo:bar baz'
If the first argument is not a string then util.format()
returns
a string that is the concatenation of all arguments separated by spaces.
Each argument is converted to a string using util.inspect()
.
util.format(1, 2, 3); // '1 2 3'
If only one argument is passed to util.format()
, it is returned as it is
without any formatting.
util.format('%% %s'); // '%% %s'
util.inherits(constructor, superConstructor)#
Note: Usage of util.inherits()
is discouraged. Please use the ES6 class
and extends
keywords to get language level inheritance support. Also note
that the two styles are semantically incompatible.
constructor
<Function>superConstructor
<Function>
Inherit the prototype methods from one constructor into another. The
prototype of constructor
will be set to a new object created from
superConstructor
.
As an additional convenience, superConstructor
will be accessible
through the constructor.super_
property.
const util = require('util');
const EventEmitter = require('events');
function MyStream() {
EventEmitter.call(this);
}
util.inherits(MyStream, EventEmitter);
MyStream.prototype.write = function(data) {
this.emit('data', data);
};
const stream = new MyStream();
console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true
stream.on('data', (data) => {
console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"
ES6 example using class
and extends
const EventEmitter = require('events');
class MyStream extends EventEmitter {
write(data) {
this.emit('data', data);
}
}
const stream = new MyStream();
stream.on('data', (data) => {
console.log(`Received data: "${data}"`);
});
stream.write('With ES6');
util.inspect(object[, options])#
object
<any> Any JavaScript primitive or Object.options
<Object>showHidden
<boolean> Iftrue
, theobject
's non-enumerable symbols and properties will be included in the formatted result. Defaults tofalse
.depth
<number> Specifies the number of times to recurse while formatting theobject
. This is useful for inspecting large complicated objects. Defaults to2
. To make it recurse indefinitely passnull
.colors
<boolean> Iftrue
, the output will be styled with ANSI color codes. Defaults tofalse
. Colors are customizable, see Customizingutil.inspect
colors.customInspect
<boolean> Iffalse
, then custominspect(depth, opts)
functions exported on theobject
being inspected will not be called. Defaults totrue
.showProxy
<boolean> Iftrue
, then objects and functions that areProxy
objects will be introspected to show theirtarget
andhandler
objects. Defaults tofalse
.maxArrayLength
<number> Specifies the maximum number of array andTypedArray
elements to include when formatting. Defaults to100
. Set tonull
to show all array elements. Set to0
or negative to show no array elements.breakLength
<number> The length at which an object's keys are split across multiple lines. Set toInfinity
to format an object as a single line. Defaults to 60 for legacy compatibility.
The util.inspect()
method returns a string representation of object
that is
primarily useful for debugging. Additional options
may be passed that alter
certain aspects of the formatted string.
The following example inspects all properties of the util
object:
const util = require('util');
console.log(util.inspect(util, { showHidden: true, depth: null }));
Values may supply their own custom inspect(depth, opts)
functions, when
called these receive the current depth
in the recursive inspection, as well as
the options object passed to util.inspect()
.
Customizing util.inspect
colors#
Color output (if enabled) of util.inspect
is customizable globally
via the util.inspect.styles
and util.inspect.colors
properties.
util.inspect.styles
is a map associating a style name to a color from
util.inspect.colors
.
The default styles and associated colors are:
number
-yellow
boolean
-yellow
string
-green
date
-magenta
regexp
-red
null
-bold
undefined
-grey
special
-cyan
(only applied to functions at this time)name
- (no styling)
The predefined color codes are: white
, grey
, black
, blue
, cyan
,
green
, magenta
, red
and yellow
. There are also bold
, italic
,
underline
and inverse
codes.
Color styling uses ANSI control codes that may not be supported on all terminals.
Custom inspection functions on Objects#
Objects may also define their own [util.inspect.custom](depth, opts)
(or, equivalently inspect(depth, opts)
) function that util.inspect()
will
invoke and use the result of when inspecting the object:
const util = require('util');
class Box {
constructor(value) {
this.value = value;
}
inspect(depth, options) {
if (depth < 0) {
return options.stylize('[Box]', 'special');
}
const newOptions = Object.assign({}, options, {
depth: options.depth === null ? null : options.depth - 1
});
// Five space padding because that's the size of "Box< ".
const padding = ' '.repeat(5);
const inner = util.inspect(this.value, newOptions)
.replace(/\n/g, `\n${padding}`);
return `${options.stylize('Box', 'special')}< ${inner} >`;
}
}
const box = new Box(true);
util.inspect(box);
// Returns: "Box< true >"
Custom [util.inspect.custom](depth, opts)
functions typically return a string
but may return a value of any type that will be formatted accordingly by
util.inspect()
.
const util = require('util');
const obj = { foo: 'this will not show up in the inspect() output' };
obj[util.inspect.custom] = function(depth) {
return { bar: 'baz' };
};
util.inspect(obj);
// Returns: "{ bar: 'baz' }"
A custom inspection method can alternatively be provided by exposing
an inspect(depth, opts)
method on the object:
const util = require('util');
const obj = { foo: 'this will not show up in the inspect() output' };
obj.inspect = function(depth) {
return { bar: 'baz' };
};
util.inspect(obj);
// Returns: "{ bar: 'baz' }"
util.inspect.custom#
A Symbol that can be used to declare custom inspect functions, see Custom inspection functions on Objects.
util.inspect.defaultOptions#
The defaultOptions
value allows customization of the default options used by
util.inspect
. This is useful for functions like console.log
or
util.format
which implicitly call into util.inspect
. It shall be set to an
object containing one or more valid util.inspect()
options. Setting
option properties directly is also supported.
const util = require('util');
const arr = Array(101).fill(0);
console.log(arr); // logs the truncated array
util.inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array
util.promisify(original)#
original
<Function>
Takes a function following the common Node.js callback style, i.e. taking a
(err, value) => ...
callback as the last argument, and returns a version
that returns promises.
For example:
const util = require('util');
const fs = require('fs');
const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});
Or, equivalently using async function
s:
const util = require('util');
const fs = require('fs');
const stat = util.promisify(fs.stat);
async function callStat() {
const stats = await stat('.');
console.log(`This directory is owned by ${stats.uid}`);
}
If there is an original[util.promisify.custom]
property present, promisify
will return its value, see Custom promisified functions.
promisify()
assumes that original
is a function taking a callback as its
final argument in all cases, and the returned function will result in undefined
behavior if it does not.
Custom promisified functions#
Using the util.promisify.custom
symbol one can override the return value of
util.promisify()
:
const util = require('util');
function doSomething(foo, callback) {
// ...
}
doSomething[util.promisify.custom] = function(foo) {
return getPromiseSomehow();
};
const promisified = util.promisify(doSomething);
console.log(promisified === doSomething[util.promisify.custom]);
// prints 'true'
This can be useful for cases where the original function does not follow the standard format of taking an error-first callback as the last argument.
util.promisify.custom#
A Symbol that can be used to declare custom promisified variants of functions, see Custom promisified functions.
Deprecated APIs#
The following APIs have been deprecated and should no longer be used. Existing applications and modules should be updated to find alternative approaches.
util._extend(target, source)#
Stability: 0 - Deprecated: Use Object.assign()
instead.
The util._extend()
method was never intended to be used outside of internal
Node.js modules. The community found and used it anyway.
It is deprecated and should not be used in new code. JavaScript comes with very
similar built-in functionality through Object.assign()
.
util.debug(string)#
Stability: 0 - Deprecated: Use console.error()
instead.
string
<string> The message to print tostderr
Deprecated predecessor of console.error
.
util.error([...strings])#
Stability: 0 - Deprecated: Use console.error()
instead.
...strings
<string> The message to print tostderr
Deprecated predecessor of console.error
.
util.isArray(object)#
Stability: 0 - Deprecated
object
<any>
Internal alias for Array.isArray
.
Returns true
if the given object
is an Array
. Otherwise, returns false
.
const util = require('util');
util.isArray([]);
// Returns: true
util.isArray(new Array());
// Returns: true
util.isArray({});
// Returns: false
util.isBoolean(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a Boolean
. Otherwise, returns false
.
const util = require('util');
util.isBoolean(1);
// Returns: false
util.isBoolean(0);
// Returns: false
util.isBoolean(false);
// Returns: true
util.isBuffer(object)#
Stability: 0 - Deprecated: Use Buffer.isBuffer()
instead.
object
<any>
Returns true
if the given object
is a Buffer
. Otherwise, returns false
.
const util = require('util');
util.isBuffer({ length: 0 });
// Returns: false
util.isBuffer([]);
// Returns: false
util.isBuffer(Buffer.from('hello world'));
// Returns: true
util.isDate(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a Date
. Otherwise, returns false
.
const util = require('util');
util.isDate(new Date());
// Returns: true
util.isDate(Date());
// false (without 'new' returns a String)
util.isDate({});
// Returns: false
util.isError(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is an Error
. Otherwise, returns
false
.
const util = require('util');
util.isError(new Error());
// Returns: true
util.isError(new TypeError());
// Returns: true
util.isError({ name: 'Error', message: 'an error occurred' });
// Returns: false
Note that this method relies on Object.prototype.toString()
behavior. It is
possible to obtain an incorrect result when the object
argument manipulates
@@toStringTag
.
const util = require('util');
const obj = { name: 'Error', message: 'an error occurred' };
util.isError(obj);
// Returns: false
obj[Symbol.toStringTag] = 'Error';
util.isError(obj);
// Returns: true
util.isFunction(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a Function
. Otherwise, returns
false
.
const util = require('util');
function Foo() {}
const Bar = () => {};
util.isFunction({});
// Returns: false
util.isFunction(Foo);
// Returns: true
util.isFunction(Bar);
// Returns: true
util.isNull(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is strictly null
. Otherwise, returns
false
.
const util = require('util');
util.isNull(0);
// Returns: false
util.isNull(undefined);
// Returns: false
util.isNull(null);
// Returns: true
util.isNullOrUndefined(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is null
or undefined
. Otherwise,
returns false
.
const util = require('util');
util.isNullOrUndefined(0);
// Returns: false
util.isNullOrUndefined(undefined);
// Returns: true
util.isNullOrUndefined(null);
// Returns: true
util.isNumber(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a Number
. Otherwise, returns false
.
const util = require('util');
util.isNumber(false);
// Returns: false
util.isNumber(Infinity);
// Returns: true
util.isNumber(0);
// Returns: true
util.isNumber(NaN);
// Returns: true
util.isObject(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is strictly an Object
and not a
Function
. Otherwise, returns false
.
const util = require('util');
util.isObject(5);
// Returns: false
util.isObject(null);
// Returns: false
util.isObject({});
// Returns: true
util.isObject(function() {});
// Returns: false
util.isPrimitive(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a primitive type. Otherwise, returns
false
.
const util = require('util');
util.isPrimitive(5);
// Returns: true
util.isPrimitive('foo');
// Returns: true
util.isPrimitive(false);
// Returns: true
util.isPrimitive(null);
// Returns: true
util.isPrimitive(undefined);
// Returns: true
util.isPrimitive({});
// Returns: false
util.isPrimitive(function() {});
// Returns: false
util.isPrimitive(/^$/);
// Returns: false
util.isPrimitive(new Date());
// Returns: false
util.isRegExp(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a RegExp
. Otherwise, returns false
.
const util = require('util');
util.isRegExp(/some regexp/);
// Returns: true
util.isRegExp(new RegExp('another regexp'));
// Returns: true
util.isRegExp({});
// Returns: false
util.isString(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a string
. Otherwise, returns false
.
const util = require('util');
util.isString('');
// Returns: true
util.isString('foo');
// Returns: true
util.isString(String('foo'));
// Returns: true
util.isString(5);
// Returns: false
util.isSymbol(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is a Symbol
. Otherwise, returns false
.
const util = require('util');
util.isSymbol(5);
// Returns: false
util.isSymbol('foo');
// Returns: false
util.isSymbol(Symbol('foo'));
// Returns: true
util.isUndefined(object)#
Stability: 0 - Deprecated
object
<any>
Returns true
if the given object
is undefined
. Otherwise, returns false
.
const util = require('util');
const foo = undefined;
util.isUndefined(5);
// Returns: false
util.isUndefined(foo);
// Returns: true
util.isUndefined(null);
// Returns: false
util.log(string)#
Stability: 0 - Deprecated: Use a third party module instead.
string
<string>
The util.log()
method prints the given string
to stdout
with an included
timestamp.
const util = require('util');
util.log('Timestamped message.');
util.print([...strings])#
Stability: 0 - Deprecated: Use console.log()
instead.
Deprecated predecessor of console.log
.
util.puts([...strings])#
Stability: 0 - Deprecated: Use console.log()
instead.
Deprecated predecessor of console.log
.
V8#
The v8
module exposes APIs that are specific to the version of V8
built into the Node.js binary. It can be accessed using:
const v8 = require('v8');
Note: The APIs and implementation are subject to change at any time.
v8.cachedDataVersionTag()#
Returns an integer representing a "version tag" derived from the V8 version,
command line flags and detected CPU features. This is useful for determining
whether a vm.Script
cachedData
buffer is compatible with this instance
of V8.
v8.getHeapSpaceStatistics()#
Returns statistics about the V8 heap spaces, i.e. the segments which make up
the V8 heap. Neither the ordering of heap spaces, nor the availability of a
heap space can be guaranteed as the statistics are provided via the V8
GetHeapSpaceStatistics
function and may change from one V8 version to the
next.
The value returned is an array of objects containing the following properties:
space_name
<string>space_size
<number>space_used_size
<number>space_available_size
<number>physical_space_size
<number>
For example:
[
{
"space_name": "new_space",
"space_size": 2063872,
"space_used_size": 951112,
"space_available_size": 80824,
"physical_space_size": 2063872
},
{
"space_name": "old_space",
"space_size": 3090560,
"space_used_size": 2493792,
"space_available_size": 0,
"physical_space_size": 3090560
},
{
"space_name": "code_space",
"space_size": 1260160,
"space_used_size": 644256,
"space_available_size": 960,
"physical_space_size": 1260160
},
{
"space_name": "map_space",
"space_size": 1094160,
"space_used_size": 201608,
"space_available_size": 0,
"physical_space_size": 1094160
},
{
"space_name": "large_object_space",
"space_size": 0,
"space_used_size": 0,
"space_available_size": 1490980608,
"physical_space_size": 0
}
]
v8.getHeapStatistics()#
Returns an object with the following properties:
total_heap_size
<number>total_heap_size_executable
<number>total_physical_size
<number>total_available_size
<number>used_heap_size
<number>heap_size_limit
<number>malloced_memory
<number>peak_malloced_memory
<number>does_zap_garbage
<number>
does_zap_garbage
is a 0/1 boolean, which signifies whether the --zap_code_space
option is enabled or not. This makes V8 overwrite heap garbage with a bit
pattern. The RSS footprint (resident memory set) gets bigger because it
continuously touches all heap pages and that makes them less likely to get
swapped out by the operating system.
For example:
{
total_heap_size: 7326976,
total_heap_size_executable: 4194304,
total_physical_size: 7326976,
total_available_size: 1152656,
used_heap_size: 3476208,
heap_size_limit: 1535115264,
malloced_memory: 16384,
peak_malloced_memory: 1127496,
does_zap_garbage: 0
}
v8.setFlagsFromString(string)#
The v8.setFlagsFromString()
method can be used to programmatically set
V8 command line flags. This method should be used with care. Changing settings
after the VM has started may result in unpredictable behavior, including
crashes and data loss; or it may simply do nothing.
The V8 options available for a version of Node.js may be determined by running
node --v8-options
. An unofficial, community-maintained list of options
and their effects is available here.
Usage:
// Print GC events to stdout for one minute.
const v8 = require('v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(function() { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
Serialization API#
Stability: 1 - Experimental
The serialization API provides means of serializing JavaScript values in a way that is compatible with the HTML structured clone algorithm. The format is backward-compatible (i.e. safe to store to disk).
Note: This API is under development, and changes (including incompatible changes to the API or wire format) may occur until this warning is removed.
v8.serialize(value)#
- Returns: <Buffer>
Uses a DefaultSerializer
to serialize value
into a buffer.
v8.deserialize(buffer)#
buffer
<Buffer> | <Uint8Array> A buffer returned byserialize()
.
Uses a DefaultDeserializer
with default options to read a JS value
from a buffer.
class: v8.Serializer#
new Serializer()#
Creates a new Serializer
object.
serializer.writeHeader()#
Writes out a header, which includes the serialization format version.
serializer.writeValue(value)#
Serializes a JavaScript value and adds the serialized representation to the internal buffer.
This throws an error if value
cannot be serialized.
serializer.releaseBuffer()#
Returns the stored internal buffer. This serializer should not be used once the buffer is released. Calling this method results in undefined behavior if a previous write has failed.
serializer.transferArrayBuffer(id, arrayBuffer)#
id
<integer> A 32-bit unsigned integer.arrayBuffer
<ArrayBuffer> AnArrayBuffer
instance.
Marks an ArrayBuffer
as havings its contents transferred out of band.
Pass the corresponding ArrayBuffer
in the deserializing context to
deserializer.transferArrayBuffer()
.
serializer.writeUint32(value)#
value
<integer>
Write a raw 32-bit unsigned integer.
For use inside of a custom serializer._writeHostObject()
.
serializer.writeUint64(hi, lo)#
Write a raw 64-bit unsigned integer, split into high and low 32-bit parts.
For use inside of a custom serializer._writeHostObject()
.
serializer.writeDouble(value)#
value
<number>
Write a JS number
value.
For use inside of a custom serializer._writeHostObject()
.
serializer.writeRawBytes(buffer)#
buffer
<Buffer> | <Uint8Array>
Write raw bytes into the serializer’s internal buffer. The deserializer
will require a way to compute the length of the buffer.
For use inside of a custom serializer._writeHostObject()
.
serializer._writeHostObject(object)#
object
<Object>
This method is called to write some kind of host object, i.e. an object created
by native C++ bindings. If it is not possible to serialize object
, a suitable
exception should be thrown.
This method is not present on the Serializer
class itself but can be provided
by subclasses.
serializer._getDataCloneError(message)#
message
<string>
This method is called to generate error objects that will be thrown when an object can not be cloned.
This method defaults to the Error
constructor and can be be overridden on
subclasses.
serializer._getSharedArrayBufferId(sharedArrayBuffer)#
sharedArrayBuffer
<SharedArrayBuffer>
This method is called when the serializer is going to serialize a
SharedArrayBuffer
object. It must return an unsigned 32-bit integer ID for
the object, using the same ID if this SharedArrayBuffer
has already been
serialized. When deserializing, this ID will be passed to
deserializer.transferArrayBuffer()
.
If the object cannot be serialized, an exception should be thrown.
This method is not present on the Serializer
class itself but can be provided
by subclasses.
serializer._setTreatArrayBufferViewsAsHostObjects(flag)#
flag
<boolean>
Indicate whether to treat TypedArray
and DataView
objects as
host objects, i.e. pass them to serializer._writeHostObject()
.
The default is not to treat those objects as host objects.
class: v8.Deserializer#
new Deserializer(buffer)#
buffer
<Buffer> | <Uint8Array> A buffer returned byserializer.releaseBuffer()
.
Creates a new Deserializer
object.
deserializer.readHeader()#
Reads and validates a header (including the format version).
May, for example, reject an invalid or unsupported wire format. In that case,
an Error
is thrown.
deserializer.readValue()#
Deserializes a JavaScript value from the buffer and returns it.
deserializer.transferArrayBuffer(id, arrayBuffer)#
id
<integer> A 32-bit unsigned integer.arrayBuffer
<ArrayBuffer> | <SharedArrayBuffer> AnArrayBuffer
instance.
Marks an ArrayBuffer
as havings its contents transferred out of band.
Pass the corresponding ArrayBuffer
in the serializing context to
serializer.transferArrayBuffer()
(or return the id
from
serializer._getSharedArrayBufferId()
in the case of SharedArrayBuffer
s).
deserializer.getWireFormatVersion()#
- Returns: <integer>
Reads the underlying wire format version. Likely mostly to be useful to
legacy code reading old wire format versions. May not be called before
.readHeader()
.
deserializer.readUint32()#
- Returns: <integer>
Read a raw 32-bit unsigned integer and return it.
For use inside of a custom deserializer._readHostObject()
.
deserializer.readUint64()#
- Returns: <Array>
Read a raw 64-bit unsigned integer and return it as an array [hi, lo]
with two 32-bit unsigned integer entries.
For use inside of a custom deserializer._readHostObject()
.
deserializer.readDouble()#
- Returns: <number>
Read a JS number
value.
For use inside of a custom deserializer._readHostObject()
.
deserializer.readRawBytes(length)#
- Returns: <Buffer>
Read raw bytes from the deserializer’s internal buffer. The length
parameter
must correspond to the length of the buffer that was passed to
serializer.writeRawBytes()
.
For use inside of a custom deserializer._readHostObject()
.
deserializer._readHostObject()#
This method is called to read some kind of host object, i.e. an object that is created by native C++ bindings. If it is not possible to deserialize the data, a suitable exception should be thrown.
This method is not present on the Deserializer
class itself but can be
provided by subclasses.
class: v8.DefaultSerializer#
A subclass of Serializer
that serializes TypedArray
(in particular Buffer
) and DataView
objects as host objects, and only
stores the part of their underlying ArrayBuffer
s that they are referring to.
class: v8.DefaultDeserializer#
A subclass of Deserializer
corresponding to the format written by
DefaultSerializer
.
VM (Executing JavaScript)#
Stability: 2 - Stable
The vm
module provides APIs for compiling and running code within V8 Virtual
Machine contexts. It can be accessed using:
const vm = require('vm');
JavaScript code can be compiled and run immediately or compiled, saved, and run later.
Note: The vm module is not a security mechanism. Do not use it to run untrusted code.
Class: vm.Script#
Instances of the vm.Script
class contain precompiled scripts that can be
executed in specific sandboxes (or "contexts").
new vm.Script(code, options)#
code
<string> The JavaScript code to compile.options
filename
<string> Specifies the filename used in stack traces produced by this script.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script.columnOffset
<number> Specifies the column number offset that is displayed in stack traces produced by this script.displayErrors
<boolean> Whentrue
, if anError
error occurs while compiling thecode
, the line of code causing the error is attached to the stack trace.timeout
<number> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown.cachedData
<Buffer> Provides an optionalBuffer
with V8's code cache data for the supplied source. When supplied, thecachedDataRejected
value will be set to eithertrue
orfalse
depending on acceptance of the data by V8.produceCachedData
<boolean> Whentrue
and nocachedData
is present, V8 will attempt to produce code cache data forcode
. Upon success, aBuffer
with V8's code cache data will be produced and stored in thecachedData
property of the returnedvm.Script
instance. ThecachedDataProduced
value will be set to eithertrue
orfalse
depending on whether code cache data is produced successfully.
Creating a new vm.Script
object compiles code
but does not run it. The
compiled vm.Script
can be run later multiple times. It is important to note
that the code
is not bound to any global object; rather, it is bound before
each run, just for that run.
script.runInContext(contextifiedSandbox[, options])#
contextifiedSandbox
<Object> A contextified object as returned by thevm.createContext()
method.options
<Object>filename
<string> Specifies the filename used in stack traces produced by this script.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script.columnOffset
<number> Specifies the column number offset that is displayed in stack traces produced by this script.displayErrors
<boolean> Whentrue
, if anError
error occurs while compiling thecode
, the line of code causing the error is attached to the stack trace.timeout
<number> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown.breakOnSigint
: iftrue
, the execution will be terminated whenSIGINT
(Ctrl+C) is received. Existing handlers for the event that have been attached viaprocess.on("SIGINT")
will be disabled during script execution, but will continue to work after that. If execution is terminated, anError
will be thrown.
Runs the compiled code contained by the vm.Script
object within the given
contextifiedSandbox
and returns the result. Running code does not have access
to local scope.
The following example compiles code that increments a global variable, sets
the value of another global variable, then execute the code multiple times.
The globals are contained in the sandbox
object.
const util = require('util');
const vm = require('vm');
const sandbox = {
animal: 'cat',
count: 2
};
const script = new vm.Script('count += 1; name = "kitty";');
const context = new vm.createContext(sandbox);
for (let i = 0; i < 10; ++i) {
script.runInContext(context);
}
console.log(util.inspect(sandbox));
// { animal: 'cat', count: 12, name: 'kitty' }
Note: Using the timeout
or breakOnSigint
options will result in new
event loops and corresponding threads being started, which have a non-zero
performance overhead.
script.runInNewContext([sandbox[, options]])#
sandbox
<Object> An object that will be contextified. Ifundefined
, a new object will be created.options
<Object>filename
<string> Specifies the filename used in stack traces produced by this script.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script.columnOffset
<number> Specifies the column number offset that is displayed in stack traces produced by this script.displayErrors
<boolean> Whentrue
, if anError
error occurs while compiling thecode
, the line of code causing the error is attached to the stack trace.timeout
<number> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown.
First contextifies the given sandbox
, runs the compiled code contained by
the vm.Script
object within the created sandbox, and returns the result.
Running code does not have access to local scope.
The following example compiles code that sets a global variable, then executes
the code multiple times in different contexts. The globals are set on and
contained within each individual sandbox
.
const util = require('util');
const vm = require('vm');
const script = new vm.Script('globalVar = "set"');
const sandboxes = [{}, {}, {}];
sandboxes.forEach((sandbox) => {
script.runInNewContext(sandbox);
});
console.log(util.inspect(sandboxes));
// [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]
script.runInThisContext(options)#
options
<Object>filename
<string> Specifies the filename used in stack traces produced by this script.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script.columnOffset
<number> Specifies the column number offset that is displayed in stack traces produced by this script.displayErrors
<boolean> Whentrue
, if anError
error occurs while compiling thecode
, the line of code causing the error is attached to the stack trace.timeout
<number> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown.
Runs the compiled code contained by the vm.Script
within the context of the
current global
object. Running code does not have access to local scope, but
does have access to the current global
object.
The following example compiles code that increments a global
variable then
executes that code multiple times:
const vm = require('vm');
global.globalVar = 0;
const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });
for (let i = 0; i < 1000; ++i) {
script.runInThisContext();
}
console.log(globalVar);
// 1000
vm.createContext([sandbox])#
sandbox
<Object>
If given a sandbox
object, the vm.createContext()
method will prepare
that sandbox so that it can be used in calls to
vm.runInContext()
or script.runInContext()
. Inside such scripts,
the sandbox
object will be the global object, retaining all of its existing
properties but also having the built-in objects and functions any standard
global object has. Outside of scripts run by the vm module, global variables
will remain unchanged.
const util = require('util');
const vm = require('vm');
global.globalVar = 3;
const sandbox = { globalVar: 1 };
vm.createContext(sandbox);
vm.runInContext('globalVar *= 2;', sandbox);
console.log(util.inspect(sandbox)); // { globalVar: 2 }
console.log(util.inspect(globalVar)); // 3
If sandbox
is omitted (or passed explicitly as undefined
), a new, empty
contextified sandbox object will be returned.
The vm.createContext()
method is primarily useful for creating a single
sandbox that can be used to run multiple scripts. For instance, if emulating a
web browser, the method can be used to create a single sandbox representing a
window's global object, then run all <script>
tags together within the context
of that sandbox.
vm.isContext(sandbox)#
sandbox
<Object>
Returns true
if the given sandbox
object has been contextified using
vm.createContext()
.
vm.runInContext(code, contextifiedSandbox[, options])#
code
<string> The JavaScript code to compile and run.contextifiedSandbox
<Object> The contextified object that will be used as theglobal
when thecode
is compiled and run.options
filename
<string> Specifies the filename used in stack traces produced by this script.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script.columnOffset
<number> Specifies the column number offset that is displayed in stack traces produced by this script.displayErrors
<boolean> Whentrue
, if anError
error occurs while compiling thecode
, the line of code causing the error is attached to the stack trace.timeout
<number> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown.
The vm.runInContext()
method compiles code
, runs it within the context of
the contextifiedSandbox
, then returns the result. Running code does not have
access to the local scope. The contextifiedSandbox
object must have been
previously contextified using the vm.createContext()
method.
The following example compiles and executes different scripts using a single contextified object:
const util = require('util');
const vm = require('vm');
const sandbox = { globalVar: 1 };
vm.createContext(sandbox);
for (let i = 0; i < 10; ++i) {
vm.runInContext('globalVar *= 2;', sandbox);
}
console.log(util.inspect(sandbox));
// { globalVar: 1024 }
vm.runInDebugContext(code)#
Stability: 0 - Deprecated. An alternative is in development.
code
<string> The JavaScript code to compile and run.
The vm.runInDebugContext()
method compiles and executes code
inside the V8
debug context. The primary use case is to gain access to the V8 Debug
object:
const vm = require('vm');
const Debug = vm.runInDebugContext('Debug');
console.log(Debug.findScript(process.emit).name); // 'events.js'
console.log(Debug.findScript(process.exit).name); // 'internal/process.js'
Note: The debug context and object are intrinsically tied to V8's debugger implementation and may change (or even be removed) without prior warning.
The Debug
object can also be made available using the V8-specific
--expose_debug_as=
command line option.
vm.runInNewContext(code[, sandbox][, options])#
code
<string> The JavaScript code to compile and run.sandbox
<Object> An object that will be contextified. Ifundefined
, a new object will be created.options
filename
<string> Specifies the filename used in stack traces produced by this script.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script.columnOffset
<number> Specifies the column number offset that is displayed in stack traces produced by this script.displayErrors
<boolean> Whentrue
, if anError
error occurs while compiling thecode
, the line of code causing the error is attached to the stack trace.timeout
<number> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown.
The vm.runInNewContext()
first contextifies the given sandbox
object (or
creates a new sandbox
if passed as undefined
), compiles the code
, runs it
within the context of the created context, then returns the result. Running code
does not have access to the local scope.
The following example compiles and executes code that increments a global
variable and sets a new one. These globals are contained in the sandbox
.
const util = require('util');
const vm = require('vm');
const sandbox = {
animal: 'cat',
count: 2
};
vm.runInNewContext('count += 1; name = "kitty"', sandbox);
console.log(util.inspect(sandbox));
// { animal: 'cat', count: 3, name: 'kitty' }
vm.runInThisContext(code[, options])#
code
<string> The JavaScript code to compile and run.options
filename
<string> Specifies the filename used in stack traces produced by this script.lineOffset
<number> Specifies the line number offset that is displayed in stack traces produced by this script.columnOffset
<number> Specifies the column number offset that is displayed in stack traces produced by this script.displayErrors
<boolean> Whentrue
, if anError
error occurs while compiling thecode
, the line of code causing the error is attached to the stack trace.timeout
<number> Specifies the number of milliseconds to executecode
before terminating execution. If execution is terminated, anError
will be thrown.
vm.runInThisContext()
compiles code
, runs it within the context of the
current global
and returns the result. Running code does not have access to
local scope, but does have access to the current global
object.
The following example illustrates using both vm.runInThisContext()
and
the JavaScript eval()
function to run the same code:
const vm = require('vm');
let localVar = 'initial value';
const vmResult = vm.runInThisContext('localVar = "vm";');
console.log('vmResult:', vmResult);
console.log('localVar:', localVar);
const evalResult = eval('localVar = "eval";');
console.log('evalResult:', evalResult);
console.log('localVar:', localVar);
// vmResult: 'vm', localVar: 'initial value'
// evalResult: 'eval', localVar: 'eval'
Because vm.runInThisContext()
does not have access to the local scope,
localVar
is unchanged. In contrast, eval()
does have access to the
local scope, so the value localVar
is changed. In this way
vm.runInThisContext()
is much like an indirect eval()
call, e.g.
(0,eval)('code')
.
Example: Running an HTTP Server within a VM#
When using either script.runInThisContext()
or vm.runInThisContext()
, the
code is executed within the current V8 global context. The code passed
to this VM context will have its own isolated scope.
In order to run a simple web server using the http
module the code passed to
the context must either call require('http')
on its own, or have a reference
to the http
module passed to it. For instance:
'use strict';
const vm = require('vm');
const code = `
(function(require) {
const http = require('http');
http.createServer((request, response) => {
response.writeHead(200, { 'Content-Type': 'text/plain' });
response.end('Hello World\\n');
}).listen(8124);
console.log('Server running at http://127.0.0.1:8124/');
})`;
vm.runInThisContext(code)(require);
Note: The require()
in the above case shares the state with the context it
is passed from. This may introduce risks when untrusted code is executed, e.g.
altering objects in the context in unwanted ways.
What does it mean to "contextify" an object?#
All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:
In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.
When the method vm.createContext()
is called, the sandbox
object that is
passed in (or a newly created object if sandbox
is undefined
) is associated
internally with a new instance of a V8 Context. This V8 Context provides the
code
run using the vm
module's methods with an isolated global environment
within which it can operate. The process of creating the V8 Context and
associating it with the sandbox
object is what this document refers to as
"contextifying" the sandbox
.
Zlib#
Stability: 2 - Stable
The zlib
module provides compression functionality implemented using Gzip and
Deflate/Inflate. It can be accessed using:
const zlib = require('zlib');
Compressing or decompressing a stream (such as a file) can be accomplished by
piping the source stream data through a zlib
stream into a destination stream:
const gzip = zlib.createGzip();
const fs = require('fs');
const inp = fs.createReadStream('input.txt');
const out = fs.createWriteStream('input.txt.gz');
inp.pipe(gzip).pipe(out);
It is also possible to compress or decompress data in a single step:
const input = '.................................';
zlib.deflate(input, (err, buffer) => {
if (!err) {
console.log(buffer.toString('base64'));
} else {
// handle error
}
});
const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
zlib.unzip(buffer, (err, buffer) => {
if (!err) {
console.log(buffer.toString());
} else {
// handle error
}
});
Compressing HTTP requests and responses#
The zlib
module can be used to implement support for the gzip
and deflate
content-encoding mechanisms defined by
HTTP.
The HTTP Accept-Encoding
header is used within an http request to identify
the compression encodings accepted by the client. The Content-Encoding
header is used to identify the compression encodings actually applied to a
message.
Note: the examples given below are drastically simplified to show
the basic concept. Using zlib
encoding can be expensive, and the results
ought to be cached. See Memory Usage Tuning for more information
on the speed/memory/compression tradeoffs involved in zlib
usage.
// client request example
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const request = http.get({ host: 'example.com',
path: '/',
port: 80,
headers: { 'Accept-Encoding': 'gzip,deflate' } });
request.on('response', (response) => {
const output = fs.createWriteStream('example.com_index.html');
switch (response.headers['content-encoding']) {
// or, just use zlib.createUnzip() to handle both cases
case 'gzip':
response.pipe(zlib.createGunzip()).pipe(output);
break;
case 'deflate':
response.pipe(zlib.createInflate()).pipe(output);
break;
default:
response.pipe(output);
break;
}
});
// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
http.createServer((request, response) => {
const raw = fs.createReadStream('index.html');
let acceptEncoding = request.headers['accept-encoding'];
if (!acceptEncoding) {
acceptEncoding = '';
}
// Note: This is not a conformant accept-encoding parser.
// See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
if (/\bdeflate\b/.test(acceptEncoding)) {
response.writeHead(200, { 'Content-Encoding': 'deflate' });
raw.pipe(zlib.createDeflate()).pipe(response);
} else if (/\bgzip\b/.test(acceptEncoding)) {
response.writeHead(200, { 'Content-Encoding': 'gzip' });
raw.pipe(zlib.createGzip()).pipe(response);
} else {
response.writeHead(200, {});
raw.pipe(response);
}
}).listen(1337);
By default, the zlib
methods will throw an error when decompressing
truncated data. However, if it is known that the data is incomplete, or
the desire is to inspect only the beginning of a compressed file, it is
possible to suppress the default error handling by changing the flushing
method that is used to decompress the last chunk of input data:
// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');
zlib.unzip(
buffer,
{ finishFlush: zlib.constants.Z_SYNC_FLUSH },
(err, buffer) => {
if (!err) {
console.log(buffer.toString());
} else {
// handle error
}
});
This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.
Memory Usage Tuning#
From zlib/zconf.h
, modified to node.js's usage:
The memory requirements for deflate are (in bytes):
(1 << (windowBits + 2)) + (1 << (memLevel + 9))
That is: 128K for windowBits=15 + 128K for memLevel = 8 (default values) plus a few kilobytes for small objects.
For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:
const options = { windowBits: 14, memLevel: 7 };
This will, however, generally degrade compression.
The memory requirements for inflate are (in bytes) 1 << windowBits
.
That is, 32K for windowBits=15 (default value) plus a few kilobytes
for small objects.
This is in addition to a single internal output slab buffer of size
chunkSize
, which defaults to 16K.
The speed of zlib
compression is affected most dramatically by the
level
setting. A higher level will result in better compression, but
will take longer to complete. A lower level will result in less
compression, but will be much faster.
In general, greater memory usage options will mean that Node.js has to make
fewer calls to zlib
because it will be able to process more data on
each write
operation. So, this is another factor that affects the
speed, at the cost of memory usage.
Flushing#
Calling .flush()
on a compression stream will make zlib
return as much
output as currently possible. This may come at the cost of degraded compression
quality, but can be useful when data needs to be available as soon as possible.
In the following example, flush()
is used to write a compressed partial
HTTP response to the client:
const zlib = require('zlib');
const http = require('http');
http.createServer((request, response) => {
// For the sake of simplicity, the Accept-Encoding checks are omitted.
response.writeHead(200, { 'content-encoding': 'gzip' });
const output = zlib.createGzip();
output.pipe(response);
setInterval(() => {
output.write(`The current time is ${Date()}\n`, () => {
// The data has been passed to zlib, but the compression algorithm may
// have decided to buffer the data for more efficient compression.
// Calling .flush() will make the data available as soon as the client
// is ready to receive it.
output.flush();
});
}, 1000);
}).listen(1337);
Constants#
All of the constants defined in zlib.h
are also defined on
require('zlib').constants
. In the normal course of operations, it will not be
necessary to use these constants. They are documented so that their presence is
not surprising. This section is taken almost directly from the
zlib documentation. See http://zlib.net/manual.html#Constants for more
details.
Note: Previously, the constants were available directly from
require('zlib')
, for instance zlib.Z_NO_FLUSH
. Accessing the constants
directly from the module is currently still possible but should be considered
deprecated.
Allowed flush values.
zlib.constants.Z_NO_FLUSH
zlib.constants.Z_PARTIAL_FLUSH
zlib.constants.Z_SYNC_FLUSH
zlib.constants.Z_FULL_FLUSH
zlib.constants.Z_FINISH
zlib.constants.Z_BLOCK
zlib.constants.Z_TREES
Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.
zlib.constants.Z_OK
zlib.constants.Z_STREAM_END
zlib.constants.Z_NEED_DICT
zlib.constants.Z_ERRNO
zlib.constants.Z_STREAM_ERROR
zlib.constants.Z_DATA_ERROR
zlib.constants.Z_MEM_ERROR
zlib.constants.Z_BUF_ERROR
zlib.constants.Z_VERSION_ERROR
Compression levels.
zlib.constants.Z_NO_COMPRESSION
zlib.constants.Z_BEST_SPEED
zlib.constants.Z_BEST_COMPRESSION
zlib.constants.Z_DEFAULT_COMPRESSION
Compression strategy.
zlib.constants.Z_FILTERED
zlib.constants.Z_HUFFMAN_ONLY
zlib.constants.Z_RLE
zlib.constants.Z_FIXED
zlib.constants.Z_DEFAULT_STRATEGY
Class Options#
Each class takes an options
object. All options are optional.
Note that some options are only relevant when compressing, and are ignored by the decompression classes.
flush
<integer> (default:zlib.constants.Z_NO_FLUSH
)finishFlush
<integer> (default:zlib.constants.Z_FINISH
)chunkSize
<integer> (default: 16*1024)windowBits
<integer>level
<integer> (compression only)memLevel
<integer> (compression only)strategy
<integer> (compression only)dictionary
<Buffer> | <TypedArray> | <DataView> (deflate/inflate only, empty dictionary by default)info
<boolean> (Iftrue
, returns an object withbuffer
andengine
)
See the description of deflateInit2
and inflateInit2
at
http://zlib.net/manual.html#Advanced for more information on these.
Class: zlib.Deflate#
Compress data using deflate.
Class: zlib.DeflateRaw#
Compress data using deflate, and do not append a zlib
header.
Class: zlib.Gunzip#
Decompress a gzip stream.
Class: zlib.Gzip#
Compress data using gzip.
Class: zlib.Inflate#
Decompress a deflate stream.
Class: zlib.InflateRaw#
Decompress a raw deflate stream.
Class: zlib.Unzip#
Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.
Class: zlib.Zlib#
Not exported by the zlib
module. It is documented here because it is the base
class of the compressor/decompressor classes.
zlib.bytesRead#
The zlib.bytesRead
property specifies the number of bytes read by the engine
before the bytes are processed (compressed or decompressed, as appropriate for
the derived class).
zlib.flush([kind], callback)#
kind
defaults to zlib.constants.Z_FULL_FLUSH
.
Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.
Calling this only flushes data from the internal zlib
state, and does not
perform flushing of any kind on the streams level. Rather, it behaves like a
normal call to .write()
, i.e. it will be queued up behind other pending
writes and will only produce output when data is being read from the stream.
zlib.params(level, strategy, callback)#
Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.
zlib.reset()#
Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.
zlib.constants#
Provides an object enumerating Zlib-related constants.
zlib.createDeflate(options)#
Creates and returns a new Deflate object with the given options.
zlib.createDeflateRaw(options)#
Creates and returns a new DeflateRaw object with the given options.
Note: The zlib library rejects requests for 256-byte windows (i.e.,
{ windowBits: 8 }
in options
). An Error
will be thrown when creating
a DeflateRaw object with this specific value of the windowBits
option.
zlib.createGunzip(options)#
Creates and returns a new Gunzip object with the given options.
zlib.createGzip(options)#
Creates and returns a new Gzip object with the given options.
zlib.createInflate(options)#
Creates and returns a new Inflate object with the given options.
zlib.createInflateRaw(options)#
Creates and returns a new InflateRaw object with the given options.
zlib.createUnzip(options)#
Creates and returns a new Unzip object with the given options.
Convenience Methods#
All of these take a Buffer
, TypedArray
, DataView
, or string as
the first argument, an optional second argument to supply options to the zlib
classes and will call the supplied callback with callback(error, result)
.
Every method has a *Sync
counterpart, which accept the same arguments, but
without a callback.
zlib.deflate(buffer[, options], callback)#
zlib.deflateSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <string>
Compress a chunk of data with Deflate.
zlib.deflateRaw(buffer[, options], callback)#
zlib.deflateRawSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <string>
Compress a chunk of data with DeflateRaw.
zlib.gunzip(buffer[, options], callback)#
zlib.gunzipSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <string>
Decompress a chunk of data with Gunzip.
zlib.gzip(buffer[, options], callback)#
zlib.gzipSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <string>
Compress a chunk of data with Gzip.
zlib.inflate(buffer[, options], callback)#
zlib.inflateSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <string>
Decompress a chunk of data with Inflate.
zlib.inflateRaw(buffer[, options], callback)#
zlib.inflateRawSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <string>
Decompress a chunk of data with InflateRaw.
zlib.unzip(buffer[, options], callback)#
zlib.unzipSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <string>
Decompress a chunk of data with Unzip.