Node.js v22.0.0-nightly20240213544cfc5ef1 documentation
- Node.js v22.0.0-nightly20240213544cfc5ef1
-
►
Table of contents
- File system
- Promise example
- Callback example
- Synchronous example
- Promises API
- Class:
FileHandle
- Event:
'close'
filehandle.appendFile(data[, options])
filehandle.chmod(mode)
filehandle.chown(uid, gid)
filehandle.close()
filehandle.createReadStream([options])
filehandle.createWriteStream([options])
filehandle.datasync()
filehandle.fd
filehandle.read(buffer, offset, length, position)
filehandle.read([options])
filehandle.read(buffer[, options])
filehandle.readableWebStream([options])
filehandle.readFile(options)
filehandle.readLines([options])
filehandle.readv(buffers[, position])
filehandle.stat([options])
filehandle.sync()
filehandle.truncate(len)
filehandle.utimes(atime, mtime)
filehandle.write(buffer, offset[, length[, position]])
filehandle.write(buffer[, options])
filehandle.write(string[, position[, encoding]])
filehandle.writeFile(data, options)
filehandle.writev(buffers[, position])
filehandle[Symbol.asyncDispose]()
- Event:
fsPromises.access(path[, mode])
fsPromises.appendFile(path, data[, options])
fsPromises.chmod(path, mode)
fsPromises.chown(path, uid, gid)
fsPromises.copyFile(src, dest[, mode])
fsPromises.cp(src, dest[, options])
fsPromises.lchmod(path, mode)
fsPromises.lchown(path, uid, gid)
fsPromises.lutimes(path, atime, mtime)
fsPromises.link(existingPath, newPath)
fsPromises.lstat(path[, options])
fsPromises.mkdir(path[, options])
fsPromises.mkdtemp(prefix[, options])
fsPromises.open(path, flags[, mode])
fsPromises.opendir(path[, options])
fsPromises.readdir(path[, options])
fsPromises.readFile(path[, options])
fsPromises.readlink(path[, options])
fsPromises.realpath(path[, options])
fsPromises.rename(oldPath, newPath)
fsPromises.rmdir(path[, options])
fsPromises.rm(path[, options])
fsPromises.stat(path[, options])
fsPromises.statfs(path[, options])
fsPromises.symlink(target, path[, type])
fsPromises.truncate(path[, len])
fsPromises.unlink(path)
fsPromises.utimes(path, atime, mtime)
fsPromises.watch(filename[, options])
fsPromises.writeFile(file, data[, options])
fsPromises.constants
- Class:
- Callback API
fs.access(path[, mode], callback)
fs.appendFile(path, data[, options], callback)
fs.chmod(path, mode, callback)
fs.chown(path, uid, gid, callback)
fs.close(fd[, callback])
fs.copyFile(src, dest[, mode], callback)
fs.cp(src, dest[, options], callback)
fs.createReadStream(path[, options])
fs.createWriteStream(path[, options])
fs.exists(path, callback)
fs.fchmod(fd, mode, callback)
fs.fchown(fd, uid, gid, callback)
fs.fdatasync(fd, callback)
fs.fstat(fd[, options], callback)
fs.fsync(fd, callback)
fs.ftruncate(fd[, len], callback)
fs.futimes(fd, atime, mtime, callback)
fs.lchmod(path, mode, callback)
fs.lchown(path, uid, gid, callback)
fs.lutimes(path, atime, mtime, callback)
fs.link(existingPath, newPath, callback)
fs.lstat(path[, options], callback)
fs.mkdir(path[, options], callback)
fs.mkdtemp(prefix[, options], callback)
fs.open(path[, flags[, mode]], callback)
fs.openAsBlob(path[, options])
fs.opendir(path[, options], callback)
fs.read(fd, buffer, offset, length, position, callback)
fs.read(fd[, options], callback)
fs.read(fd, buffer[, options], callback)
fs.readdir(path[, options], callback)
fs.readFile(path[, options], callback)
fs.readlink(path[, options], callback)
fs.readv(fd, buffers[, position], callback)
fs.realpath(path[, options], callback)
fs.realpath.native(path[, options], callback)
fs.rename(oldPath, newPath, callback)
fs.rmdir(path[, options], callback)
fs.rm(path[, options], callback)
fs.stat(path[, options], callback)
fs.statfs(path[, options], callback)
fs.symlink(target, path[, type], callback)
fs.truncate(path[, len], callback)
fs.unlink(path, callback)
fs.unwatchFile(filename[, listener])
fs.utimes(path, atime, mtime, callback)
fs.watch(filename[, options][, listener])
fs.watchFile(filename[, options], listener)
fs.write(fd, buffer, offset[, length[, position]], callback)
fs.write(fd, buffer[, options], callback)
fs.write(fd, string[, position[, encoding]], callback)
fs.writeFile(file, data[, options], callback)
fs.writev(fd, buffers[, position], callback)
- Synchronous API
fs.accessSync(path[, mode])
fs.appendFileSync(path, data[, options])
fs.chmodSync(path, mode)
fs.chownSync(path, uid, gid)
fs.closeSync(fd)
fs.copyFileSync(src, dest[, mode])
fs.cpSync(src, dest[, options])
fs.existsSync(path)
fs.fchmodSync(fd, mode)
fs.fchownSync(fd, uid, gid)
fs.fdatasyncSync(fd)
fs.fstatSync(fd[, options])
fs.fsyncSync(fd)
fs.ftruncateSync(fd[, len])
fs.futimesSync(fd, atime, mtime)
fs.lchmodSync(path, mode)
fs.lchownSync(path, uid, gid)
fs.lutimesSync(path, atime, mtime)
fs.linkSync(existingPath, newPath)
fs.lstatSync(path[, options])
fs.mkdirSync(path[, options])
fs.mkdtempSync(prefix[, options])
fs.opendirSync(path[, options])
fs.openSync(path[, flags[, mode]])
fs.readdirSync(path[, options])
fs.readFileSync(path[, options])
fs.readlinkSync(path[, options])
fs.readSync(fd, buffer, offset, length[, position])
fs.readSync(fd, buffer[, options])
fs.readvSync(fd, buffers[, position])
fs.realpathSync(path[, options])
fs.realpathSync.native(path[, options])
fs.renameSync(oldPath, newPath)
fs.rmdirSync(path[, options])
fs.rmSync(path[, options])
fs.statSync(path[, options])
fs.statfsSync(path[, options])
fs.symlinkSync(target, path[, type])
fs.truncateSync(path[, len])
fs.unlinkSync(path)
fs.utimesSync(path, atime, mtime)
fs.writeFileSync(file, data[, options])
fs.writeSync(fd, buffer, offset[, length[, position]])
fs.writeSync(fd, buffer[, options])
fs.writeSync(fd, string[, position[, encoding]])
fs.writevSync(fd, buffers[, position])
- Common Objects
- Class:
fs.Dir
- Class:
fs.Dirent
- Class:
fs.FSWatcher
- Class:
fs.StatWatcher
- Class:
fs.ReadStream
- Class:
fs.Stats
stats.isBlockDevice()
stats.isCharacterDevice()
stats.isDirectory()
stats.isFIFO()
stats.isFile()
stats.isSocket()
stats.isSymbolicLink()
stats.dev
stats.ino
stats.mode
stats.nlink
stats.uid
stats.gid
stats.rdev
stats.size
stats.blksize
stats.blocks
stats.atimeMs
stats.mtimeMs
stats.ctimeMs
stats.birthtimeMs
stats.atimeNs
stats.mtimeNs
stats.ctimeNs
stats.birthtimeNs
stats.atime
stats.mtime
stats.ctime
stats.birthtime
- Stat time values
- Class:
fs.StatFs
- Class:
fs.WriteStream
fs.constants
- Class:
- Notes
- File system
-
►
Index
- Assertion testing
- Asynchronous context tracking
- Async hooks
- Buffer
- C++ addons
- C/C++ addons with Node-API
- C++ embedder API
- Child processes
- Cluster
- Command-line options
- Console
- Corepack
- Crypto
- Debugger
- Deprecated APIs
- Diagnostics Channel
- DNS
- Domain
- Errors
- Events
- File system
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules: CommonJS modules
- Modules: ECMAScript modules
- Modules:
node:module
API - Modules: Packages
- Net
- OS
- Path
- Performance hooks
- Permissions
- Process
- Punycode
- Query strings
- Readline
- REPL
- Report
- Single executable applications
- Stream
- String decoder
- Test runner
- Timers
- TLS/SSL
- Trace events
- TTY
- UDP/datagram
- URL
- Utilities
- V8
- VM
- WASI
- Web Crypto API
- Web Streams API
- Worker threads
- Zlib
- ► Other versions
- ► Options
File system#
Source Code: lib/fs.js
The node:fs
module enables interacting with the file system in a
way modeled on standard POSIX functions.
To use the promise-based APIs:
import * as fs from 'node:fs/promises';
const fs = require('node:fs/promises');
To use the callback and sync APIs:
import * as fs from 'node:fs';
const fs = require('node:fs');
All file system operations have synchronous, callback, and promise-based forms, and are accessible using both CommonJS syntax and ES6 Modules (ESM).
Promise example#
Promise-based operations return a promise that is fulfilled when the asynchronous operation is complete.
import { unlink } from 'node:fs/promises';
try {
await unlink('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (error) {
console.error('there was an error:', error.message);
}
const { unlink } = require('node:fs/promises');
(async function(path) {
try {
await unlink(path);
console.log(`successfully deleted ${path}`);
} catch (error) {
console.error('there was an error:', error.message);
}
})('/tmp/hello');
Callback example#
The callback form takes a completion callback function as its last
argument and invokes the operation asynchronously. The arguments passed to
the completion callback depend on the method, but the first argument is always
reserved for an exception. If the operation is completed successfully, then
the first argument is null
or undefined
.
import { unlink } from 'node:fs';
unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
const { unlink } = require('node:fs');
unlink('/tmp/hello', (err) => {
if (err) throw err;
console.log('successfully deleted /tmp/hello');
});
The callback-based versions of the node:fs
module APIs are preferable over
the use of the promise APIs when maximal performance (both in terms of
execution time and memory allocation) is required.
Synchronous example#
The synchronous APIs block the Node.js event loop and further JavaScript
execution until the operation is complete. Exceptions are thrown immediately
and can be handled using try…catch
, or can be allowed to bubble up.
import { unlinkSync } from 'node:fs';
try {
unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
}
const { unlinkSync } = require('node:fs');
try {
unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');
} catch (err) {
// handle the error
}
Promises API#
The fs/promises
API provides asynchronous file system methods that return
promises.
The promise APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.
Class: FileHandle
#
A <FileHandle> object is an object wrapper for a numeric file descriptor.
Instances of the <FileHandle> object are created by the fsPromises.open()
method.
All <FileHandle> objects are <EventEmitter>s.
If a <FileHandle> is not closed using the filehandle.close()
method, it will
try to automatically close the file descriptor and emit a process warning,
helping to prevent memory leaks. Please do not rely on this behavior because
it can be unreliable and the file may not be closed. Instead, always explicitly
close <FileHandle>s. Node.js may change this behavior in the future.
Event: 'close'
#
The 'close'
event is emitted when the <FileHandle> has been closed and can no
longer be used.
filehandle.appendFile(data[, options])
#
data
<string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable> | <Stream>options
<Object> | <string>- Returns: <Promise> Fulfills with
undefined
upon success.
Alias of filehandle.writeFile()
.
When operating on file handles, the mode cannot be changed from what it was set
to with fsPromises.open()
. Therefore, this is equivalent to
filehandle.writeFile()
.
filehandle.chmod(mode)
#
Modifies the permissions on the file. See chmod(2)
.
filehandle.chown(uid, gid)
#
uid
<integer> The file's new owner's user id.gid
<integer> The file's new group's group id.- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the ownership of the file. A wrapper for chown(2)
.
filehandle.close()
#
- Returns: <Promise> Fulfills with
undefined
upon success.
Closes the file handle after waiting for any pending operation on the handle to complete.
import { open } from 'node:fs/promises';
let filehandle;
try {
filehandle = await open('thefile.txt', 'r');
} finally {
await filehandle?.close();
}
filehandle.createReadStream([options])
#
options
<Object>- Returns: <fs.ReadStream>
Unlike the 16 KiB default highWaterMark
for a <stream.Readable>, the stream
returned by this method has a default highWaterMark
of 64 KiB.
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, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. If start
is
omitted or undefined
, filehandle.createReadStream()
reads sequentially from
the current file position. The encoding
can be any one of those accepted by
<Buffer>.
If the FileHandle
points to a character device that only supports blocking
reads (such as keyboard or sound card), read operations do not finish until data
is available. This can prevent the process from exiting and the stream from
closing naturally.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
import { open } from 'node:fs/promises';
const fd = await open('/dev/input/event0');
// Create a stream from some character device.
const stream = fd.createReadStream();
setTimeout(() => {
stream.close(); // This may not close the stream.
// Artificially marking end-of-stream, as if the underlying resource had
// indicated end-of-file by itself, allows the stream to close.
// This does not cancel pending read operations, and if there is such an
// operation, the process may still not be able to exit successfully
// until it finishes.
stream.push(null);
stream.read(0);
}, 100);
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.
An example to read the last 10 bytes of a file which is 100 bytes long:
import { open } from 'node:fs/promises';
const fd = await open('sample.txt');
fd.createReadStream({ start: 90, end: 99 });
filehandle.createWriteStream([options])
#
options
<Object>- Returns: <fs.WriteStream>
options
may also include a start
option to allow writing data at some
position past the beginning of the file, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. Modifying a file rather than
replacing it may require the flags
open
option to be set to r+
rather than
the default r
. The encoding
can be any one of those accepted by <Buffer>.
If autoClose
is set to true (default behavior) on 'error'
or 'finish'
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.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
filehandle.datasync()
#
- Returns: <Promise> Fulfills with
undefined
upon success.
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2)
documentation for details.
Unlike filehandle.sync
this method does not flush modified metadata.
filehandle.fd
#
- <number> The numeric file descriptor managed by the <FileHandle> object.
filehandle.read(buffer, offset, length, position)
#
buffer
<Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.offset
<integer> The location in the buffer at which to start filling. Default:0
length
<integer> The number of bytes to read. Default:buffer.byteLength - offset
position
<integer> | <bigint> | <null> The location where to begin reading data from the file. Ifnull
or-1
, data will be read from the current file position, and the position will be updated. Ifposition
is a non-negative integer, the current file position will remain unchanged. Default::null
- Returns: <Promise> Fulfills upon success with an object with two properties:
bytesRead
<integer> The number of bytes readbuffer
<Buffer> | <TypedArray> | <DataView> A reference to the passed inbuffer
argument.
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
filehandle.read([options])
#
options
<Object>buffer
<Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read. Default:Buffer.alloc(16384)
offset
<integer> The location in the buffer at which to start filling. Default:0
length
<integer> The number of bytes to read. Default:buffer.byteLength - offset
position
<integer> | <bigint> | <null> The location where to begin reading data from the file. Ifnull
or-1
, data will be read from the current file position, and the position will be updated. Ifposition
is a non-negative integer, the current file position will remain unchanged. Default::null
- Returns: <Promise> Fulfills upon success with an object with two properties:
bytesRead
<integer> The number of bytes readbuffer
<Buffer> | <TypedArray> | <DataView> A reference to the passed inbuffer
argument.
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
filehandle.read(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.options
<Object>offset
<integer> The location in the buffer at which to start filling. Default:0
length
<integer> The number of bytes to read. Default:buffer.byteLength - offset
position
<integer> | <bigint> | <null> The location where to begin reading data from the file. Ifnull
or-1
, data will be read from the current file position, and the position will be updated. Ifposition
is a non-negative integer, the current file position will remain unchanged. Default::null
- Returns: <Promise> Fulfills upon success with an object with two properties:
bytesRead
<integer> The number of bytes readbuffer
<Buffer> | <TypedArray> | <DataView> A reference to the passed inbuffer
argument.
Reads data from the file and stores that in the given buffer.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
filehandle.readableWebStream([options])
#
-
options
<Object>type
<string> | <undefined> Whether to open a normal or a'bytes'
stream. Default:undefined
-
Returns: <ReadableStream>
Returns a ReadableStream
that may be used to read the files data.
An error will be thrown if this method is called more than once or is called
after the FileHandle
is closed or closing.
import {
open,
} from 'node:fs/promises';
const file = await open('./some/file/to/read');
for await (const chunk of file.readableWebStream())
console.log(chunk);
await file.close();
const {
open,
} = require('node:fs/promises');
(async () => {
const file = await open('./some/file/to/read');
for await (const chunk of file.readableWebStream())
console.log(chunk);
await file.close();
})();
While the ReadableStream
will read the file to completion, it will not
close the FileHandle
automatically. User code must still call the
fileHandle.close()
method.
filehandle.readFile(options)
#
options
<Object> | <string>encoding
<string> | <null> Default:null
signal
<AbortSignal> allows aborting an in-progress readFile
- Returns: <Promise> Fulfills upon a successful read with the contents of the
file. If no encoding is specified (using
options.encoding
), the data is returned as a <Buffer> object. Otherwise, the data will be a string.
Asynchronously reads the entire contents of a file.
If options
is a string, then it specifies the encoding
.
The <FileHandle> has to support reading.
If one or more filehandle.read()
calls are made on a file handle and then a
filehandle.readFile()
call is made, the data will be read from the current
position till the end of the file. It doesn't always read from the beginning
of the file.
filehandle.readLines([options])
#
options
<Object>- Returns: <readline.InterfaceConstructor>
Convenience method to create a readline
interface and stream over the file.
See filehandle.createReadStream()
for the options.
import { open } from 'node:fs/promises';
const file = await open('./some/file/to/read');
for await (const line of file.readLines()) {
console.log(line);
}
const { open } = require('node:fs/promises');
(async () => {
const file = await open('./some/file/to/read');
for await (const line of file.readLines()) {
console.log(line);
}
})();
filehandle.readv(buffers[, position])
#
buffers
<Buffer[]> | <TypedArray[]> | <DataView[]>position
<integer> | <null> The offset from the beginning of the file where the data should be read from. Ifposition
is not anumber
, the data will be read from the current position. Default:null
- Returns: <Promise> Fulfills upon success an object containing two properties:
bytesRead
<integer> the number of bytes readbuffers
<Buffer[]> | <TypedArray[]> | <DataView[]> property containing a reference to thebuffers
input.
Read from a file and write to an array of <ArrayBufferView>s
filehandle.stat([options])
#
options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with an <fs.Stats> for the file.
filehandle.sync()
#
- Returns: <Promise> Fulfills with
undefined
upon success.
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2)
documentation for more detail.
filehandle.truncate(len)
#
Truncates the file.
If the file was larger than len
bytes, only the first len
bytes will be
retained in the file.
The following example retains only the first four bytes of the file:
import { open } from 'node:fs/promises';
let filehandle = null;
try {
filehandle = await open('temp.txt', 'r+');
await filehandle.truncate(4);
} finally {
await filehandle?.close();
}
If the file previously was shorter than len
bytes, it is extended, and the
extended part is filled with null bytes ('\0'
):
If len
is negative then 0
will be used.
filehandle.utimes(atime, mtime)
#
Change the file system timestamps of the object referenced by the <FileHandle> then fulfills the promise with no arguments upon success.
filehandle.write(buffer, offset[, length[, position]])
#
buffer
<Buffer> | <TypedArray> | <DataView>offset
<integer> The start position from withinbuffer
where the data to write begins.length
<integer> The number of bytes frombuffer
to write. Default:buffer.byteLength - offset
position
<integer> | <null> The offset from the beginning of the file where the data frombuffer
should be written. Ifposition
is not anumber
, the data will be written at the current position. See the POSIXpwrite(2)
documentation for more detail. Default:null
- Returns: <Promise>
Write buffer
to the file.
The promise is fulfilled with an object containing two properties:
bytesWritten
<integer> the number of bytes writtenbuffer
<Buffer> | <TypedArray> | <DataView> a reference to thebuffer
written.
It is unsafe to use filehandle.write()
multiple times on the same file
without waiting for the promise to be fulfilled (or rejected). For this
scenario, use filehandle.createWriteStream()
.
On Linux, positional writes do not 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.
filehandle.write(buffer[, options])
#
buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>- Returns: <Promise>
Write buffer
to the file.
Similar to the above filehandle.write
function, this version takes an
optional options
object. If no options
object is specified, it will
default with the above values.
filehandle.write(string[, position[, encoding]])
#
string
<string>position
<integer> | <null> The offset from the beginning of the file where the data fromstring
should be written. Ifposition
is not anumber
the data will be written at the current position. See the POSIXpwrite(2)
documentation for more detail. Default:null
encoding
<string> The expected string encoding. Default:'utf8'
- Returns: <Promise>
Write string
to the file. If string
is not a string, the promise is
rejected with an error.
The promise is fulfilled with an object containing two properties:
bytesWritten
<integer> the number of bytes writtenbuffer
<string> a reference to thestring
written.
It is unsafe to use filehandle.write()
multiple times on the same file
without waiting for the promise to be fulfilled (or rejected). For this
scenario, use filehandle.createWriteStream()
.
On Linux, positional writes do not 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.
filehandle.writeFile(data, options)
#
data
<string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable> | <Stream>options
<Object> | <string>- Returns: <Promise>
Asynchronously writes data to a file, replacing the file if it already exists.
data
can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.
The promise is fulfilled with no arguments upon success.
If options
is a string, then it specifies the encoding
.
The <FileHandle> has to support writing.
It is unsafe to use filehandle.writeFile()
multiple times on the same file
without waiting for the promise to be fulfilled (or rejected).
If one or more filehandle.write()
calls are made on a file handle and then a
filehandle.writeFile()
call is made, the data will be written from the
current position till the end of the file. It doesn't always write from the
beginning of the file.
filehandle.writev(buffers[, position])
#
buffers
<Buffer[]> | <TypedArray[]> | <DataView[]>position
<integer> | <null> The offset from the beginning of the file where the data frombuffers
should be written. Ifposition
is not anumber
, the data will be written at the current position. Default:null
- Returns: <Promise>
Write an array of <ArrayBufferView>s to the file.
The promise is fulfilled with an object containing a two properties:
bytesWritten
<integer> the number of bytes writtenbuffers
<Buffer[]> | <TypedArray[]> | <DataView[]> a reference to thebuffers
input.
It is unsafe to call writev()
multiple times on the same file without waiting
for the promise to be fulfilled (or rejected).
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.
filehandle[Symbol.asyncDispose]()
#
An alias for filehandle.close()
.
fsPromises.access(path[, mode])
#
path
<string> | <Buffer> | <URL>mode
<integer> Default:fs.constants.F_OK
- Returns: <Promise> Fulfills with
undefined
upon success.
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. mode
should be either the value fs.constants.F_OK
or a mask consisting of the bitwise OR of any of fs.constants.R_OK
,
fs.constants.W_OK
, and fs.constants.X_OK
(e.g.
fs.constants.W_OK | fs.constants.R_OK
). Check File access constants for
possible values of mode
.
If the accessibility check is successful, the promise is fulfilled with no
value. If any of the accessibility checks fail, the promise is rejected
with an <Error> object. The following example checks if the file
/etc/passwd
can be read and written by the current process.
import { access, constants } from 'node:fs/promises';
try {
await access('/etc/passwd', constants.R_OK | constants.W_OK);
console.log('can access');
} catch {
console.error('cannot access');
}
Using fsPromises.access()
to check for the accessibility of a file before
calling fsPromises.open()
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.
fsPromises.appendFile(path, data[, options])
#
path
<string> | <Buffer> | <URL> | <FileHandle> filename or <FileHandle>data
<string> | <Buffer>options
<Object> | <string>- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a <Buffer>.
If options
is a string, then it specifies the encoding
.
The mode
option only affects the newly created file. See fs.open()
for more details.
The path
may be specified as a <FileHandle> that has been opened
for appending (using fsPromises.open()
).
fsPromises.chmod(path, mode)
#
path
<string> | <Buffer> | <URL>mode
<string> | <integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the permissions of a file.
fsPromises.chown(path, uid, gid)
#
path
<string> | <Buffer> | <URL>uid
<integer>gid
<integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the ownership of a file.
fsPromises.copyFile(src, dest[, mode])
#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationmode
<integer> Optional modifiers that specify the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g.fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
) Default:0
.fs.constants.COPYFILE_EXCL
: The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously copies src
to dest
. By default, dest
is overwritten if it
already exists.
No guarantees are made about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, an attempt will be made to remove the destination.
import { copyFile, constants } from 'node:fs/promises';
try {
await copyFile('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
} catch {
console.error('The file could not be copied');
}
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
try {
await copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
console.log('source.txt was copied to destination.txt');
} catch {
console.error('The file could not be copied');
}
fsPromises.cp(src, dest[, options])
#
src
<string> | <URL> source path to copy.dest
<string> | <URL> destination path to copy to.options
<Object>dereference
<boolean> dereference symlinks. Default:false
.errorOnExist
<boolean> whenforce
isfalse
, and the destination exists, throw an error. Default:false
.filter
<Function> Function to filter copied files/directories. Returntrue
to copy the item,false
to ignore it. When ignoring a directory, all of its contents will be skipped as well. Can also return aPromise
that resolves totrue
orfalse
Default:undefined
.force
<boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use theerrorOnExist
option to change this behavior. Default:true
.mode
<integer> modifiers for copy operation. Default:0
. Seemode
flag offsPromises.copyFile()
.preserveTimestamps
<boolean> Whentrue
timestamps fromsrc
will be preserved. Default:false
.recursive
<boolean> copy directories recursively Default:false
verbatimSymlinks
<boolean> Whentrue
, path resolution for symlinks will be skipped. Default:false
- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously copies the entire directory structure from src
to dest
,
including subdirectories and files.
When copying a directory to another directory, globs are not supported and
behavior is similar to cp dir1/ dir2/
.
fsPromises.lchmod(path, mode)
#
path
<string> | <Buffer> | <URL>mode
<integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the permissions on a symbolic link.
This method is only implemented on macOS.
fsPromises.lchown(path, uid, gid)
#
path
<string> | <Buffer> | <URL>uid
<integer>gid
<integer>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the ownership on a symbolic link.
fsPromises.lutimes(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>- Returns: <Promise> Fulfills with
undefined
upon success.
Changes the access and modification times of a file in the same way as
fsPromises.utimes()
, with the difference that if the path refers to a
symbolic link, then the link is not dereferenced: instead, the timestamps of
the symbolic link itself are changed.
fsPromises.link(existingPath, newPath)
#
existingPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>- Returns: <Promise> Fulfills with
undefined
upon success.
Creates a new link from the existingPath
to the newPath
. See the POSIX
link(2)
documentation for more detail.
fsPromises.lstat(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with the <fs.Stats> object for the given
symbolic link
path
.
Equivalent to fsPromises.stat()
unless path
refers to a symbolic link,
in which case the link itself is stat-ed, not the file that it refers to.
Refer to the POSIX lstat(2)
document for more detail.
fsPromises.mkdir(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object> | <integer>- Returns: <Promise> Upon success, fulfills with
undefined
ifrecursive
isfalse
, or the first directory path created ifrecursive
istrue
.
Asynchronously creates a directory.
The optional options
argument can be an integer specifying mode
(permission
and sticky bits), or an object with a mode
property and a recursive
property indicating whether parent directories should be created. Calling
fsPromises.mkdir()
when path
is a directory that exists results in a
rejection only when recursive
is false.
import { mkdir } from 'node:fs/promises';
try {
const projectFolder = new URL('./test/project/', import.meta.url);
const createDir = await mkdir(projectFolder, { recursive: true });
console.log(`created ${createDir}`);
} catch (err) {
console.error(err.message);
}
const { mkdir } = require('node:fs/promises');
const { join } = require('node:path');
async function makeDirectory() {
const projectFolder = join(__dirname, 'test', 'project');
const dirCreation = await mkdir(projectFolder, { recursive: true });
console.log(dirCreation);
return dirCreation;
}
makeDirectory().catch(console.error);
fsPromises.mkdtemp(prefix[, options])
#
prefix
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <Promise> Fulfills with a string containing the file system path of the newly created temporary directory.
Creates a unique temporary directory. A unique directory name is generated by
appending six random characters to the end of the provided prefix
. Due to
platform inconsistencies, avoid trailing X
characters in prefix
. Some
platforms, notably the BSDs, can return more than six random characters, and
replace trailing X
characters in prefix
with random characters.
The optional options
argument can be a string specifying an encoding, or an
object with an encoding
property specifying the character encoding to use.
import { mkdtemp } from 'node:fs/promises';
import { join } from 'node:path';
import { tmpdir } from 'node:os';
try {
await mkdtemp(join(tmpdir(), 'foo-'));
} catch (err) {
console.error(err);
}
The fsPromises.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('node:path').sep
).
fsPromises.open(path, flags[, mode])
#
path
<string> | <Buffer> | <URL>flags
<string> | <number> See support of file systemflags
. Default:'r'
.mode
<string> | <integer> Sets the file mode (permission and sticky bits) if the file is created. Default:0o666
(readable and writable)- Returns: <Promise> Fulfills with a <FileHandle> object.
Opens a <FileHandle>.
Refer to the POSIX open(2)
documentation for more detail.
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.
fsPromises.opendir(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>encoding
<string> | <null> Default:'utf8'
bufferSize
<number> Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default:32
recursive
<boolean> ResolvedDir
will be an <AsyncIterable> containing all sub files and directories. Default:false
- Returns: <Promise> Fulfills with an <fs.Dir>.
Asynchronously open a directory for iterative scanning. See the POSIX
opendir(3)
documentation for more detail.
Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.
The encoding
option sets the encoding for the path
while opening the
directory and subsequent read operations.
Example using async iteration:
import { opendir } from 'node:fs/promises';
try {
const dir = await opendir('./');
for await (const dirent of dir)
console.log(dirent.name);
} catch (err) {
console.error(err);
}
When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.
fsPromises.readdir(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>- Returns: <Promise> Fulfills with an array of the names of the files in
the directory excluding
'.'
and'..'
.
Reads the contents of a directory.
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. If the encoding
is set to 'buffer'
, the filenames returned
will be passed as <Buffer> objects.
If options.withFileTypes
is set to true
, the returned array will contain
<fs.Dirent> objects.
import { readdir } from 'node:fs/promises';
try {
const files = await readdir(path);
for (const file of files)
console.log(file);
} catch (err) {
console.error(err);
}
fsPromises.readFile(path[, options])
#
path
<string> | <Buffer> | <URL> | <FileHandle> filename orFileHandle
options
<Object> | <string>encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.signal
<AbortSignal> allows aborting an in-progress readFile
- Returns: <Promise> Fulfills with the contents of the file.
Asynchronously reads the entire contents of a file.
If no encoding is specified (using options.encoding
), the data is returned
as a <Buffer> object. Otherwise, the data will be a string.
If options
is a string, then it specifies the encoding.
When the path
is a directory, the behavior of fsPromises.readFile()
is
platform-specific. On macOS, Linux, and Windows, the promise will be rejected
with an error. On FreeBSD, a representation of the directory's contents will be
returned.
An example of reading a package.json
file located in the same directory of the
running code:
import { readFile } from 'node:fs/promises';
try {
const filePath = new URL('./package.json', import.meta.url);
const contents = await readFile(filePath, { encoding: 'utf8' });
console.log(contents);
} catch (err) {
console.error(err.message);
}
const { readFile } = require('node:fs/promises');
const { resolve } = require('node:path');
async function logFile() {
try {
const filePath = resolve('./package.json');
const contents = await readFile(filePath, { encoding: 'utf8' });
console.log(contents);
} catch (err) {
console.error(err.message);
}
}
logFile();
It is possible to abort an ongoing readFile
using an <AbortSignal>. If a
request is aborted the promise returned is rejected with an AbortError
:
import { readFile } from 'node:fs/promises';
try {
const controller = new AbortController();
const { signal } = controller;
const promise = readFile(fileName, { signal });
// Abort the request before the promise settles.
controller.abort();
await promise;
} catch (err) {
// When a request is aborted - err is an AbortError
console.error(err);
}
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.readFile
performs.
Any specified <FileHandle> has to support reading.
fsPromises.readlink(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <Promise> Fulfills with the
linkString
upon success.
Reads the contents of the symbolic link referred to by path
. See the POSIX
readlink(2)
documentation for more detail. The promise is fulfilled with the
linkString
upon success.
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 returned. If the encoding
is set to 'buffer'
, the link path
returned will be passed as a <Buffer> object.
fsPromises.realpath(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <Promise> Fulfills with the resolved path upon success.
Determines the actual location of path
using the same semantics as the
fs.realpath.native()
function.
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. If the encoding
is set to 'buffer'
, the path returned will be
passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fsPromises.rename(oldPath, newPath)
#
oldPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>- Returns: <Promise> Fulfills with
undefined
upon success.
Renames oldPath
to newPath
.
fsPromises.rmdir(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js retries the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default:false
. Deprecated.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
- Returns: <Promise> Fulfills with
undefined
upon success.
Removes the directory identified by path
.
Using fsPromises.rmdir()
on a file (not a directory) results in the
promise being rejected with an ENOENT
error on Windows and an ENOTDIR
error on POSIX.
To get a behavior similar to the rm -rf
Unix command, use
fsPromises.rm()
with options { recursive: true, force: true }
.
fsPromises.rm(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>force
<boolean> Whentrue
, exceptions will be ignored ifpath
does not exist. Default:false
.maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js will retry the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode operations are retried on failure. Default:false
.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
- Returns: <Promise> Fulfills with
undefined
upon success.
Removes files and directories (modeled on the standard POSIX rm
utility).
fsPromises.stat(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with the <fs.Stats> object for the
given
path
.
fsPromises.statfs(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.StatFs> object should bebigint
. Default:false
.
- Returns: <Promise> Fulfills with the <fs.StatFs> object for the
given
path
.
fsPromises.symlink(target, path[, type])
#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> | <null> Default:null
- Returns: <Promise> Fulfills with
undefined
upon success.
Creates a symbolic link.
The type
argument is only used on Windows platforms and can be one of 'dir'
,
'file'
, or 'junction'
. If the type
argument is not a string, Node.js will
autodetect target
type and use 'file'
or 'dir'
. If the target
does not
exist, 'file'
will be used. Windows junction points require the destination
path to be absolute. When using 'junction'
, the target
argument will
automatically be normalized to absolute path. Junction points on NTFS volumes
can only point to directories.
fsPromises.truncate(path[, len])
#
path
<string> | <Buffer> | <URL>len
<integer> Default:0
- Returns: <Promise> Fulfills with
undefined
upon success.
Truncates (shortens or extends the length) of the content at path
to len
bytes.
fsPromises.unlink(path)
#
If path
refers to a symbolic link, then the link is removed without affecting
the file or directory to which that link refers. If the path
refers to a file
path that is not a symbolic link, the file is deleted. See the POSIX unlink(2)
documentation for more detail.
fsPromises.utimes(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>- Returns: <Promise> Fulfills with
undefined
upon success.
Change the file system timestamps of the object referenced by path
.
The atime
and mtime
arguments follow these rules:
- Values can be either numbers representing Unix epoch time,
Date
s, or a numeric string like'123456789.0'
. - If the value can not be converted to a number, or is
NaN
,Infinity
, or-Infinity
, anError
will be thrown.
fsPromises.watch(filename[, options])
#
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'
.signal
<AbortSignal> An <AbortSignal> used to signal when the watcher should stop.
- Returns: <AsyncIterator> of objects with the properties:
Returns an async iterator that watches for changes on filename
, where filename
is either a file or a directory.
const { watch } = require('node:fs/promises');
const ac = new AbortController();
const { signal } = ac;
setTimeout(() => ac.abort(), 10000);
(async () => {
try {
const watcher = watch(__filename, { signal });
for await (const event of watcher)
console.log(event);
} catch (err) {
if (err.name === 'AbortError')
return;
throw err;
}
})();
On most platforms, 'rename'
is emitted whenever a filename appears or
disappears in the directory.
All the caveats for fs.watch()
also apply to fsPromises.watch()
.
fsPromises.writeFile(file, data[, options])
#
file
<string> | <Buffer> | <URL> | <FileHandle> filename orFileHandle
data
<string> | <Buffer> | <TypedArray> | <DataView> | <AsyncIterable> | <Iterable> | <Stream>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.flush
<boolean> If all data is successfully written to the file, andflush
istrue
,filehandle.sync()
is used to flush the data. Default:false
.signal
<AbortSignal> allows aborting an in-progress writeFile
- Returns: <Promise> Fulfills with
undefined
upon success.
Asynchronously writes data to a file, replacing the file if it already exists.
data
can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.
The encoding
option is ignored if data
is a buffer.
If options
is a string, then it specifies the encoding.
The mode
option only affects the newly created file. See fs.open()
for more details.
Any specified <FileHandle> has to support writing.
It is unsafe to use fsPromises.writeFile()
multiple times on the same file
without waiting for the promise to be settled.
Similarly to fsPromises.readFile
- fsPromises.writeFile
is a convenience
method that performs multiple write
calls internally to write the buffer
passed to it. For performance sensitive code consider using
fs.createWriteStream()
or filehandle.createWriteStream()
.
It is possible to use an <AbortSignal> to cancel an fsPromises.writeFile()
.
Cancelation is "best effort", and some amount of data is likely still
to be written.
import { writeFile } from 'node:fs/promises';
import { Buffer } from 'node:buffer';
try {
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
const promise = writeFile('message.txt', data, { signal });
// Abort the request before the promise settles.
controller.abort();
await promise;
} catch (err) {
// When a request is aborted - err is an AbortError
console.error(err);
}
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.writeFile
performs.
fsPromises.constants
#
Returns an object containing commonly used constants for file system
operations. The object is the same as fs.constants
. See FS constants
for more details.
Callback API#
The callback APIs perform all operations asynchronously, without blocking the event loop, then invoke a callback function upon completion or error.
The callback APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.
fs.access(path[, mode], callback)
#
path
<string> | <Buffer> | <URL>mode
<integer> Default:fs.constants.F_OK
callback
<Function>err
<Error>
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. mode
should be either the value fs.constants.F_OK
or a mask consisting of the bitwise OR of any of fs.constants.R_OK
,
fs.constants.W_OK
, and fs.constants.X_OK
(e.g.
fs.constants.W_OK | fs.constants.R_OK
). Check File access constants for
possible values of mode
.
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 an Error
object. The following examples check if
package.json
exists, and if it is readable or writable.
import { access, constants } from 'node:fs';
const file = 'package.json';
// Check if the file exists in the current directory.
access(file, constants.F_OK, (err) => {
console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});
// Check if the file is readable.
access(file, constants.R_OK, (err) => {
console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});
// Check if the file is writable.
access(file, constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});
// Check if the file is readable and writable.
access(file, constants.R_OK | constants.W_OK, (err) => {
console.log(`${file} ${err ? 'is not' : 'is'} readable and writable`);
});
Do not use fs.access()
to check for the accessibility of a file before calling
fs.open()
, fs.readFile()
, or fs.writeFile()
. 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.
write (NOT RECOMMENDED)
import { access, open, close } from 'node:fs';
access('myfile', (err) => {
if (!err) {
console.error('myfile already exists');
return;
}
open('myfile', 'wx', (err, fd) => {
if (err) throw err;
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
});
write (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
read (NOT RECOMMENDED)
import { access, open, close } from 'node:fs';
access('myfile', (err) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
open('myfile', 'r', (err, fd) => {
if (err) throw err;
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
});
read (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
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 will not be used directly, for example when its accessibility is a signal from another process.
On Windows, access-control policies (ACLs) on a directory may limit access to
a file or directory. The fs.access()
function, however, does not check the
ACL and therefore may report that a path is accessible even if the ACL restricts
the user from reading or writing to it.
fs.appendFile(path, data[, options], callback)
#
path
<string> | <Buffer> | <URL> | <number> filename or file descriptordata
<string> | <Buffer>options
<Object> | <string>callback
<Function>err
<Error>
Asynchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a <Buffer>.
The mode
option only affects the newly created file. See fs.open()
for more details.
import { appendFile } from 'node: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:
import { appendFile } from 'node:fs';
appendFile('message.txt', 'data to append', 'utf8', callback);
The path
may be specified as a numeric file descriptor that has been opened
for appending (using fs.open()
or fs.openSync()
). The file descriptor will
not be closed automatically.
import { open, close, appendFile } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('message.txt', 'a', (err, fd) => {
if (err) throw err;
try {
appendFile(fd, 'data to append', 'utf8', (err) => {
closeFd(fd);
if (err) throw err;
});
} catch (err) {
closeFd(fd);
throw err;
}
});
fs.chmod(path, mode, callback)
#
Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.
See the POSIX chmod(2)
documentation for more detail.
import { chmod } from 'node:fs';
chmod('my_file.txt', 0o775, (err) => {
if (err) throw err;
console.log('The permissions for file "my_file.txt" have been changed!');
});
File modes#
The mode
argument used in both the fs.chmod()
and fs.chmodSync()
methods is a numeric bitmask created using a logical OR of the following
constants:
Constant | Octal | Description |
---|---|---|
fs.constants.S_IRUSR | 0o400 | read by owner |
fs.constants.S_IWUSR | 0o200 | write by owner |
fs.constants.S_IXUSR | 0o100 | execute/search by owner |
fs.constants.S_IRGRP | 0o40 | read by group |
fs.constants.S_IWGRP | 0o20 | write by group |
fs.constants.S_IXGRP | 0o10 | execute/search by group |
fs.constants.S_IROTH | 0o4 | read by others |
fs.constants.S_IWOTH | 0o2 | write by others |
fs.constants.S_IXOTH | 0o1 | execute/search by others |
An easier method of constructing the mode
is to use a sequence of three
octal digits (e.g. 765
). The left-most digit (7
in the example), specifies
the permissions for the file owner. The middle digit (6
in the example),
specifies permissions for the group. The right-most digit (5
in the example),
specifies the permissions for others.
Number | Description |
---|---|
7 | read, write, and execute |
6 | read and write |
5 | read and execute |
4 | read only |
3 | write and execute |
2 | write only |
1 | execute only |
0 | no permission |
For example, the octal value 0o765
means:
- The owner may read, write, and execute the file.
- The group may read and write the file.
- Others may read and execute the file.
When using raw numbers where file modes are expected, any value larger than
0o777
may result in platform-specific behaviors that are not supported to work
consistently. Therefore constants like S_ISVTX
, S_ISGID
, or S_ISUID
are
not exposed in fs.constants
.
Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner, or others is not implemented.
fs.chown(path, uid, gid, callback)
#
Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.
See the POSIX chown(2)
documentation for more detail.
fs.close(fd[, callback])
#
fd
<integer>callback
<Function>err
<Error>
Closes the file descriptor. No arguments other than a possible exception are given to the completion callback.
Calling fs.close()
on any file descriptor (fd
) that is currently in use
through any other fs
operation may lead to undefined behavior.
See the POSIX close(2)
documentation for more detail.
fs.copyFile(src, dest[, mode], callback)
#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationmode
<integer> modifiers for copy operation. Default:0
.callback
<Function>
Asynchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. No arguments other than a possible exception are given to the
callback function. Node.js makes no guarantees about the atomicity of the copy
operation. If an error occurs after the destination file has been opened for
writing, Node.js will attempt to remove the destination.
mode
is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
: The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
import { copyFile, constants } from 'node:fs';
function callback(err) {
if (err) throw err;
console.log('source.txt was copied to destination.txt');
}
// destination.txt will be created or overwritten by default.
copyFile('source.txt', 'destination.txt', callback);
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL, callback);
fs.cp(src, dest[, options], callback)
#
src
<string> | <URL> source path to copy.dest
<string> | <URL> destination path to copy to.options
<Object>dereference
<boolean> dereference symlinks. Default:false
.errorOnExist
<boolean> whenforce
isfalse
, and the destination exists, throw an error. Default:false
.filter
<Function> Function to filter copied files/directories. Returntrue
to copy the item,false
to ignore it. When ignoring a directory, all of its contents will be skipped as well. Can also return aPromise
that resolves totrue
orfalse
Default:undefined
.force
<boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use theerrorOnExist
option to change this behavior. Default:true
.mode
<integer> modifiers for copy operation. Default:0
. Seemode
flag offs.copyFile()
.preserveTimestamps
<boolean> Whentrue
timestamps fromsrc
will be preserved. Default:false
.recursive
<boolean> copy directories recursively Default:false
verbatimSymlinks
<boolean> Whentrue
, path resolution for symlinks will be skipped. Default:false
callback
<Function>
Asynchronously copies the entire directory structure from src
to dest
,
including subdirectories and files.
When copying a directory to another directory, globs are not supported and
behavior is similar to cp dir1/ dir2/
.
fs.createReadStream(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>flags
<string> See support of file systemflags
. Default:'r'
.encoding
<string> Default:null
fd
<integer> | <FileHandle> Default:null
mode
<integer> Default:0o666
autoClose
<boolean> Default:true
emitClose
<boolean> Default:true
start
<integer>end
<integer> Default:Infinity
highWaterMark
<integer> Default:64 * 1024
fs
<Object> | <null> Default:null
signal
<AbortSignal> | <null> Default:null
- Returns: <fs.ReadStream>
Unlike the 16 KiB default highWaterMark
for a <stream.Readable>, the stream
returned by this method has a default highWaterMark
of 64 KiB.
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, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. 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. fd
should be blocking; non-blocking fd
s should be passed to
<net.Socket>.
If fd
points to a character device that only supports blocking reads
(such as keyboard or sound card), read operations do not finish until data is
available. This can prevent the process from exiting and the stream from
closing naturally.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
By providing the fs
option, it is possible to override the corresponding fs
implementations for open
, read
, and close
. When providing the fs
option,
an override for read
is required. If no fd
is provided, an override for
open
is also required. If autoClose
is true
, an override for close
is
also required.
import { createReadStream } from 'node:fs';
// Create a stream from some character device.
const stream = createReadStream('/dev/input/event0');
setTimeout(() => {
stream.close(); // This may not close the stream.
// Artificially marking end-of-stream, as if the underlying resource had
// indicated end-of-file by itself, allows the stream to close.
// This does not cancel pending read operations, and if there is such an
// operation, the process may still not be able to exit successfully
// until it finishes.
stream.push(null);
stream.read(0);
}, 100);
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:
import { createReadStream } from 'node:fs';
createReadStream('sample.txt', { start: 90, end: 99 });
If options
is a string, then it specifies the encoding.
fs.createWriteStream(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>flags
<string> See support of file systemflags
. Default:'w'
.encoding
<string> Default:'utf8'
fd
<integer> | <FileHandle> Default:null
mode
<integer> Default:0o666
autoClose
<boolean> Default:true
emitClose
<boolean> Default:true
start
<integer>fs
<Object> | <null> Default:null
signal
<AbortSignal> | <null> Default:null
highWaterMark
<number> Default:16384
flush
<boolean> Iftrue
, the underlying file descriptor is flushed prior to closing it. Default:false
.
- Returns: <fs.WriteStream>
options
may also include a start
option to allow writing data at some
position past the beginning of the file, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. Modifying a file rather than
replacing it may require the flags
option to be set to r+
rather than the
default w
. The encoding
can be any one of those accepted by <Buffer>.
If autoClose
is set to true (default behavior) on 'error'
or 'finish'
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.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
By providing the fs
option it is possible to override the corresponding fs
implementations for open
, write
, writev
, and close
. Overriding write()
without writev()
can reduce performance as some optimizations (_writev()
)
will be disabled. When providing the fs
option, overrides for at least one of
write
and writev
are required. If no fd
option is supplied, an override
for open
is also required. If autoClose
is true
, an override for close
is also required.
Like <fs.ReadStream>, if fd
is specified, <fs.WriteStream> will ignore the
path
argument and will use the specified file descriptor. This means that no
'open'
event will be emitted. 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)
#
path
<string> | <Buffer> | <URL>callback
<Function>exists
<boolean>
Test whether or not the given path exists by checking with the file system.
Then call the callback
argument with either true or false:
import { exists } from 'node:fs';
exists('/etc/passwd', (e) => {
console.log(e ? 'it exists' : 'no passwd!');
});
The parameters for this callback are 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.
write (NOT RECOMMENDED)
import { exists, open, close } from 'node:fs';
exists('myfile', (e) => {
if (e) {
console.error('myfile already exists');
} else {
open('myfile', 'wx', (err, fd) => {
if (err) throw err;
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
}
});
write (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'wx', (err, fd) => {
if (err) {
if (err.code === 'EEXIST') {
console.error('myfile already exists');
return;
}
throw err;
}
try {
writeMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
read (NOT RECOMMENDED)
import { open, close, exists } from 'node:fs';
exists('myfile', (e) => {
if (e) {
open('myfile', 'r', (err, fd) => {
if (err) throw err;
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
} else {
console.error('myfile does not exist');
}
});
read (RECOMMENDED)
import { open, close } from 'node:fs';
open('myfile', 'r', (err, fd) => {
if (err) {
if (err.code === 'ENOENT') {
console.error('myfile does not exist');
return;
}
throw err;
}
try {
readMyData(fd);
} finally {
close(fd, (err) => {
if (err) throw err;
});
}
});
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.fchmod(fd, mode, callback)
#
fd
<integer>mode
<string> | <integer>callback
<Function>err
<Error>
Sets the permissions on the file. No arguments other than a possible exception are given to the completion callback.
See the POSIX fchmod(2)
documentation for more detail.
fs.fchown(fd, uid, gid, callback)
#
fd
<integer>uid
<integer>gid
<integer>callback
<Function>err
<Error>
Sets the owner of the file. No arguments other than a possible exception are given to the completion callback.
See the POSIX fchown(2)
documentation for more detail.
fs.fdatasync(fd, callback)
#
fd
<integer>callback
<Function>err
<Error>
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2)
documentation for details. No arguments other than a possible
exception are given to the completion callback.
fs.fstat(fd[, options], callback)
#
fd
<integer>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.Stats>
Invokes the callback with the <fs.Stats> for the file descriptor.
See the POSIX fstat(2)
documentation for more detail.
fs.fsync(fd, callback)
#
fd
<integer>callback
<Function>err
<Error>
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2)
documentation for more detail. No arguments other
than a possible exception are given to the completion callback.
fs.ftruncate(fd[, len], callback)
#
fd
<integer>len
<integer> Default:0
callback
<Function>err
<Error>
Truncates the file descriptor. No arguments other than a possible exception are given to the completion callback.
See the POSIX ftruncate(2)
documentation for more detail.
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:
import { open, close, ftruncate } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('temp.txt', 'r+', (err, fd) => {
if (err) throw err;
try {
ftruncate(fd, 4, (err) => {
closeFd(fd);
if (err) throw err;
});
} catch (err) {
closeFd(fd);
if (err) throw err;
}
});
If the file previously was shorter than len
bytes, it is extended, and the
extended part is filled with null bytes ('\0'
):
If len
is negative then 0
will be used.
fs.futimes(fd, atime, mtime, callback)
#
fd
<integer>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>callback
<Function>err
<Error>
Change the file system timestamps of the object referenced by the supplied file
descriptor. See fs.utimes()
.
fs.lchmod(path, mode, callback)
#
path
<string> | <Buffer> | <URL>mode
<integer>callback
<Function>err
<Error> | <AggregateError>
Changes the permissions on a symbolic link. No arguments other than a possible exception are given to the completion callback.
This method is only implemented on macOS.
See the POSIX lchmod(2)
documentation for more detail.
fs.lchown(path, uid, gid, callback)
#
Set the owner of the symbolic link. No arguments other than a possible exception are given to the completion callback.
See the POSIX lchown(2)
documentation for more detail.
fs.lutimes(path, atime, mtime, callback)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>callback
<Function>err
<Error>
Changes the access and modification times of a file in the same way as
fs.utimes()
, with the difference that if the path refers to a symbolic
link, then the link is not dereferenced: instead, the timestamps of the
symbolic link itself are changed.
No arguments other than a possible exception are given to the completion callback.
fs.link(existingPath, newPath, callback)
#
existingPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>callback
<Function>err
<Error>
Creates a new link from the existingPath
to the newPath
. See the POSIX
link(2)
documentation for more detail. No arguments other than a possible
exception are given to the completion callback.
fs.lstat(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.Stats>
Retrieves the <fs.Stats> for the symbolic link referred to by the path.
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.
See the POSIX lstat(2)
documentation for more details.
fs.mkdir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object> | <integer>callback
<Function>err
<Error>path
<string> | <undefined> Present only if a directory is created withrecursive
set totrue
.
Asynchronously creates a directory.
The callback is given a possible exception and, if recursive
is true
, the
first directory path created, (err[, path])
.
path
can still be undefined
when recursive
is true
, if no directory was
created (for instance, if it was previously created).
The optional options
argument can be an integer specifying mode
(permission
and sticky bits), or an object with a mode
property and a recursive
property indicating whether parent directories should be created. Calling
fs.mkdir()
when path
is a directory that exists results in an error only
when recursive
is false. If recursive
is false and the directory exists,
an EEXIST
error occurs.
import { mkdir } from 'node:fs';
// Create ./tmp/a/apple, regardless of whether ./tmp and ./tmp/a exist.
mkdir('./tmp/a/apple', { recursive: true }, (err) => {
if (err) throw err;
});
On Windows, using fs.mkdir()
on the root directory even with recursion will
result in an error:
import { mkdir } from 'node:fs';
mkdir('/', { recursive: true }, (err) => {
// => [Error: EPERM: operation not permitted, mkdir 'C:\']
});
See the POSIX mkdir(2)
documentation for more details.
fs.mkdtemp(prefix[, options], callback)
#
prefix
<string> | <Buffer> | <URL>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. Due to platform
inconsistencies, avoid trailing X
characters in prefix
. Some platforms,
notably the BSDs, can return more than six random characters, and replace
trailing X
characters in prefix
with random characters.
The created directory 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.
import { mkdtemp } from 'node:fs';
import { join } from 'node:path';
import { tmpdir } from 'node:os';
mkdtemp(join(tmpdir(), 'foo-'), (err, directory) => {
if (err) throw err;
console.log(directory);
// Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
});
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('node:path').sep
).
import { tmpdir } from 'node:os';
import { mkdtemp } from 'node:fs';
// The parent directory for the new temporary directory
const tmpDir = tmpdir();
// This method is *INCORRECT*:
mkdtemp(tmpDir, (err, directory) => {
if (err) throw err;
console.log(directory);
// Will print something similar to `/tmpabc123`.
// A new temporary directory is created at the file system root
// rather than *within* the /tmp directory.
});
// This method is *CORRECT*:
import { sep } from 'node:path';
mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
if (err) throw err;
console.log(directory);
// Will print something similar to `/tmp/abc123`.
// A new temporary directory is created within
// the /tmp directory.
});
fs.open(path[, flags[, mode]], callback)
#
path
<string> | <Buffer> | <URL>flags
<string> | <number> See support of file systemflags
. Default:'r'
.mode
<string> | <integer> Default:0o666
(readable and writable)callback
<Function>
Asynchronous file open. See the POSIX open(2)
documentation for more details.
mode
sets the file mode (permission and sticky bits), but only if the file was
created. On Windows, only the write permission can be manipulated; see
fs.chmod()
.
The callback gets two arguments (err, 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:
fs.writeFile()
, fs.readFile()
, etc.
fs.openAsBlob(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>type
<string> An optional mime type for the blob.
- Return: <Promise> containing <Blob>
Returns a <Blob> whose data is backed by the given file.
The file must not be modified after the <Blob> is created. Any modifications
will cause reading the <Blob> data to fail with a DOMException
error.
Synchronous stat operations on the file when the Blob
is created, and before
each read in order to detect whether the file data has been modified on disk.
import { openAsBlob } from 'node:fs';
const blob = await openAsBlob('the.file.txt');
const ab = await blob.arrayBuffer();
blob.stream();
const { openAsBlob } = require('node:fs');
(async () => {
const blob = await openAsBlob('the.file.txt');
const ab = await blob.arrayBuffer();
blob.stream();
})();
fs.opendir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>callback
<Function>
Asynchronously open a directory. See the POSIX opendir(3)
documentation for
more details.
Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.
The encoding
option sets the encoding for the path
while opening the
directory and subsequent read operations.
fs.read(fd, buffer, offset, length, position, callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView> The buffer that the data will be written to.offset
<integer> The position inbuffer
to write the data to.length
<integer> The number of bytes to read.position
<integer> | <bigint> | <null> Specifies where to begin reading from in the file. Ifposition
isnull
or-1
, data will be read from the current file position, and the file position will be updated. Ifposition
is a non-negative integer, the file position will be unchanged.callback
<Function>
Read data from the file specified by fd
.
The callback is given the three arguments, (err, bytesRead, buffer)
.
If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.
If this method is invoked as its util.promisify()
ed version, it returns
a promise for an Object
with bytesRead
and buffer
properties.
fs.read(fd[, options], callback)
#
fd
<integer>options
<Object>buffer
<Buffer> | <TypedArray> | <DataView> Default:Buffer.alloc(16384)
offset
<integer> Default:0
length
<integer> Default:buffer.byteLength - offset
position
<integer> | <bigint> | <null> Default:null
callback
<Function>
Similar to the fs.read()
function, this version takes an optional
options
object. If no options
object is specified, it will default with the
above values.
fs.read(fd, buffer[, options], callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView> The buffer that the data will be written to.options
<Object>callback
<Function>
Similar to the fs.read()
function, this version takes an optional
options
object. If no options
object is specified, it will default with the
above values.
fs.readdir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>callback
<Function>err
<Error>files
<string[]> | <Buffer[]> | <fs.Dirent[]>
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 '..'
.
See the POSIX readdir(3)
documentation for more details.
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.
If options.withFileTypes
is set to true
, the files
array will contain
<fs.Dirent> objects.
fs.readFile(path[, options], callback)
#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions
<Object> | <string>encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.signal
<AbortSignal> allows aborting an in-progress readFile
callback
<Function>err
<Error> | <AggregateError>data
<string> | <Buffer>
Asynchronously reads the entire contents of a file.
import { readFile } from 'node: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:
import { readFile } from 'node:fs';
readFile('/etc/passwd', 'utf8', callback);
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.
import { readFile } from 'node:fs';
// macOS, Linux, and Windows
readFile('<directory>', (err, data) => {
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
});
// FreeBSD
readFile('<directory>', (err, data) => {
// => null, <data>
});
It is possible to abort an ongoing request using an AbortSignal
. If a
request is aborted the callback is called with an AbortError
:
import { readFile } from 'node:fs';
const controller = new AbortController();
const signal = controller.signal;
readFile(fileInfo[0].name, { signal }, (err, buf) => {
// ...
});
// When you want to abort the request
controller.abort();
The fs.readFile()
function buffers the entire file. To minimize memory costs,
when possible prefer streaming via fs.createReadStream()
.
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.readFile
performs.
File descriptors#
- Any specified file descriptor has to support reading.
- If a file descriptor is specified as the
path
, it will not be closed automatically. - The reading will begin at the current position. For example, if the file
already had
'Hello World'
and six bytes are read with the file descriptor, the call tofs.readFile()
with the same file descriptor, would give'World'
, rather than'Hello World'
.
Performance Considerations#
The fs.readFile()
method asynchronously reads the contents of a file into
memory one chunk at a time, allowing the event loop to turn between each chunk.
This allows the read operation to have less impact on other activity that may
be using the underlying libuv thread pool but means that it will take longer
to read a complete file into memory.
The additional read overhead can vary broadly on different systems and depends on the type of file being read. If the file type is not a regular file (a pipe for instance) and Node.js is unable to determine an actual file size, each read operation will load on 64 KiB of data. For regular files, each read will process 512 KiB of data.
For applications that require as-fast-as-possible reading of file contents, it
is better to use fs.read()
directly and for application code to manage
reading the full contents of the file itself.
The Node.js GitHub issue #25741 provides more information and a detailed
analysis on the performance of fs.readFile()
for multiple file sizes in
different Node.js versions.
fs.readlink(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Reads the contents of the symbolic link referred to by path
. The callback gets
two arguments (err, linkString)
.
See the POSIX readlink(2)
documentation for more details.
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.readv(fd, buffers[, position], callback)
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
callback
<Function>err
<Error>bytesRead
<integer>buffers
<ArrayBufferView[]>
Read from a file specified by fd
and write to an array of ArrayBufferView
s
using readv()
.
position
is the offset from the beginning of the file from where data
should be read. If typeof position !== 'number'
, the data will be read
from the current position.
The callback will be given three arguments: err
, bytesRead
, and
buffers
. bytesRead
is how many bytes were read from the file.
If this method is invoked as its util.promisify()
ed version, it returns
a promise for an Object
with bytesRead
and buffers
properties.
fs.realpath(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
callback
<Function>
Asynchronously computes the canonical pathname by resolving .
, ..
, and
symbolic links.
A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames.
This function behaves like realpath(3)
, with some exceptions:
-
No case conversion is performed on case-insensitive file systems.
-
The maximum number of symbolic links is platform-independent and generally (much) higher than what the native
realpath(3)
implementation supports.
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.
If path
resolves to a socket or a pipe, the function will return a system
dependent name for that object.
fs.realpath.native(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)
.
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.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fs.rename(oldPath, newPath, callback)
#
oldPath
<string> | <Buffer> | <URL>newPath
<string> | <Buffer> | <URL>callback
<Function>err
<Error>
Asynchronously rename file at oldPath
to the pathname provided
as newPath
. In the case that newPath
already exists, it will
be overwritten. If there is a directory at newPath
, an error will
be raised instead. No arguments other than a possible exception are
given to the completion callback.
See also: rename(2)
.
import { rename } from 'node:fs';
rename('oldFile.txt', 'newFile.txt', (err) => {
if (err) throw err;
console.log('Rename complete!');
});
fs.rmdir(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js retries the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default:false
. Deprecated.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
callback
<Function>err
<Error>
Asynchronous rmdir(2)
. No arguments other than a possible exception are given
to the completion callback.
Using fs.rmdir()
on a file (not a directory) results in an ENOENT
error on
Windows and an ENOTDIR
error on POSIX.
To get a behavior similar to the rm -rf
Unix command, use fs.rm()
with options { recursive: true, force: true }
.
fs.rm(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>force
<boolean> Whentrue
, exceptions will be ignored ifpath
does not exist. Default:false
.maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js will retry the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive removal. In recursive mode operations are retried on failure. Default:false
.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
callback
<Function>err
<Error>
Asynchronously removes files and directories (modeled on the standard POSIX rm
utility). No arguments other than a possible exception are given to the
completion callback.
fs.stat(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.Stats>
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.
fs.stat()
follows symbolic links. Use fs.lstat()
to look at the
links themselves.
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.
For example, given the following directory structure:
- txtDir
-- file.txt
- app.js
The next program will check for the stats of the given paths:
import { stat } from 'node:fs';
const pathsToCheck = ['./txtDir', './txtDir/file.txt'];
for (let i = 0; i < pathsToCheck.length; i++) {
stat(pathsToCheck[i], (err, stats) => {
console.log(stats.isDirectory());
console.log(stats);
});
}
The resulting output will resemble:
true
Stats {
dev: 16777220,
mode: 16877,
nlink: 3,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 14214262,
size: 96,
blocks: 0,
atimeMs: 1561174653071.963,
mtimeMs: 1561174614583.3518,
ctimeMs: 1561174626623.5366,
birthtimeMs: 1561174126937.2893,
atime: 2019-06-22T03:37:33.072Z,
mtime: 2019-06-22T03:36:54.583Z,
ctime: 2019-06-22T03:37:06.624Z,
birthtime: 2019-06-22T03:28:46.937Z
}
false
Stats {
dev: 16777220,
mode: 33188,
nlink: 1,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 14214074,
size: 8,
blocks: 8,
atimeMs: 1561174616618.8555,
mtimeMs: 1561174614584,
ctimeMs: 1561174614583.8145,
birthtimeMs: 1561174007710.7478,
atime: 2019-06-22T03:36:56.619Z,
mtime: 2019-06-22T03:36:54.584Z,
ctime: 2019-06-22T03:36:54.584Z,
birthtime: 2019-06-22T03:26:47.711Z
}
fs.statfs(path[, options], callback)
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.StatFs> object should bebigint
. Default:false
.
callback
<Function>err
<Error>stats
<fs.StatFs>
Asynchronous statfs(2)
. Returns information about the mounted file system which
contains path
. The callback gets two arguments (err, stats)
where stats
is an <fs.StatFs> object.
In case of an error, the err.code
will be one of Common System Errors.
fs.symlink(target, path[, type], callback)
#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> | <null> Default:null
callback
<Function>err
<Error>
Creates the link called path
pointing to target
. No arguments other than a
possible exception are given to the completion callback.
See the POSIX symlink(2)
documentation for more details.
The type
argument is only available on Windows and ignored on other platforms.
It can be set to 'dir'
, 'file'
, or 'junction'
. If the type
argument is
not a string, Node.js will autodetect target
type and use 'file'
or 'dir'
.
If the target
does not exist, 'file'
will be used. Windows junction points
require the destination path to be absolute. When using 'junction'
, the
target
argument will automatically be normalized to absolute path. Junction
points on NTFS volumes can only point to directories.
Relative targets are relative to the link's parent directory.
import { symlink } from 'node:fs';
symlink('./mew', './mewtwo', callback);
The above example creates a symbolic link mewtwo
which points to mew
in the
same directory:
$ tree .
.
├── mew
└── mewtwo -> ./mew
fs.truncate(path[, len], callback)
#
path
<string> | <Buffer> | <URL>len
<integer> Default:0
callback
<Function>err
<Error> | <AggregateError>
Truncates the file. 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.
import { truncate } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
});
const { truncate } = require('node:fs');
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was truncated');
});
Passing a file descriptor is deprecated and may result in an error being thrown in the future.
See the POSIX truncate(2)
documentation for more details.
fs.unlink(path, callback)
#
path
<string> | <Buffer> | <URL>callback
<Function>err
<Error>
Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.
import { unlink } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
unlink('path/file.txt', (err) => {
if (err) throw err;
console.log('path/file.txt was deleted');
});
fs.unlink()
will not work on a directory, empty or otherwise. To remove a
directory, use fs.rmdir()
.
See the POSIX unlink(2)
documentation for more details.
fs.unwatchFile(filename[, listener])
#
filename
<string> | <Buffer> | <URL>listener
<Function> Optional, a listener previously attached usingfs.watchFile()
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.
Using 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)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>callback
<Function>err
<Error>
Change the file system timestamps of the object referenced by path
.
The atime
and mtime
arguments follow these rules:
- Values can be either numbers representing Unix epoch time in seconds,
Date
s, or a numeric string like'123456789.0'
. - If the value can not be converted to a number, or is
NaN
,Infinity
, or-Infinity
, anError
will be thrown.
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'
.signal
<AbortSignal> allows closing the watcher with an AbortSignal.
listener
<Function> | <undefined> Default:undefined
- Returns: <fs.FSWatcher>
Watch for changes on filename
, where filename
is either a file or a
directory.
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.
On most platforms, 'rename'
is emitted whenever a filename appears or
disappears in the directory.
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
.
If a signal
is passed, aborting the corresponding AbortController will close
the returned <fs.FSWatcher>.
Caveats#
The fs.watch
API is not 100% consistent across platforms, and is
unavailable in some situations.
On Windows, no events will be emitted if the watched directory is moved or
renamed. An EPERM
error is reported when the watched directory is deleted.
Availability#
This feature depends on the underlying operating system providing a way to be notified of file system changes.
- On Linux systems, this uses
inotify(7)
. - On BSD systems, this uses
kqueue(2)
. - On macOS, this uses
kqueue(2)
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. - On IBM i systems, this feature is not supported.
If the underlying functionality is not available for some reason, then
fs.watch()
will not be able to function and may throw an exception.
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 or Docker.
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.
AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation).
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
.
import { watch } from 'node: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>current
<fs.Stats>previous
<fs.Stats>
- Returns: <fs.StatWatcher>
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 listener
gets two arguments the current stat object and the previous
stat object:
import { watchFile } from 'node: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
. If the bigint
option is true
,
the numeric values in these objects are specified as BigInt
s.
To be notified when the file was modified, not just accessed, it is necessary
to compare curr.mtimeMs
and prev.mtimeMs
.
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). 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.
Using 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.
When a file being watched by fs.watchFile()
disappears and reappears,
then the contents of previous
in the second callback event (the file's
reappearance) will be the same as the contents of previous
in the first
callback event (its disappearance).
This happens when:
- the file is deleted, followed by a restore
- the file is renamed and then renamed a second time back to its original name
fs.write(fd, buffer, offset[, length[, position]], callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>offset
<integer> Default:0
length
<integer> Default:buffer.byteLength - offset
position
<integer> | <null> Default:null
callback
<Function>err
<Error>bytesWritten
<integer>buffer
<Buffer> | <TypedArray> | <DataView>
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.
It is unsafe to use fs.write()
multiple times on the same file without waiting
for the callback. For this scenario, fs.createWriteStream()
is
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, buffer[, options], callback)
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>callback
<Function>err
<Error>bytesWritten
<integer>buffer
<Buffer> | <TypedArray> | <DataView>
Write buffer
to the file specified by fd
.
Similar to the above fs.write
function, this version takes an
optional options
object. If no options
object is specified, it will
default with the above values.
fs.write(fd, string[, position[, encoding]], callback)
#
fd
<integer>string
<string>position
<integer> | <null> Default:null
encoding
<string> Default:'utf8'
callback
<Function>
Write string
to the file specified by fd
. If string
is not a string,
an exception is thrown.
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. Bytes
written is not necessarily the same as string characters written. See
Buffer.byteLength
.
It is unsafe to use fs.write()
multiple times on the same file without waiting
for the callback. For this scenario, fs.createWriteStream()
is
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.
On Windows, if the file descriptor is connected to the console (e.g. fd == 1
or stdout
) a string containing non-ASCII characters will not be rendered
properly by default, regardless of the encoding used.
It is possible to configure the console to render UTF-8 properly by changing the
active codepage with the chcp 65001
command. See the chcp docs for more
details.
fs.writeFile(file, data[, options], callback)
#
file
<string> | <Buffer> | <URL> | <integer> filename or file descriptordata
<string> | <Buffer> | <TypedArray> | <DataView>options
<Object> | <string>encoding
<string> | <null> Default:'utf8'
mode
<integer> Default:0o666
flag
<string> See support of file systemflags
. Default:'w'
.flush
<boolean> If all data is successfully written to the file, andflush
istrue
,fs.fsync()
is used to flush the data. Default:false
.signal
<AbortSignal> allows aborting an in-progress writeFile
callback
<Function>err
<Error> | <AggregateError>
When file
is a filename, asynchronously writes data to the file, replacing the
file if it already exists. data
can be a string or a buffer.
When file
is a file descriptor, the behavior is similar to calling
fs.write()
directly (which is recommended). See the notes below on using
a file descriptor.
The encoding
option is ignored if data
is a buffer.
The mode
option only affects the newly created file. See fs.open()
for more details.
import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';
const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, (err) => {
if (err) throw err;
console.log('The file has been saved!');
});
If options
is a string, then it specifies the encoding:
import { writeFile } from 'node:fs';
writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
It is unsafe to use fs.writeFile()
multiple times on the same file without
waiting for the callback. For this scenario, fs.createWriteStream()
is
recommended.
Similarly to fs.readFile
- fs.writeFile
is a convenience method that
performs multiple write
calls internally to write the buffer passed to it.
For performance sensitive code consider using fs.createWriteStream()
.
It is possible to use an <AbortSignal> to cancel an fs.writeFile()
.
Cancelation is "best effort", and some amount of data is likely still
to be written.
import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';
const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, { signal }, (err) => {
// When a request is aborted - the callback is called with an AbortError
});
// When the request should be aborted
controller.abort();
Aborting an ongoing request does not abort individual operating
system requests but rather the internal buffering fs.writeFile
performs.
Using fs.writeFile()
with file descriptors#
When file
is a file descriptor, the behavior is almost identical to directly
calling fs.write()
like:
import { write } from 'node:fs';
import { Buffer } from 'node:buffer';
write(fd, Buffer.from(data, options.encoding), callback);
The difference from directly calling fs.write()
is that under some unusual
conditions, fs.write()
might write only part of the buffer and need to be
retried to write the remaining data, whereas fs.writeFile()
retries until
the data is entirely written (or an error occurs).
The implications of this are a common source of confusion. In the file descriptor case, the file is not replaced! The data is not necessarily written to the beginning of the file, and the file's original data may remain before and/or after the newly written data.
For example, if fs.writeFile()
is called twice in a row, first to write the
string 'Hello'
, then to write the string ', World'
, the file would contain
'Hello, World'
, and might contain some of the file's original data (depending
on the size of the original file, and the position of the file descriptor). If
a file name had been used instead of a descriptor, the file would be guaranteed
to contain only ', World'
.
fs.writev(fd, buffers[, position], callback)
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
callback
<Function>err
<Error>bytesWritten
<integer>buffers
<ArrayBufferView[]>
Write an array of ArrayBufferView
s to the file specified by fd
using
writev()
.
position
is 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.
The callback will be given three arguments: err
, bytesWritten
, and
buffers
. bytesWritten
is how many bytes were written from buffers
.
If this method is util.promisify()
ed, it returns a promise for an
Object
with bytesWritten
and buffers
properties.
It is unsafe to use fs.writev()
multiple times on the same file without
waiting for the callback. For this scenario, use fs.createWriteStream()
.
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.
Synchronous API#
The synchronous APIs perform all operations synchronously, blocking the event loop until the operation completes or fails.
fs.accessSync(path[, mode])
#
Synchronously 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. mode
should be either the value
fs.constants.F_OK
or a mask consisting of the bitwise OR of any of
fs.constants.R_OK
, fs.constants.W_OK
, and fs.constants.X_OK
(e.g.
fs.constants.W_OK | fs.constants.R_OK
). Check File access constants for
possible values of mode
.
If any of the accessibility checks fail, an Error
will be thrown. Otherwise,
the method will return undefined
.
import { accessSync, constants } from 'node:fs';
try {
accessSync('etc/passwd', constants.R_OK | constants.W_OK);
console.log('can read/write');
} catch (err) {
console.error('no access!');
}
fs.appendFileSync(path, data[, options])
#
path
<string> | <Buffer> | <URL> | <number> filename or file descriptordata
<string> | <Buffer>options
<Object> | <string>
Synchronously append data to a file, creating the file if it does not yet
exist. data
can be a string or a <Buffer>.
The mode
option only affects the newly created file. See fs.open()
for more details.
import { appendFileSync } from 'node:fs';
try {
appendFileSync('message.txt', 'data to append');
console.log('The "data to append" was appended to file!');
} catch (err) {
/* Handle the error */
}
If options
is a string, then it specifies the encoding:
import { appendFileSync } from 'node:fs';
appendFileSync('message.txt', 'data to append', 'utf8');
The path
may be specified as a numeric file descriptor that has been opened
for appending (using fs.open()
or fs.openSync()
). The file descriptor will
not be closed automatically.
import { openSync, closeSync, appendFileSync } from 'node:fs';
let fd;
try {
fd = openSync('message.txt', 'a');
appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
/* Handle the error */
} finally {
if (fd !== undefined)
closeSync(fd);
}
fs.chmodSync(path, mode)
#
For detailed information, see the documentation of the asynchronous version of
this API: fs.chmod()
.
See the POSIX chmod(2)
documentation for more detail.
fs.chownSync(path, uid, gid)
#
Synchronously changes owner and group of a file. Returns undefined
.
This is the synchronous version of fs.chown()
.
See the POSIX chown(2)
documentation for more detail.
fs.closeSync(fd)
#
fd
<integer>
Closes the file descriptor. Returns undefined
.
Calling fs.closeSync()
on any file descriptor (fd
) that is currently in use
through any other fs
operation may lead to undefined behavior.
See the POSIX close(2)
documentation for more detail.
fs.copyFileSync(src, dest[, mode])
#
src
<string> | <Buffer> | <URL> source filename to copydest
<string> | <Buffer> | <URL> destination filename of the copy operationmode
<integer> modifiers for copy operation. Default:0
.
Synchronously copies src
to dest
. By default, dest
is overwritten if it
already exists. Returns undefined
. Node.js makes no guarantees about the
atomicity of the copy operation. If an error occurs after the destination file
has been opened for writing, Node.js will attempt to remove the destination.
mode
is an optional integer that specifies the behavior
of the copy operation. It is possible to create a mask consisting of the bitwise
OR of two or more values (e.g.
fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE
).
fs.constants.COPYFILE_EXCL
: The copy operation will fail ifdest
already exists.fs.constants.COPYFILE_FICLONE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.fs.constants.COPYFILE_FICLONE_FORCE
: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
import { copyFileSync, constants } from 'node:fs';
// destination.txt will be created or overwritten by default.
copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');
// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFileSync('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
fs.cpSync(src, dest[, options])
#
src
<string> | <URL> source path to copy.dest
<string> | <URL> destination path to copy to.options
<Object>dereference
<boolean> dereference symlinks. Default:false
.errorOnExist
<boolean> whenforce
isfalse
, and the destination exists, throw an error. Default:false
.filter
<Function> Function to filter copied files/directories. Returntrue
to copy the item,false
to ignore it. When ignoring a directory, all of its contents will be skipped as well. Default:undefined
force
<boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use theerrorOnExist
option to change this behavior. Default:true
.mode
<integer> modifiers for copy operation. Default:0
. Seemode
flag offs.copyFileSync()
.preserveTimestamps
<boolean> Whentrue
timestamps fromsrc
will be preserved. Default:false
.recursive
<boolean> copy directories recursively Default:false
verbatimSymlinks
<boolean> Whentrue
, path resolution for symlinks will be skipped. Default:false
Synchronously copies the entire directory structure from src
to dest
,
including subdirectories and files.
When copying a directory to another directory, globs are not supported and
behavior is similar to cp dir1/ dir2/
.
fs.existsSync(path)
#
Returns true
if the path exists, false
otherwise.
For detailed information, see the documentation of the asynchronous version of
this API: fs.exists()
.
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.
import { existsSync } from 'node:fs';
if (existsSync('/etc/passwd'))
console.log('The path exists.');
fs.fchmodSync(fd, mode)
#
Sets the permissions on the file. Returns undefined
.
See the POSIX fchmod(2)
documentation for more detail.
fs.fchownSync(fd, uid, gid)
#
fd
<integer>uid
<integer> The file's new owner's user id.gid
<integer> The file's new group's group id.
Sets the owner of the file. Returns undefined
.
See the POSIX fchown(2)
documentation for more detail.
fs.fdatasyncSync(fd)
#
fd
<integer>
Forces all currently queued I/O operations associated with the file to the
operating system's synchronized I/O completion state. Refer to the POSIX
fdatasync(2)
documentation for details. Returns undefined
.
fs.fstatSync(fd[, options])
#
fd
<integer>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.
- Returns: <fs.Stats>
Retrieves the <fs.Stats> for the file descriptor.
See the POSIX fstat(2)
documentation for more detail.
fs.fsyncSync(fd)
#
fd
<integer>
Request that all data for the open file descriptor is flushed to the storage
device. The specific implementation is operating system and device specific.
Refer to the POSIX fsync(2)
documentation for more detail. Returns undefined
.
fs.ftruncateSync(fd[, len])
#
Truncates the file descriptor. Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.ftruncate()
.
fs.futimesSync(fd, atime, mtime)
#
Synchronous version of fs.futimes()
. Returns undefined
.
fs.lchmodSync(path, mode)
#
Changes the permissions on a symbolic link. Returns undefined
.
This method is only implemented on macOS.
See the POSIX lchmod(2)
documentation for more detail.
fs.lchownSync(path, uid, gid)
#
path
<string> | <Buffer> | <URL>uid
<integer> The file's new owner's user id.gid
<integer> The file's new group's group id.
Set the owner for the path. Returns undefined
.
See the POSIX lchown(2)
documentation for more details.
fs.lutimesSync(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>
Change the file system timestamps of the symbolic link referenced by path
.
Returns undefined
, or throws an exception when parameters are incorrect or
the operation fails. This is the synchronous version of fs.lutimes()
.
fs.linkSync(existingPath, newPath)
#
Creates a new link from the existingPath
to the newPath
. See the POSIX
link(2)
documentation for more detail. Returns undefined
.
fs.lstatSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.throwIfNoEntry
<boolean> Whether an exception will be thrown if no file system entry exists, rather than returningundefined
. Default:true
.
- Returns: <fs.Stats>
Retrieves the <fs.Stats> for the symbolic link referred to by path
.
See the POSIX lstat(2)
documentation for more details.
fs.mkdirSync(path[, options])
#
Synchronously creates a directory. Returns undefined
, or if recursive
is
true
, the first directory path created.
This is the synchronous version of fs.mkdir()
.
See the POSIX mkdir(2)
documentation for more details.
fs.mkdtempSync(prefix[, options])
#
prefix
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <string>
Returns the created directory path.
For detailed information, see the documentation of the asynchronous version of
this API: fs.mkdtemp()
.
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.opendirSync(path[, options])
#
Synchronously open a directory. See opendir(3)
.
Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.
The encoding
option sets the encoding for the path
while opening the
directory and subsequent read operations.
fs.openSync(path[, flags[, mode]])
#
path
<string> | <Buffer> | <URL>flags
<string> | <number> Default:'r'
. See support of file systemflags
.mode
<string> | <integer> Default:0o666
- Returns: <number>
Returns an integer representing the file descriptor.
For detailed information, see the documentation of the asynchronous version of
this API: fs.open()
.
fs.readdirSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>- Returns: <string[]> | <Buffer[]> | <fs.Dirent[]>
Reads the contents of the directory.
See the POSIX readdir(3)
documentation for more details.
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 returned. If the encoding
is set to 'buffer'
,
the filenames returned will be passed as <Buffer> objects.
If options.withFileTypes
is set to true
, the result will contain
<fs.Dirent> objects.
fs.readFileSync(path[, options])
#
path
<string> | <Buffer> | <URL> | <integer> filename or file descriptoroptions
<Object> | <string>encoding
<string> | <null> Default:null
flag
<string> See support of file systemflags
. Default:'r'
.
- Returns: <string> | <Buffer>
Returns the contents of the path
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.readFile()
.
If the encoding
option is specified then this function returns a
string. Otherwise it returns a buffer.
Similar to fs.readFile()
, when the path is a directory, the behavior of
fs.readFileSync()
is platform-specific.
import { readFileSync } from 'node:fs';
// macOS, Linux, and Windows
readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]
// FreeBSD
readFileSync('<directory>'); // => <data>
fs.readlinkSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <string> | <Buffer>
Returns the symbolic link's string value.
See the POSIX readlink(2)
documentation for more details.
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 returned. 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
<Buffer> | <TypedArray> | <DataView>offset
<integer>length
<integer>position
<integer> | <bigint> | <null> Default:null
- Returns: <number>
Returns the number of bytesRead
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.read()
.
fs.readSync(fd, buffer[, options])
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>- Returns: <number>
Returns the number of bytesRead
.
Similar to the above fs.readSync
function, this version takes an optional options
object.
If no options
object is specified, it will default with the above values.
For detailed information, see the documentation of the asynchronous version of
this API: fs.read()
.
fs.readvSync(fd, buffers[, position])
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
- Returns: <number> The number of bytes read.
For detailed information, see the documentation of the asynchronous version of
this API: fs.readv()
.
fs.realpathSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <string> | <Buffer>
Returns the resolved pathname.
For detailed information, see the documentation of the asynchronous version of
this API: fs.realpath()
.
fs.realpathSync.native(path[, options])
#
path
<string> | <Buffer> | <URL>options
<string> | <Object>encoding
<string> Default:'utf8'
- Returns: <string> | <Buffer>
Synchronous realpath(3)
.
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 returned. If the encoding
is set to 'buffer'
,
the path returned will be passed as a <Buffer> object.
On Linux, when Node.js is linked against musl libc, the procfs file system must
be mounted on /proc
in order for this function to work. Glibc does not have
this restriction.
fs.renameSync(oldPath, newPath)
#
Renames the file from oldPath
to newPath
. Returns undefined
.
See the POSIX rename(2)
documentation for more details.
fs.rmdirSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js retries the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default:false
. Deprecated.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
Synchronous rmdir(2)
. Returns undefined
.
Using fs.rmdirSync()
on a file (not a directory) results in an ENOENT
error
on Windows and an ENOTDIR
error on POSIX.
To get a behavior similar to the rm -rf
Unix command, use fs.rmSync()
with options { recursive: true, force: true }
.
fs.rmSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>force
<boolean> Whentrue
, exceptions will be ignored ifpath
does not exist. Default:false
.maxRetries
<integer> If anEBUSY
,EMFILE
,ENFILE
,ENOTEMPTY
, orEPERM
error is encountered, Node.js will retry the operation with a linear backoff wait ofretryDelay
milliseconds longer on each try. This option represents the number of retries. This option is ignored if therecursive
option is nottrue
. Default:0
.recursive
<boolean> Iftrue
, perform a recursive directory removal. In recursive mode operations are retried on failure. Default:false
.retryDelay
<integer> The amount of time in milliseconds to wait between retries. This option is ignored if therecursive
option is nottrue
. Default:100
.
Synchronously removes files and directories (modeled on the standard POSIX rm
utility). Returns undefined
.
fs.statSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.Stats> object should bebigint
. Default:false
.throwIfNoEntry
<boolean> Whether an exception will be thrown if no file system entry exists, rather than returningundefined
. Default:true
.
- Returns: <fs.Stats>
Retrieves the <fs.Stats> for the path.
fs.statfsSync(path[, options])
#
path
<string> | <Buffer> | <URL>options
<Object>bigint
<boolean> Whether the numeric values in the returned <fs.StatFs> object should bebigint
. Default:false
.
- Returns: <fs.StatFs>
Synchronous statfs(2)
. Returns information about the mounted file system which
contains path
.
In case of an error, the err.code
will be one of Common System Errors.
fs.symlinkSync(target, path[, type])
#
target
<string> | <Buffer> | <URL>path
<string> | <Buffer> | <URL>type
<string> | <null> Default:null
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.symlink()
.
fs.truncateSync(path[, len])
#
Truncates the file. Returns undefined
. A file descriptor can also be
passed as the first argument. In this case, fs.ftruncateSync()
is called.
Passing a file descriptor is deprecated and may result in an error being thrown in the future.
fs.unlinkSync(path)
#
Synchronous unlink(2)
. Returns undefined
.
fs.utimesSync(path, atime, mtime)
#
path
<string> | <Buffer> | <URL>atime
<number> | <string> | <Date>mtime
<number> | <string> | <Date>
Returns undefined
.
For detailed information, see the documentation of the asynchronous version of
this API: fs.utimes()
.
fs.writeFileSync(file, data[, options])
#
file
<string> | <Buffer> | <URL> | <integer> filename or file descriptordata
<string> | <Buffer> | <TypedArray> | <DataView>options
<Object> | <string>
Returns undefined
.
The mode
option only affects the newly created file. See fs.open()
for more details.
For detailed information, see the documentation of the asynchronous version of
this API: fs.writeFile()
.
fs.writeSync(fd, buffer, offset[, length[, position]])
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>offset
<integer> Default:0
length
<integer> Default:buffer.byteLength - offset
position
<integer> | <null> Default:null
- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, buffer...)
.
fs.writeSync(fd, buffer[, options])
#
fd
<integer>buffer
<Buffer> | <TypedArray> | <DataView>options
<Object>- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, buffer...)
.
fs.writeSync(fd, string[, position[, encoding]])
#
fd
<integer>string
<string>position
<integer> | <null> Default:null
encoding
<string> Default:'utf8'
- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.write(fd, string...)
.
fs.writevSync(fd, buffers[, position])
#
fd
<integer>buffers
<ArrayBufferView[]>position
<integer> | <null> Default:null
- Returns: <number> The number of bytes written.
For detailed information, see the documentation of the asynchronous version of
this API: fs.writev()
.
Common Objects#
The common objects are shared by all of the file system API variants (promise, callback, and synchronous).
Class: fs.Dir
#
A class representing a directory stream.
Created by fs.opendir()
, fs.opendirSync()
, or
fsPromises.opendir()
.
import { opendir } from 'node:fs/promises';
try {
const dir = await opendir('./');
for await (const dirent of dir)
console.log(dirent.name);
} catch (err) {
console.error(err);
}
When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.
dir.close()
#
- Returns: <Promise>
Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
A promise is returned that will be fulfilled after the resource has been closed.
dir.close(callback)
#
callback
<Function>err
<Error>
Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
The callback
will be called after the resource handle has been closed.
dir.closeSync()
#
Synchronously close the directory's underlying resource handle. Subsequent reads will result in errors.
dir.path
#
The read-only path of this directory as was provided to fs.opendir()
,
fs.opendirSync()
, or fsPromises.opendir()
.
dir.read()
#
- Returns: <Promise> containing <fs.Dirent> | <null>
Asynchronously read the next directory entry via readdir(3)
as an
<fs.Dirent>.
A promise is returned that will be fulfilled with an <fs.Dirent>, or null
if there are no more directory entries to read.
Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir.read(callback)
#
callback
<Function>err
<Error>dirent
<fs.Dirent> | <null>
Asynchronously read the next directory entry via readdir(3)
as an
<fs.Dirent>.
After the read is completed, the callback
will be called with an
<fs.Dirent>, or null
if there are no more directory entries to read.
Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir.readSync()
#
- Returns: <fs.Dirent> | <null>
Synchronously read the next directory entry as an <fs.Dirent>. See the
POSIX readdir(3)
documentation for more detail.
If there are no more directory entries to read, null
will be returned.
Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
dir[Symbol.asyncIterator]()
#
- Returns: <AsyncIterator> of <fs.Dirent>
Asynchronously iterates over the directory until all entries have
been read. Refer to the POSIX readdir(3)
documentation for more detail.
Entries returned by the async iterator are always an <fs.Dirent>.
The null
case from dir.read()
is handled internally.
See <fs.Dir> for an example.
Directory entries returned by this iterator are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.
Class: fs.Dirent
#
A representation of a directory entry, which can be a file or a subdirectory within the directory, as returned by reading from an <fs.Dir>. The directory entry is a combination of the file name and file type pairs.
Additionally, when fs.readdir()
or fs.readdirSync()
is called with
the withFileTypes
option set to true
, the resulting array is filled with
<fs.Dirent> objects, rather than strings or <Buffer>s.
dirent.isBlockDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a block device.
dirent.isCharacterDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a character device.
dirent.isDirectory()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a file system
directory.
dirent.isFIFO()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a first-in-first-out
(FIFO) pipe.
dirent.isFile()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a regular file.
dirent.isSocket()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a socket.
dirent.isSymbolicLink()
#
- Returns: <boolean>
Returns true
if the <fs.Dirent> object describes a symbolic link.
dirent.name
#
The file name that this <fs.Dirent> object refers to. The type of this
value is determined by the options.encoding
passed to fs.readdir()
or
fs.readdirSync()
.
dirent.parentPath
#
The path to the parent directory of the file this <fs.Dirent> object refers to.
dirent.path
#
dirent.parentPath
instead.Alias for dirent.parentPath
.
Class: fs.FSWatcher
#
- Extends <EventEmitter>
A successful call to fs.watch()
method will return a new <fs.FSWatcher>
object.
All <fs.FSWatcher> objects emit a 'change'
event whenever a specific watched
file is modified.
Event: 'change'
#
eventType
<string> The type of change event that has occurredfilename
<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 UTF-8 string.
import { watch } from 'node:fs';
// Example when handled through fs.watch() listener
watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
if (filename) {
console.log(filename);
// Prints: <Buffer ...>
}
});
Event: 'close'
#
Emitted when the watcher stops watching for changes. The closed <fs.FSWatcher> object is no longer usable in the event handler.
Event: 'error'
#
error
<Error>
Emitted when an error occurs while watching the file. The errored <fs.FSWatcher> object is no longer usable in the event handler.
watcher.close()
#
Stop watching for changes on the given <fs.FSWatcher>. Once stopped, the <fs.FSWatcher> object is no longer usable.
watcher.ref()
#
- Returns: <fs.FSWatcher>
When called, requests that the Node.js event loop not exit so long as the
<fs.FSWatcher> is active. Calling watcher.ref()
multiple times will have
no effect.
By default, all <fs.FSWatcher> objects are "ref'ed", making it normally
unnecessary to call watcher.ref()
unless watcher.unref()
had been
called previously.
watcher.unref()
#
- Returns: <fs.FSWatcher>
When called, the active <fs.FSWatcher> 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 <fs.FSWatcher> object's
callback is invoked. Calling watcher.unref()
multiple times will have
no effect.
Class: fs.StatWatcher
#
- Extends <EventEmitter>
A successful call to fs.watchFile()
method will return a new <fs.StatWatcher>
object.
watcher.ref()
#
- Returns: <fs.StatWatcher>
When called, requests that the Node.js event loop not exit so long as the
<fs.StatWatcher> is active. Calling watcher.ref()
multiple times will have
no effect.
By default, all <fs.StatWatcher> objects are "ref'ed", making it normally
unnecessary to call watcher.ref()
unless watcher.unref()
had been
called previously.
watcher.unref()
#
- Returns: <fs.StatWatcher>
When called, the active <fs.StatWatcher> 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 <fs.StatWatcher> object's
callback is invoked. Calling watcher.unref()
multiple times will have
no effect.
Class: fs.ReadStream
#
- Extends: <stream.Readable>
Instances of <fs.ReadStream> are created and returned using the
fs.createReadStream()
function.
Event: 'close'
#
Emitted when the <fs.ReadStream>'s underlying file descriptor has been closed.
Event: 'open'
#
fd
<integer> Integer file descriptor used by the <fs.ReadStream>.
Emitted when the <fs.ReadStream>'s file descriptor has been opened.
Event: 'ready'
#
Emitted when the <fs.ReadStream> is ready to be used.
Fires immediately after 'open'
.
readStream.bytesRead
#
The number of bytes that have been 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>. If fd
is specified, then
readStream.path
will be undefined
.
readStream.pending
#
This property is true
if the underlying file has not been opened yet,
i.e. before the 'ready'
event is emitted.
Class: fs.Stats
#
A <fs.Stats> object provides information about a file.
Objects returned from fs.stat()
, fs.lstat()
, fs.fstat()
, and
their synchronous counterparts are of this type.
If bigint
in the options
passed to those methods is true, the numeric values
will be bigint
instead of number
, and the object will contain additional
nanosecond-precision properties suffixed with Ns
.
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 }
bigint
version:
BigIntStats {
dev: 2114n,
ino: 48064969n,
mode: 33188n,
nlink: 1n,
uid: 85n,
gid: 100n,
rdev: 0n,
size: 527n,
blksize: 4096n,
blocks: 8n,
atimeMs: 1318289051000n,
mtimeMs: 1318289051000n,
ctimeMs: 1318289051000n,
birthtimeMs: 1318289051000n,
atimeNs: 1318289051000000000n,
mtimeNs: 1318289051000000000n,
ctimeNs: 1318289051000000000n,
birthtimeNs: 1318289051000000000n,
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 }
stats.isBlockDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a block device.
stats.isCharacterDevice()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a character device.
stats.isDirectory()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a file system directory.
If the <fs.Stats> object was obtained from calling fs.lstat()
on a
symbolic link which resolves to a directory, this method will return false
.
This is because fs.lstat()
returns information
about a symbolic link itself and not the path it resolves to.
stats.isFIFO()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a first-in-first-out (FIFO)
pipe.
stats.isFile()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a regular file.
stats.isSocket()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a socket.
stats.isSymbolicLink()
#
- Returns: <boolean>
Returns true
if the <fs.Stats> object describes a symbolic link.
This method is only valid when using fs.lstat()
.
stats.dev
#
The numeric identifier of the device containing the file.
stats.ino
#
The file system specific "Inode" number for the file.
stats.mode
#
A bit-field describing the file type and mode.
stats.nlink
#
The number of hard-links that exist for the file.
stats.uid
#
The numeric user identifier of the user that owns the file (POSIX).
stats.gid
#
The numeric group identifier of the group that owns the file (POSIX).
stats.rdev
#
A numeric device identifier if the file represents a device.
stats.size
#
The size of the file in bytes.
If the underlying file system does not support getting the size of the file,
this will be 0
.
stats.blksize
#
The file system block size for i/o operations.
stats.blocks
#
The number of blocks allocated for this file.
stats.atimeMs
#
The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.
stats.mtimeMs
#
The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.
stats.ctimeMs
#
The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.
stats.birthtimeMs
#
The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.
stats.atimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the last time this file was accessed expressed in
nanoseconds since the POSIX Epoch.
stats.mtimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the last time this file was modified expressed in
nanoseconds since the POSIX Epoch.
stats.ctimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the last time the file status was changed expressed
in nanoseconds since the POSIX Epoch.
stats.birthtimeNs
#
Only present when bigint: true
is passed into the method that generates
the object.
The timestamp indicating the creation time of this file expressed in
nanoseconds since the POSIX Epoch.
stats.atime
#
The timestamp indicating the last time this file was accessed.
stats.mtime
#
The timestamp indicating the last time this file was modified.
stats.ctime
#
The timestamp indicating the last time the file status was changed.
stats.birthtime
#
The timestamp indicating the creation time of this file.
Stat time values#
The atimeMs
, mtimeMs
, ctimeMs
, birthtimeMs
properties are
numeric values that hold the corresponding times in milliseconds. Their
precision is platform specific. When bigint: true
is passed into the
method that generates the object, the properties will be bigints,
otherwise they will be numbers.
The atimeNs
, mtimeNs
, ctimeNs
, birthtimeNs
properties are
bigints that hold the corresponding times in nanoseconds. They are
only present when bigint: true
is passed into the method that generates
the object. 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.
The times in the stat object have the following semantics:
atime
"Access Time": Time when file data last accessed. Changed by themknod(2)
,utimes(2)
, andread(2)
system calls.mtime
"Modified Time": Time when file data last modified. Changed by themknod(2)
,utimes(2)
, andwrite(2)
system calls.ctime
"Change Time": Time when file status was last changed (inode data modification). Changed by thechmod(2)
,chown(2)
,link(2)
,mknod(2)
,rename(2)
,unlink(2)
,utimes(2)
,read(2)
, andwrite(2)
system calls.birthtime
"Birth Time": Time of file creation. Set once when the file is created. On file systems where birthtime is not available, this field may instead hold either thectime
or1970-01-01T00:00Z
(ie, Unix epoch timestamp0
). 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 theutimes(2)
system call.
Prior to Node.js 0.12, the ctime
held the birthtime
on Windows systems. As
of 0.12, ctime
is not "creation time", and on Unix systems, it never was.
Class: fs.StatFs
#
Provides information about a mounted file system.
Objects returned from fs.statfs()
and its synchronous counterpart are of
this type. If bigint
in the options
passed to those methods is true
, the
numeric values will be bigint
instead of number
.
StatFs {
type: 1397114950,
bsize: 4096,
blocks: 121938943,
bfree: 61058895,
bavail: 61058895,
files: 999,
ffree: 1000000
}
bigint
version:
StatFs {
type: 1397114950n,
bsize: 4096n,
blocks: 121938943n,
bfree: 61058895n,
bavail: 61058895n,
files: 999n,
ffree: 1000000n
}
statfs.bavail
#
Free blocks available to unprivileged users.
statfs.bfree
#
Free blocks in file system.
statfs.blocks
#
Total data blocks in file system.
statfs.bsize
#
Optimal transfer block size.
statfs.ffree
#
Free file nodes in file system.
statfs.files
#
Total file nodes in file system.
statfs.type
#
Type of file system.
Class: fs.WriteStream
#
- Extends <stream.Writable>
Instances of <fs.WriteStream> are created and returned using the
fs.createWriteStream()
function.
Event: 'close'
#
Emitted when the <fs.WriteStream>'s underlying file descriptor has been closed.
Event: 'open'
#
fd
<integer> Integer file descriptor used by the <fs.WriteStream>.
Emitted when the <fs.WriteStream>'s file is opened.
Event: 'ready'
#
Emitted when the <fs.WriteStream> is ready to be used.
Fires immediately after 'open'
.
writeStream.bytesWritten
#
The number of bytes written so far. Does not include data that is still queued for writing.
writeStream.close([callback])
#
callback
<Function>err
<Error>
Closes writeStream
. Optionally accepts a
callback that will be executed once the writeStream
is closed.
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>.
writeStream.pending
#
This property is true
if the underlying file has not been opened yet,
i.e. before the 'ready'
event is emitted.
fs.constants
#
Returns an object containing commonly used constants for file system operations.
FS constants#
The following constants are exported by fs.constants
and fsPromises.constants
.
Not every constant will be available on every operating system; this is especially important for Windows, where many of the POSIX specific definitions are not available. For portable applications it is recommended to check for their presence before use.
To use more than one constant, use the bitwise OR |
operator.
Example:
import { open, constants } from 'node:fs';
const {
O_RDWR,
O_CREAT,
O_EXCL,
} = constants;
open('/path/to/my/file', O_RDWR | O_CREAT | O_EXCL, (err, fd) => {
// ...
});
File access constants#
The following constants are meant for use as the mode
parameter passed to
fsPromises.access()
, fs.access()
, and fs.accessSync()
.
Constant | Description |
---|---|
F_OK |
Flag indicating that the file is visible to the calling process.
This is useful for determining if a file exists, but says nothing
about rwx permissions. Default if no mode is specified. |
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. This has no effect on Windows
(will behave like fs.constants.F_OK ). |
The definitions are also available on Windows.
File copy constants#
The following constants are meant for use with fs.copyFile()
.
Constant | Description |
---|---|
COPYFILE_EXCL |
If present, the copy operation will fail with an error if the destination path already exists. |
COPYFILE_FICLONE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used. |
COPYFILE_FICLONE_FORCE |
If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error. |
The definitions are also available on Windows.
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 synchronized I/O with write operations waiting for file integrity. |
O_DSYNC |
Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity. |
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. |
UV_FS_O_FILEMAP |
When set, a memory file mapping is used to access the file. This flag is available on Windows operating systems only. On other operating systems, this flag is ignored. |
On Windows, only O_APPEND
, O_CREAT
, O_EXCL
, O_RDONLY
, O_RDWR
,
O_TRUNC
, O_WRONLY
, and UV_FS_O_FILEMAP
are available.
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. |
On Windows, only S_IFCHR
, S_IFDIR
, S_IFLNK
, S_IFMT
, and S_IFREG
,
are available.
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. |
On Windows, only S_IRUSR
and S_IWUSR
are available.
Notes#
Ordering of callback and promise-based operations#
Because they are executed asynchronously by the underlying thread pool, there is no guaranteed ordering when using either the callback or promise-based methods.
For example, the following is prone to error because the fs.stat()
operation might complete before the fs.rename()
operation:
const fs = require('node:fs');
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 is important to correctly order the operations by awaiting the results of one before invoking the other:
import { rename, stat } from 'node:fs/promises';
const oldPath = '/tmp/hello';
const newPath = '/tmp/world';
try {
await rename(oldPath, newPath);
const stats = await stat(newPath);
console.log(`stats: ${JSON.stringify(stats)}`);
} catch (error) {
console.error('there was an error:', error.message);
}
const { rename, stat } = require('node:fs/promises');
(async function(oldPath, newPath) {
try {
await rename(oldPath, newPath);
const stats = await stat(newPath);
console.log(`stats: ${JSON.stringify(stats)}`);
} catch (error) {
console.error('there was an error:', error.message);
}
})('/tmp/hello', '/tmp/world');
Or, when using the callback APIs, move the fs.stat()
call into the callback
of the fs.rename()
operation:
import { rename, stat } from 'node:fs';
rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
});
const { rename, stat } = require('node:fs/promises');
rename('/tmp/hello', '/tmp/world', (err) => {
if (err) throw err;
stat('/tmp/world', (err, stats) => {
if (err) throw err;
console.log(`stats: ${JSON.stringify(stats)}`);
});
});
File paths#
Most fs
operations accept file paths that may be specified in the form of
a string, a <Buffer>, or a <URL> object using the file:
protocol.
String paths#
String paths are interpreted as UTF-8 character sequences identifying
the absolute or relative filename. Relative paths will be resolved relative
to the current working directory as determined by calling process.cwd()
.
Example using an absolute path on POSIX:
import { open } from 'node:fs/promises';
let fd;
try {
fd = await open('/open/some/file.txt', 'r');
// Do something with the file
} finally {
await fd?.close();
}
Example using a relative path on POSIX (relative to process.cwd()
):
import { open } from 'node:fs/promises';
let fd;
try {
fd = await open('file.txt', 'r');
// Do something with the file
} finally {
await fd?.close();
}
File URL paths#
For most node:fs
module functions, the path
or filename
argument may be
passed as a <URL> object using the file:
protocol.
import { readFileSync } from 'node:fs';
readFileSync(new URL('file:///tmp/hello'));
file:
URLs are always absolute paths.
Platform-specific considerations#
On Windows, file:
<URL>s with a host name convert to UNC paths, while file:
<URL>s with drive letters convert to local absolute paths. file:
<URL>s
with no host name and no drive letter will result in an error:
import { readFileSync } from 'node:fs';
// On Windows :
// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
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
readFileSync(new URL('file:///C:/tmp/hello'));
// - WHATWG file URLs without hostname must have a drive letters
readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute
file:
<URL>s with drive letters must use :
as a separator just after
the drive letter. Using another separator will result in an error.
On all other platforms, file:
<URL>s with a host name are unsupported and
will result in an error:
import { readFileSync } from 'node:fs';
// On other platforms:
// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
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
readFileSync(new URL('file:///tmp/hello'));
A file:
<URL> having encoded slash characters will result in an error on all
platforms:
import { readFileSync } from 'node:fs';
// On Windows
readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
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
readFileSync(new URL('file:///p/a/t/h/%2F'));
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:
<URL>s having encoded backslash will result in an error:
import { readFileSync } from 'node:fs';
// On Windows
readFileSync(new URL('file:///C:/path/%5C'));
readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
Buffer paths#
Paths specified using a <Buffer> are useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems, it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string paths, <Buffer> paths may be relative or absolute:
Example using an absolute path on POSIX:
import { open } from 'node:fs/promises';
import { Buffer } from 'node:buffer';
let fd;
try {
fd = await open(Buffer.from('/open/some/file.txt'), 'r');
// Do something with the file
} finally {
await fd?.close();
}
Per-drive working directories on Windows#
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.
File descriptors#
On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the differences between operating systems and assigns all open files a numeric file descriptor.
The callback-based fs.open()
, and synchronous fs.openSync()
methods open a
file and allocate a new file descriptor. Once allocated, the file descriptor may
be used to read data from, write data to, or request information about the file.
Operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.
import { open, close, fstat } from 'node:fs';
function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}
open('/open/some/file.txt', 'r', (err, fd) => {
if (err) throw err;
try {
fstat(fd, (err, stat) => {
if (err) {
closeFd(fd);
throw err;
}
// use stat
closeFd(fd);
});
} catch (err) {
closeFd(fd);
throw err;
}
});
The promise-based APIs use a <FileHandle> object in place of the numeric file descriptor. These objects are better managed by the system to ensure that resources are not leaked. However, it is still required that they are closed when operations are completed:
import { open } from 'node:fs/promises';
let file;
try {
file = await open('/open/some/file.txt', 'r');
const stat = await file.stat();
// use stat
} finally {
await file.close();
}
Threadpool usage#
All callback and promise-based file system APIs (with the exception of
fs.FSWatcher()
) use libuv's threadpool. This can have surprising and negative
performance implications for some applications. See the
UV_THREADPOOL_SIZE
documentation for more information.
File system flags#
The following flags are available wherever the flag
option takes a
string.
-
'a'
: Open file for appending. The file is created if it does not exist. -
'ax'
: Like'a'
but fails if the path exists. -
'a+'
: Open file for reading and appending. The file is created if it does not exist. -
'ax+'
: Like'a+'
but fails if the path exists. -
'as'
: Open file for appending in synchronous mode. The file is created if it does not exist. -
'as+'
: Open file for reading and appending in synchronous mode. The file is created if it does not exist. -
'r'
: Open file for reading. An exception occurs if the file does not exist. -
'rs'
: Open file for reading in synchronous mode. 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.
This doesn't turn
fs.open()
orfsPromises.open()
into a synchronous blocking call. If synchronous operation is desired, something likefs.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 if the path 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 if the path exists.
flag
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
.
The exclusive flag 'x'
(O_EXCL
flag in open(2)
) causes the operation to
return an error if the path already exists. On POSIX, if the path is a symbolic
link, using O_EXCL
returns an error even if the link is to a path that does
not exist. The exclusive flag might not work with network file systems.
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.
Modifying a file rather than replacing it may require the flag
option to be
set to 'r+'
rather than the default 'w'
.
The behavior of some flags are platform-specific. As such, opening a directory
on macOS and Linux with the 'a+'
flag, as in the example below, will return an
error. In contrast, on Windows and FreeBSD, a file descriptor or a FileHandle
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>
});
On Windows, opening an existing hidden file using the 'w'
flag (either
through fs.open()
, fs.writeFile()
, or fsPromises.open()
) will fail with
EPERM
. Existing hidden files can be opened for writing with the 'r+'
flag.
A call to fs.ftruncate()
or filehandle.truncate()
can be used to reset
the file contents.