Table of Contents

• Console
  • Asynchronous vs Synchronous Consoles
  • Class: Console
    • new Console(stdout[, stderr])
    • console.assert(value[, message][, ...args])
    • console.dir(obj[, options])
    • console.error([data][, ...args])
    • console.info([data][, ...args])
    • console.log([data][, ...args])
    • console.time(label)
    • console.timeEnd(label)
    • console.trace(message[, ...args])
    • console.warn([data][, ...args])

Console#

Stability: 2 - Stable

The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

• A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
• A global console instance configured to write to stdout and stderr. Because this object is global, it can be used without calling require('console').

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

While the API for the Console class is designed fundamentally around the browser console object, the Console in Node.js is not intended to duplicate the browser's functionality exactly.

Asynchronous vs Synchronous Consoles#

The console functions are usually asynchronous unless the destination is a file. Disks are fast and operating systems normally employ write-back caching; it should be a very rare occurrence indeed that a write blocks, but it is possible.

Additionally, console functions are blocking when outputting to TTYs (terminals) on OS X as a workaround for the OS's very small, 1kb buffer size. This is to prevent interleaving between stdout and stderr.

Class: Console#

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('console').Console or console.Console:

const Console = require('console').Console;
const Console = console.Console;

new Console(stdout[, stderr])#

Creates a new Console by passing one or two writable stream instances. stdout is a writable stream to print log or info output. stderr is used for warning or error output. If stderr isn't passed, warning and error output will be sent to stdout.

const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
const logger = new Console(output, errorOutput);
// use it like console
var count = 5;
logger.log('count: %d', count);
// in stdout.log: count 5

The global console is a special Console whose output is sent to process.stdout and process.stderr. It is equivalent to calling:

new Console(process.stdout, process.stderr);

console.assert(value[, message][, ...args])#

A simple assertion test that verifies whether value is truthy. If it is not, an AssertionError is thrown. If provided, the error message is formatted using util.format() and used as the error message.

console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work

Note: the console.assert() method is implemented differently in Node.js than the console.assert() method available in browsers.

Specifically, in browsers, calling console.assert() with a falsy assertion will cause the message to be printed to the console without interrupting execution of subsequent code. In Node.js, however, a falsy assertion will cause an AssertionError to be thrown.

Functionality approximating that implemented by browsers can be implemented by extending Node.js' console and overriding the console.assert() method.

In the following example, a simple module is created that extends and overrides the default behavior of console in Node.js.

'use strict';

// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.setPrototypeOf({
  assert(assertion, message, ...args) {
    try {
      console.assert(assertion, message, ...args);
    } catch (err) {
      console.error(err.stack);
    }
  }
}, console);

module.exports = myConsole;

This can then be used as a direct replacement for the built in console:

const console = require('./myConsole');
console.assert(false, 'this message will print, but no error thrown');
console.log('this will also print');

console.dir(obj[, options])#

Uses util.inspect() on obj and prints the resulting string to stdout. This function bypasses any custom inspect() function defined on obj. An optional options object may be passed to alter certain aspects of the formatted string:

• showHidden - if true then the object's non-enumerable and symbol properties will be shown too. Defaults to false.

• depth - tells util.inspect() how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to 2. To make it recurse indefinitely, pass null.

• colors - if true, then the output will be styled with ANSI color codes. Defaults to false. Colors are customizable; see customizing util.inspect() colors.

console.error([data][, ...args])#

Prints to stderr with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.info([data][, ...args])#

The console.info() function is an alias for console.log().

console.log([data][, ...args])#

Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

var count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.time(label)#

Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label. Use the same label when you call console.timeEnd() to stop the timer and output the elapsed time in milliseconds to stdout. Timer durations are accurate to the sub-millisecond.

console.timeEnd(label)#

Stops a timer that was previously started by calling console.time() and prints the result to stdout:

console.time('100-elements');
for (var i = 0; i < 100; i++) {
  ;
}
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms

Note: As of Node.js v6.0.0, console.timeEnd() deletes the timer to avoid leaking it. On older versions, the timer persisted. This allowed console.timeEnd() to be called multiple times for the same label. This functionality was unintended and is no longer supported.

console.trace(message[, ...args])#

Prints to stderr the string 'Trace :', followed by the util.format() formatted message and stack trace to the current position in the code.

console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
//  Trace: Show me
//    at repl:2:9
//    at REPLServer.defaultEval (repl.js:248:27)
//    at bound (domain.js:287:14)
//    at REPLServer.runBound [as eval] (domain.js:300:12)
//    at REPLServer.<anonymous> (repl.js:412:12)
//    at emitOne (events.js:82:20)
//    at REPLServer.emit (events.js:169:7)
//    at REPLServer.Interface._onLine (readline.js:210:10)
//    at REPLServer.Interface._line (readline.js:549:8)
//    at REPLServer.Interface._ttyWrite (readline.js:826:14)

console.warn([data][, ...args])#

The console.warn() function is an alias for console.error().