Console | Node.js v8.0.0 Documentation

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# 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 `process.stdout` and `process.stderr`. The global `console` can be used without calling `require('console')`.

Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.

Example using the global `console`:

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

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

Example using the `Console` class:

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

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

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

## Class: Console#

History

Version | Changes
--- | ---
v8.0.0 | Errors that occur while writing to the underlying streams will now be ignored.

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])#

- `stdout` <Writable>
- `stderr` <Writable>

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

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

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

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

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

Added in: v0.1.101

- `value` <any>
- `message` <any>
- `...args` <any>

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

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

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

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

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

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

    'use strict';

    // Creates a simple extension of console with a
    // new impl for assert without monkey-patching.
    const myConsole = Object.create(console, {
      assert: {
        value: function assert(assertion, message, ...args) {
          try {
            console.assert(assertion, message, ...args);
          } catch (err) {
            console.error(err.stack);
          }
        },
        configurable: true,
        enumerable: true,
        writable: true,
      },
    });

    module.exports = myConsole;

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

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

### console.dir(obj[, options])#

Added in: v0.1.101

- `obj` <any>
- `options` <Object>
  - `showHidden` <boolean>
  - `depth` <number>
  - `colors` <boolean>

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])#

Added in: v0.1.100

- `data` <any>
- `...args` <any>

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

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

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

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

Added in: v0.1.100

- `data` <any>
- `...args` <any>

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

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

Added in: v0.1.100

- `data` <any>
- `...args` <any>

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

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

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)#

Added in: v0.1.104

- `label` <string>

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

### console.timeEnd(label)#

History

Version | Changes
--- | ---
v6.0.0 | This method no longer supports multiple calls that don’t map to individual `console.time()` calls; see below for details.
v0.1.104 | Added in: v0.1.104

- `label` <string>

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

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

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

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

Added in: v0.1.104

- `message` <any>
- `...args` <any>

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

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

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

Added in: v0.1.100

- `data` <any>
- `...args` <any>

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