Console Node.js v6.2.2 Manual & 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 `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][, ...])#

Added in: v0.1.101

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])#

Added in: v0.1.101

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][, ...])#

Added in: v0.1.100

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][, ...])#

Added in: v0.1.100

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

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

Added in: v0.1.100

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)#

Added in: v0.1.104

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)#

Added in: v0.1.104

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[, ...])#

Added in: v0.1.104

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][, ...])#

Added in: v0.1.100

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