✏️ Editing: console.md Read Only

# 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`:

```js
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:

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

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

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

## Class: Console

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

```js
const { Console } = require('console');
```

```js
const { Console } = console;
```

### new Console(stdout[, stderr])
* `stdout` {stream.Writable}
* `stderr` {stream.Writable}

Creates a new `Console` with 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 provided, `stdout` is used for `stderr`.

```js
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:

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

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

* `value` {any}
* `message` {any}
* `...args` {any}

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

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

The `console.assert()` method is implemented differently in Node.js than the
`console.assert()` method [available in browsers][web-api-assert].

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.

```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:

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

### console.clear()

When `stdout` is a TTY, calling `console.clear()` will attempt to clear the
TTY. When `stdout` is not a TTY, this method does nothing.

The specific operation of `console.clear()` can vary across operating systems
and terminal types. For most Linux operating systems, `console.clear()`
operates similarly to the `clear` shell command. On Windows, `console.clear()`
will clear only the output in the current terminal viewport for the Node.js
binary.

### console.count([label])

* `label` {string} The display label for the counter. Defaults to `'default'`.

Maintains an internal counter specific to `label` and outputs to `stdout` the
number of times `console.count()` has been called with the given `label`.

```js
> console.count()
default: 1
undefined
> console.count('default')
default: 2
undefined
> console.count('abc')
abc: 1
undefined
> console.count('xyz')
xyz: 1
undefined
> console.count('abc')
abc: 2
undefined
> console.count()
default: 3
undefined
>
```

### console.countReset([label='default'])

* `label` {string} The display label for the counter. Defaults to `'default'`.

Resets the internal counter specific to `label`.

```js
> console.count('abc');
abc: 1
undefined
> console.countReset('abc');
undefined
> console.count('abc');
abc: 1
undefined
>
```

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

* `data` {any}
* `...args` {any}

The `console.debug()` function is an alias for [`console.log()`][].

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

* `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.dirxml(...data)

* `...data` {any}

This method calls `console.log()` passing it the arguments received.
Please note that this method does not produce any XML formatting.

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

* `data` {any}
* `...args` {any}

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

```js
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.group([...label])

* `...label` {any}

Increases indentation of subsequent lines by two spaces.

If one or more `label`s are provided, those are printed first without the
additional indentation.

### console.groupCollapsed()

An alias for [`console.group()`][].

### console.groupEnd()

Decreases indentation of subsequent lines by two spaces.

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

* `data` {any}
* `...args` {any}

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

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

* `data` {any}
* `...args` {any}

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

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

See [`util.format()`][] for more information.

### console.time(label)

* `label` {string} Defaults to `'default'`.

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

### console.timeEnd(label)

* `label` {string} Defaults to `'default'`.

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

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

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

* `message` {any}
* `...args` {any}

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

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

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

* `data` {any}
* `...args` {any}

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

## Inspector only methods
The following methods are exposed by the V8 engine in the general API but do
not display anything unless used in conjunction with the [inspector][]
(`--inspect` flag).

### console.markTimeline(label)

* `label` {string} Defaults to `'default'`.

This method does not display anything unless used in the inspector. The
`console.markTimeline()` method is the deprecated form of 
[`console.timeStamp()`][].

### console.profile([label])

* `label` {string}

This method does not display anything unless used in the inspector. The
`console.profile()` method starts a JavaScript CPU profile with an optional
label until [`console.profileEnd()`][] is called. The profile is then added to
the **Profile** panel of the inspector.
```js
console.profile('MyLabel');
// Some code
console.profileEnd();
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.
```

### console.profileEnd()

This method does not display anything unless used in the inspector. Stops the
current JavaScript CPU profiling session if one has been started and prints
the report to the **Profiles** panel of the inspector. See
[`console.profile()`][] for an example.

### console.table(array[, columns])

* `array` {Array|Object}
* `columns` {Array}

This method does not display anything unless used in the inspector. Prints to
`stdout` the array `array` formatted as a table.

### console.timeStamp([label])

* `label` {string}

This method does not display anything unless used in the inspector. The
`console.timeStamp()` method adds an event with the label `label` to the
**Timeline** panel of the inspector.

### console.timeline([label])

* `label` {string} Defaults to `'default'`.

This method does not display anything unless used in the inspector. The
`console.timeline()` method is the deprecated form of [`console.time()`][].

### console.timelineEnd([label])

* `label` {string} Defaults to `'default'`.

This method does not display anything unless used in the inspector. The
`console.timelineEnd()` method is the deprecated form of 
[`console.timeEnd()`][].

[`console.error()`]: #console_console_error_data_args
[`console.group()`]: #console_console_group_label
[`console.log()`]: #console_console_log_data_args
[`console.profile()`]: #console_console_profile_label
[`console.profileEnd()`]: #console_console_profileend
[`console.time()`]: #console_console_time_label
[`console.timeEnd()`]: #console_console_timeend_label
[`console.timeStamp()`]: #console_console_timestamp_label
[`process.stderr`]: process.html#process_process_stderr
[`process.stdout`]: process.html#process_process_stdout
[`util.format()`]: util.html#util_util_format_format_args
[`util.inspect()`]: util.html#util_util_inspect_object_options
[customizing `util.inspect()` colors]: util.html#util_customizing_util_inspect_colors
[inspector]: debugger.html
[note on process I/O]: process.html#process_a_note_on_process_i_o
[web-api-assert]: https://developer.mozilla.org/en-US/docs/Web/API/console/assert