xref: /third_party/node/doc/api/console.md

# Console

<!--introduced_in=v0.10.13-->

> Stability: 2 - Stable

<!-- source_link=lib/console.js -->

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 message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

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`
<!-- YAML
changes:
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/9744
    description: Errors that occur while writing to the underlying streams
                 will now be ignored by default.
-->

<!--type=class-->

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][, ignoreErrors])`
### `new Console(options)`
<!-- YAML
changes:
  - version: v14.2.0
    pr-url: https://github.com/nodejs/node/pull/32964
    description: The `groupIndentation` option was introduced.
  - version: v11.7.0
    pr-url: https://github.com/nodejs/node/pull/24978
    description: The `inspectOptions` option is introduced.
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/19372
    description: The `Console` constructor now supports an `options` argument,
                 and the `colorMode` option was introduced.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/9744
    description: The `ignoreErrors` option was introduced.
-->

* `options` {Object}
  * `stdout` {stream.Writable}
  * `stderr` {stream.Writable}
  * `ignoreErrors` {boolean} Ignore errors when writing to the underlying
    streams. **Default:** `true`.
  * `colorMode` {boolean|string} Set color support for this `Console` instance.
    Setting to `true` enables coloring while inspecting values. Setting to
    `false` disables coloring while inspecting values. Setting to
    `'auto'` makes color support depend on the value of the `isTTY` property
    and the value returned by `getColorDepth()` on the respective stream. This
    option can not be used, if `inspectOptions.colors` is set as well.
    **Default:** `'auto'`.
  * `inspectOptions` {Object} Specifies options that are passed along to
    [`util.inspect()`][].
  * `groupIndentation` {number} Set group indentation.
    **Default:** `2`.

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({ stdout: output, stderr: 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({ stdout: process.stdout, stderr: process.stderr });
```

### `console.assert(value[, ...message])`
<!-- YAML
added: v0.1.101
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/17706
    description: The implementation is now spec compliant and does not throw
                 anymore.
-->

* `value` {any} The value tested for being truthy.
* `...message` {any} All arguments besides `value` are used as error message.

`console.assert()` writes a message if `value` is [falsy][] or omitted. It only
writes a message and does not otherwise affect execution. The output always
starts with `"Assertion failed"`. If provided, `message` is formatted using
[`util.format()`][].

If `value` is [truthy][], nothing happens.

```js
console.assert(true, 'does nothing');

console.assert(false, 'Whoops %s work', 'didn\'t');
// Assertion failed: Whoops didn't work

console.assert();
// Assertion failed
```

### `console.clear()`
<!-- YAML
added: v8.3.0
-->

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])`
<!-- YAML
added: v8.3.0
-->

* `label` {string} The display label for the counter. **Default:** `'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`.

<!-- eslint-skip -->
```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])`
<!-- YAML
added: v8.3.0
-->

* `label` {string} The display label for the counter. **Default:** `'default'`.

Resets the internal counter specific to `label`.

<!-- eslint-skip -->
```js
> console.count('abc');
abc: 1
undefined
> console.countReset('abc');
undefined
> console.count('abc');
abc: 1
undefined
>
```

### `console.debug(data[, ...args])`
<!-- YAML
added: v8.0.0
changes:
  - version: v8.10.0
    pr-url: https://github.com/nodejs/node/pull/17033
    description: "`console.debug` is now an alias for `console.log`."
-->

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

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

### `console.dir(obj[, options])`
<!-- YAML
added: v0.1.101
-->

* `obj` {any}
* `options` {Object}
  * `showHidden` {boolean} If `true` then the object's non-enumerable and symbol
    properties will be shown too. **Default:** `false`.
  * `depth` {number} Tells [`util.inspect()`][] how many times to recurse while
    formatting the object. This is useful for inspecting large complicated
    objects. To make it recurse indefinitely, pass `null`. **Default:** `2`.
  * `colors` {boolean} If `true`, then the output will be styled with ANSI color
     codes. Colors are customizable;
     see [customizing `util.inspect()` colors][]. **Default:** `false`.

Uses [`util.inspect()`][] on `obj` and prints the resulting string to `stdout`.
This function bypasses any custom `inspect()` function defined on `obj`.

### `console.dirxml(...data)`
<!-- YAML
added: v8.0.0
changes:
  - version: v9.3.0
    pr-url: https://github.com/nodejs/node/pull/17152
    description: "`console.dirxml` now calls `console.log` for its arguments."
-->

* `...data` {any}

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

### `console.error([data][, ...args])`
<!-- YAML
added: 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()`][]).

```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])`
<!-- YAML
added: v8.5.0
-->

* `...label` {any}

Increases indentation of subsequent lines by spaces for `groupIndentation`
length.

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

### `console.groupCollapsed()`
<!-- YAML
  added: v8.5.0
-->

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

### `console.groupEnd()`
<!-- YAML
added: v8.5.0
-->

Decreases indentation of subsequent lines by spaces for `groupIndentation`
length.

### `console.info([data][, ...args])`
<!-- YAML
added: v0.1.100
-->

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

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

### `console.log([data][, ...args])`
<!-- YAML
added: 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()`][]).

```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.table(tabularData[, properties])`
<!-- YAML
added: v10.0.0
-->

* `tabularData` {any}
* `properties` {string[]} Alternate properties for constructing the table.

Try to construct a table with the columns of the properties of `tabularData`
(or use `properties`) and rows of `tabularData` and log it. Falls back to just
logging the argument if it can’t be parsed as tabular.

```js
// These can't be parsed as tabular data
console.table(Symbol());
// Symbol()

console.table(undefined);
// undefined

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }]);
// ┌─────────┬─────┬─────┐
// │ (index) │  a  │  b  │
// ├─────────┼─────┼─────┤
// │    0    │  1  │ 'Y' │
// │    1    │ 'Z' │  2  │
// └─────────┴─────┴─────┘

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }], ['a']);
// ┌─────────┬─────┐
// │ (index) │  a  │
// ├─────────┼─────┤
// │    0    │  1  │
// │    1    │ 'Z' │
// └─────────┴─────┘
```

### `console.time([label])`
<!-- YAML
added: v0.1.104
-->

* `label` {string} **Default:** `'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
suitable time units to `stdout`. For example, if the elapsed
time is 3869ms, `console.timeEnd()` displays "3.869s".

### `console.timeEnd([label])`
<!-- YAML
added: v0.1.104
changes:
  - version: v13.0.0
    pr-url: https://github.com/nodejs/node/pull/29251
    description: The elapsed time is displayed with a suitable time unit.
  - version: v6.0.0
    pr-url: https://github.com/nodejs/node/pull/5901
    description: This method no longer supports multiple calls that don’t map
                 to individual `console.time()` calls; see below for details.
-->

* `label` {string} **Default:** `'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.timeLog([label][, ...data])`
<!-- YAML
added: v10.7.0
-->

* `label` {string} **Default:** `'default'`
* `...data` {any}

For a timer that was previously started by calling [`console.time()`][], prints
the elapsed time and other `data` arguments to `stdout`:

```js
console.time('process');
const value = expensiveProcess1(); // Returns 42
console.timeLog('process', value);
// Prints "process: 365.227ms 42".
doExpensiveProcess2(value);
console.timeEnd('process');
```

### `console.trace([message][, ...args])`
<!-- YAML
added: 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.

```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])`
<!-- YAML
added: v0.1.100
-->

* `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.profile([label])`
<!-- YAML
added: v8.0.0
-->

* `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('MyLabel');
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.
```

### `console.profileEnd([label])`
<!-- YAML
added: v8.0.0
-->

* `label` {string}

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.

If this method is called without a label, the most recently started profile is
stopped.

### `console.timeStamp([label])`
<!-- YAML
added: v8.0.0
-->

* `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.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_label
[`console.time()`]: #console_console_time_label
[`console.timeEnd()`]: #console_console_timeend_label
[`process.stderr`]: process.md#process_process_stderr
[`process.stdout`]: process.md#process_process_stdout
[`util.format()`]: util.md#util_util_format_format_args
[`util.inspect()`]: util.md#util_util_inspect_object_options
[customizing `util.inspect()` colors]: util.md#util_customizing_util_inspect_colors
[falsy]: https://developer.mozilla.org/en-US/docs/Glossary/Falsy
[inspector]: debugger.md
[note on process I/O]: process.md#process_a_note_on_process_i_o
[truthy]: https://developer.mozilla.org/en-US/docs/Glossary/Truthy