nodejs/doc/api/util.md

# Util

<!--introduced_in=v0.10.0-->

> Stability: 2 - Stable

The `util` module supports the needs of Node.js internal APIs. Many of the
utilities are useful for application and module developers as well. To access
it:

```js
const util = require('util');
```

## `util.callbackify(original)`
<!-- YAML
added: v8.2.0
-->

* `original` {Function} An `async` function
* Returns: {Function} a callback style function

Takes an `async` function (or a function that returns a `Promise`) and returns a
function following the error-first callback style, i.e. taking
an `(err, value) => ...` callback as the last argument. In the callback, the
first argument will be the rejection reason (or `null` if the `Promise`
resolved), and the second argument will be the resolved value.

```js
const util = require('util');

async function fn() {
  return 'hello world';
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
  if (err) throw err;
  console.log(ret);
});
```

Will print:

```txt
hello world
```

The callback is executed asynchronously, and will have a limited stack trace.
If the callback throws, the process will emit an [`'uncaughtException'`][]
event, and if not handled will exit.

Since `null` has a special meaning as the first argument to a callback, if a
wrapped function rejects a `Promise` with a falsy value as a reason, the value
is wrapped in an `Error` with the original value stored in a field named
`reason`.

```js
function fn() {
  return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
  // When the Promise was rejected with `null` it is wrapped with an Error and
  // the original value is stored in `reason`.
  err && err.hasOwnProperty('reason') && err.reason === null;  // true
});
```

## `util.debuglog(section)`
<!-- YAML
added: v0.11.3
-->

* `section` {string} A string identifying the portion of the application for
  which the `debuglog` function is being created.
* Returns: {Function} The logging function

The `util.debuglog()` method is used to create a function that conditionally
writes debug messages to `stderr` based on the existence of the `NODE_DEBUG`
environment variable. If the `section` name appears within the value of that
environment variable, then the returned function operates similar to
[`console.error()`][]. If not, then the returned function is a no-op.

```js
const util = require('util');
const debuglog = util.debuglog('foo');

debuglog('hello from foo [%d]', 123);
```

If this program is run with `NODE_DEBUG=foo` in the environment, then
it will output something like:

```txt
FOO 3245: hello from foo [123]
```

where `3245` is the process id. If it is not run with that
environment variable set, then it will not print anything.

The `section` supports wildcard also:

```js
const util = require('util');
const debuglog = util.debuglog('foo-bar');

debuglog('hi there, it\'s foo-bar [%d]', 2333);
```

if it is run with `NODE_DEBUG=foo*` in the environment, then it will output
something like:

```txt
FOO-BAR 3257: hi there, it's foo-bar [2333]
```

Multiple comma-separated `section` names may be specified in the `NODE_DEBUG`
environment variable: `NODE_DEBUG=fs,net,tls`.

## `util.deprecate(fn, msg[, code])`
<!-- YAML
added: v0.8.0
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/16393
    description: Deprecation warnings are only emitted once for each code.
-->

* `fn` {Function} The function that is being deprecated.
* `msg` {string} A warning message to display when the deprecated function is
  invoked.
* `code` {string} A deprecation code. See the [list of deprecated APIs][] for a
  list of codes.
* Returns: {Function} The deprecated function wrapped to emit a warning.

The `util.deprecate()` method wraps `fn` (which may be a function or class) in
such a way that it is marked as deprecated.

```js
const util = require('util');

exports.obsoleteFunction = util.deprecate(() => {
  // Do something here.
}, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');
```

When called, `util.deprecate()` will return a function that will emit a
`DeprecationWarning` using the [`'warning'`][] event. The warning will
be emitted and printed to `stderr` the first time the returned function is
called. After the warning is emitted, the wrapped function is called without
emitting a warning.

If the same optional `code` is supplied in multiple calls to `util.deprecate()`,
the warning will be emitted only once for that `code`.

```js
const util = require('util');

const fn1 = util.deprecate(someFunction, someMessage, 'DEP0001');
const fn2 = util.deprecate(someOtherFunction, someOtherMessage, 'DEP0001');
fn1(); // Emits a deprecation warning with code DEP0001
fn2(); // Does not emit a deprecation warning because it has the same code
```

If either the `--no-deprecation` or `--no-warnings` command line flags are
used, or if the `process.noDeprecation` property is set to `true` *prior* to
the first deprecation warning, the `util.deprecate()` method does nothing.

If the `--trace-deprecation` or `--trace-warnings` command line flags are set,
or the `process.traceDeprecation` property is set to `true`, a warning and a
stack trace are printed to `stderr` the first time the deprecated function is
called.

If the `--throw-deprecation` command line flag is set, or the
`process.throwDeprecation` property is set to `true`, then an exception will be
thrown when the deprecated function is called.

The `--throw-deprecation` command line flag and `process.throwDeprecation`
property take precedence over `--trace-deprecation` and
`process.traceDeprecation`.

## `util.format(format[, ...args])`
<!-- YAML
added: v0.5.3
changes:
  - version: v12.11.0
    pr-url: https://github.com/nodejs/node/pull/29606
    description: The `%c` specifier is ignored now.
  - version: v11.4.0
    pr-url: https://github.com/nodejs/node/pull/23708
    description: The `%d`, `%f` and `%i` specifiers now support Symbols
                 properly.
  - version: v12.0.0
    pr-url: https://github.com/nodejs/node/pull/23162
    description: The `format` argument is now only taken as such if it actually
                 contains format specifiers.
  - version: v12.0.0
    pr-url: https://github.com/nodejs/node/pull/23162
    description: If the `format` argument is not a format string, the output
                 string's formatting is no longer dependent on the type of the
                 first argument. This change removes previously present quotes
                 from strings that were being output when the first argument
                 was not a string.
  - version: v11.4.0
    pr-url: https://github.com/nodejs/node/pull/24806
    description: The `%o` specifier's `depth` has default depth of 4 again.
  - version: v11.0.0
    pr-url: https://github.com/nodejs/node/pull/17907
    description: The `%o` specifier's `depth` option will now fall back to the
                 default depth.
  - version: v10.12.0
    pr-url: https://github.com/nodejs/node/pull/22097
    description: The `%d` and `%i` specifiers now support BigInt.
  - version: v8.4.0
    pr-url: https://github.com/nodejs/node/pull/14558
    description: The `%o` and `%O` specifiers are supported now.
-->

* `format` {string} A `printf`-like format string.

The `util.format()` method returns a formatted string using the first argument
as a `printf`-like format string which can contain zero or more format
specifiers. Each specifier is replaced with the converted value from the
corresponding argument. Supported specifiers are:

* `%s`: `String` will be used to convert all values except `BigInt`, `Object`
  and `-0`. `BigInt` values will be represented with an `n` and Objects that
  have no user defined `toString` function are inspected using `util.inspect()`
  with options `{ depth: 0, colors: false, compact: 3 }`.
* `%d`: `Number` will be used to convert all values except `BigInt` and
  `Symbol`.
* `%i`: `parseInt(value, 10)` is used for all values except `BigInt` and
  `Symbol`.
* `%f`: `parseFloat(value)` is used for all values expect `Symbol`.
* `%j`: JSON. Replaced with the string `'[Circular]'` if the argument contains
  circular references.
* `%o`: `Object`. A string representation of an object with generic JavaScript
  object formatting. Similar to `util.inspect()` with options
  `{ showHidden: true, showProxy: true }`. This will show the full object
  including non-enumerable properties and proxies.
* `%O`: `Object`. A string representation of an object with generic JavaScript
  object formatting. Similar to `util.inspect()` without options. This will show
  the full object not including non-enumerable properties and proxies.
* `%c`: `CSS`. This specifier is currently ignored, and will skip any CSS
  passed in.
* `%%`: single percent sign (`'%'`). This does not consume an argument.
* Returns: {string} The formatted string

If a specifier does not have a corresponding argument, it is not replaced:

```js
util.format('%s:%s', 'foo');
// Returns: 'foo:%s'
```

Values that are not part of the format string are formatted using
`util.inspect()` if their type is not `string`.

If there are more arguments passed to the `util.format()` method than the
number of specifiers, the extra arguments are concatenated to the returned
string, separated by spaces:

```js
util.format('%s:%s', 'foo', 'bar', 'baz');
// Returns: 'foo:bar baz'
```

If the first argument does not contain a valid format specifier, `util.format()`
returns a string that is the concatenation of all arguments separated by spaces:

```js
util.format(1, 2, 3);
// Returns: '1 2 3'
```

If only one argument is passed to `util.format()`, it is returned as it is
without any formatting:

```js
util.format('%% %s');
// Returns: '%% %s'
```

`util.format()` is a synchronous method that is intended as a debugging tool.
Some input values can have a significant performance overhead that can block the
event loop. Use this function with care and never in a hot code path.

## `util.formatWithOptions(inspectOptions, format[, ...args])`
<!-- YAML
added: v10.0.0
-->

* `inspectOptions` {Object}
* `format` {string}

This function is identical to [`util.format()`][], except in that it takes
an `inspectOptions` argument which specifies options that are passed along to
[`util.inspect()`][].

```js
util.formatWithOptions({ colors: true }, 'See object %O', { foo: 42 });
// Returns 'See object { foo: 42 }', where `42` is colored as a number
// when printed to a terminal.
```

## `util.getSystemErrorName(err)`
<!-- YAML
added: v9.7.0
-->

* `err` {number}
* Returns: {string}

Returns the string name for a numeric error code that comes from a Node.js API.
The mapping between error codes and error names is platform-dependent.
See [Common System Errors][] for the names of common errors.

```js
fs.access('file/that/does/not/exist', (err) => {
  const name = util.getSystemErrorName(err.errno);
  console.error(name);  // ENOENT
});
```

## `util.inherits(constructor, superConstructor)`
<!-- YAML
added: v0.3.0
changes:
  - version: v5.0.0
    pr-url: https://github.com/nodejs/node/pull/3455
    description: The `constructor` parameter can refer to an ES6 class now.
-->

* `constructor` {Function}
* `superConstructor` {Function}

Usage of `util.inherits()` is discouraged. Please use the ES6 `class` and
`extends` keywords to get language level inheritance support. Also note
that the two styles are [semantically incompatible][].

Inherit the prototype methods from one [constructor][] into another. The
prototype of `constructor` will be set to a new object created from
`superConstructor`.

This mainly adds some input validation on top of
`Object.setPrototypeOf(constructor.prototype, superConstructor.prototype)`.
As an additional convenience, `superConstructor` will be accessible
through the `constructor.super_` property.

```js
const util = require('util');
const EventEmitter = require('events');

function MyStream() {
  EventEmitter.call(this);
}

util.inherits(MyStream, EventEmitter);

MyStream.prototype.write = function(data) {
  this.emit('data', data);
};

const stream = new MyStream();

console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"
```

ES6 example using `class` and `extends`:

```js
const EventEmitter = require('events');

class MyStream extends EventEmitter {
  write(data) {
    this.emit('data', data);
  }
}

const stream = new MyStream();

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('With ES6');
```

## `util.inspect(object[, options])`
## `util.inspect(object[, showHidden[, depth[, colors]]])`
<!-- YAML
added: v0.3.0
changes:
  - version: v13.5.0
    pr-url: https://github.com/nodejs/node/pull/30768
    description: User defined prototype properties are inspected in case
                 `showHidden` is `true`.
  - version: v13.0.0
    pr-url: https://github.com/nodejs/node/pull/27685
    description: Circular references now include a marker to the reference.
  - version: v12.0.0
    pr-url: https://github.com/nodejs/node/pull/27109
    description: The `compact` options default is changed to `3` and the
                 `breakLength` options default is changed to `80`.
  - version: v11.11.0
    pr-url: https://github.com/nodejs/node/pull/26269
    description: The `compact` option accepts numbers for a new output mode.
  - version: v12.0.0
    pr-url: https://github.com/nodejs/node/pull/24971
    description: Internal properties no longer appear in the context argument
                 of a custom inspection function.
  - version: v11.7.0
    pr-url: https://github.com/nodejs/node/pull/25006
    description: ArrayBuffers now also show their binary contents.
  - version: v11.5.0
    pr-url: https://github.com/nodejs/node/pull/24852
    description: The `getters` option is supported now.
  - version: v11.4.0
    pr-url: https://github.com/nodejs/node/pull/24326
    description: The `depth` default changed back to `2`.
  - version: v11.0.0
    pr-url: https://github.com/nodejs/node/pull/22846
    description: The `depth` default changed to `20`.
  - version: v10.12.0
    pr-url: https://github.com/nodejs/node/pull/22788
    description: The `sorted` option is supported now.
  - version: v11.0.0
    pr-url: https://github.com/nodejs/node/pull/22756
    description: The inspection output is now limited to about 128 MB. Data
                 above that size will not be fully inspected.
  - version: v10.6.0
    pr-url: https://github.com/nodejs/node/pull/20725
    description: Inspecting linked lists and similar objects is now possible
                 up to the maximum call stack size.
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/19259
    description: The `WeakMap` and `WeakSet` entries can now be inspected
                 as well.
  - version: v9.9.0
    pr-url: https://github.com/nodejs/node/pull/17576
    description: The `compact` option is supported now.
  - version: v6.6.0
    pr-url: https://github.com/nodejs/node/pull/8174
    description: Custom inspection functions can now return `this`.
  - version: v6.3.0
    pr-url: https://github.com/nodejs/node/pull/7499
    description: The `breakLength` option is supported now.
  - version: v6.1.0
    pr-url: https://github.com/nodejs/node/pull/6334
    description: The `maxArrayLength` option is supported now; in particular,
                 long arrays are truncated by default.
  - version: v6.1.0
    pr-url: https://github.com/nodejs/node/pull/6465
    description: The `showProxy` option is supported now.
-->

* `object` {any} Any JavaScript primitive or `Object`.
* `options` {Object}
  * `showHidden` {boolean} If `true`, `object`'s non-enumerable symbols and
    properties are included in the formatted result. [`WeakMap`][] and
    [`WeakSet`][] entries are also included as well as user defined prototype
    properties (excluding method properties). **Default:** `false`.
  * `depth` {number} Specifies the number of times to recurse while formatting
    `object`. This is useful for inspecting large objects. To recurse up to
    the maximum call stack size pass `Infinity` or `null`.
    **Default:** `2`.
  * `colors` {boolean} If `true`, the output is styled with ANSI color
    codes. Colors are customizable. See [Customizing `util.inspect` colors][].
    **Default:** `false`.
  * `customInspect` {boolean} If `false`,
    `[util.inspect.custom](depth, opts)` functions are not invoked.
    **Default:** `true`.
  * `showProxy` {boolean} If `true`, `Proxy` inspection includes
    the [`target` and `handler`][] objects. **Default:** `false`.
  * `maxArrayLength` {integer} Specifies the maximum number of `Array`,
    [`TypedArray`][], [`WeakMap`][] and [`WeakSet`][] elements to include when
    formatting. Set to `null` or `Infinity` to show all elements. Set to `0` or
    negative to show no elements. **Default:** `100`.
  * `breakLength` {integer} The length at which input values are split across
    multiple lines. Set to `Infinity` to format the input as a single line
    (in combination with `compact` set to `true` or any number >= `1`).
    **Default:** `80`.
  * `compact` {boolean|integer} Setting this to `false` causes each object key
    to be displayed on a new line. It will also add new lines to text that is
    longer than `breakLength`. If set to a number, the most `n` inner elements
    are united on a single line as long as all properties fit into
    `breakLength`. Short array elements are also grouped together. No
    text will be reduced below 16 characters, no matter the `breakLength` size.
    For more information, see the example below. **Default:** `3`.
  * `sorted` {boolean|Function} If set to `true` or a function, all properties
    of an object, and `Set` and `Map` entries are sorted in the resulting
    string. If set to `true` the [default sort][] is used. If set to a function,
    it is used as a [compare function][].
  * `getters` {boolean|string} If set to `true`, getters are inspected. If set
    to `'get'`, only getters without a corresponding setter are inspected. If
    set to `'set'`, only getters with a corresponding setter are inspected.
    This might cause side effects depending on the getter function.
    **Default:** `false`.
* Returns: {string} The representation of `object`.

The `util.inspect()` method returns a string representation of `object` that is
intended for debugging. The output of `util.inspect` may change at any time
and should not be depended upon programmatically. Additional `options` may be
passed that alter the result.
`util.inspect()` will use the constructor's name and/or `@@toStringTag` to make
an identifiable tag for an inspected value.

```js
class Foo {
  get [Symbol.toStringTag]() {
    return 'bar';
  }
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz);       // '[foo] {}'
```

Circular references point to their anchor by using a reference index:

```js
const { inspect } = require('util');

const obj = {};
obj.a = [obj];
obj.b = {};
obj.b.inner = obj.b;
obj.b.obj = obj;

console.log(inspect(obj));
// <ref *1> {
//   a: [ [Circular *1] ],
//   b: <ref *2> { inner: [Circular *2], obj: [Circular *1] }
// }
```

The following example inspects all properties of the `util` object:

```js
const util = require('util');

console.log(util.inspect(util, { showHidden: true, depth: null }));
```

The following example highlights the effect of the `compact` option:

```js
const util = require('util');

const o = {
  a: [1, 2, [[
    'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do ' +
      'eiusmod tempor incididunt ut labore et dolore magna aliqua.',
    'test',
    'foo']], 4],
  b: new Map([['za', 1], ['zb', 'test']])
};
console.log(util.inspect(o, { compact: true, depth: 5, breakLength: 80 }));

// { a:
//   [ 1,
//     2,
//     [ [ 'Lorem ipsum dolor sit amet, consectetur [...]', // A long line
//           'test',
//           'foo' ] ],
//     4 ],
//   b: Map(2) { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false changes the output to be more reader friendly.
console.log(util.inspect(o, { compact: false, depth: 5, breakLength: 80 }));

// {
//   a: [
//     1,
//     2,
//     [
//       [
//         'Lorem ipsum dolor sit amet, consectetur ' +
//           'adipiscing elit, sed do eiusmod tempor ' +
//           'incididunt ut labore et dolore magna ' +
//           'aliqua.,
//         'test',
//         'foo'
//       ]
//     ],
//     4
//   ],
//   b: Map(2) {
//     'za' => 1,
//     'zb' => 'test'
//   }
// }

// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.
// Reducing the `breakLength` will split the "Lorem ipsum" text in smaller
// chunks.
```

The `showHidden` option allows [`WeakMap`][] and [`WeakSet`][] entries to be
inspected. If there are more entries than `maxArrayLength`, there is no
guarantee which entries are displayed. That means retrieving the same
[`WeakSet`][] entries twice may result in different output. Furthermore, entries
with no remaining strong references may be garbage collected at any time.

```js
const { inspect } = require('util');

const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);

console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }
```

The `sorted` option ensures that an object's property insertion order does not
impact the result of `util.inspect()`.

```js
const { inspect } = require('util');
const assert = require('assert');

const o1 = {
  b: [2, 3, 1],
  a: '`a` comes before `b`',
  c: new Set([2, 3, 1])
};
console.log(inspect(o1, { sorted: true }));
// { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
// { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

const o2 = {
  c: new Set([2, 1, 3]),
  a: '`a` comes before `b`',
  b: [2, 3, 1]
};
assert.strict.equal(
  inspect(o1, { sorted: true }),
  inspect(o2, { sorted: true })
);
```

`util.inspect()` is a synchronous method intended for debugging. Its maximum
output length is approximately 128 MB. Inputs that result in longer output will
be truncated.

### Customizing `util.inspect` colors

<!-- type=misc -->

Color output (if enabled) of `util.inspect` is customizable globally
via the `util.inspect.styles` and `util.inspect.colors` properties.

`util.inspect.styles` is a map associating a style name to a color from
`util.inspect.colors`.

The default styles and associated colors are:

* `bigint`: `yellow`
* `boolean`: `yellow`
* `date`: `magenta`
* `module`: `underline`
* `name`: (no styling)
* `null`: `bold`
* `number`: `yellow`
* `regexp`: `red`
* `special`: `cyan` (e.g., `Proxies`)
* `string`: `green`
* `symbol`: `green`
* `undefined`: `grey`

Color styling uses ANSI control codes that may not be supported on all
terminals. To verify color support use [`tty.hasColors()`][].

Predefined control codes are listed below (grouped as "Modifiers", "Foreground
colors", and "Background colors").

#### Modifiers

Modifier support varies throughout different terminals. They will mostly be
ignored, if not supported.

* `reset` - Resets all (color) modifiers to their defaults
* **bold** - Make text bold
* _italic_ - Make text italic
* <span style="border-bottom: 1px;">underline</span> - Make text underlined
* ~~strikethrough~~ - Puts a horizontal line through the center of the text
  (Alias: `strikeThrough`, `crossedout`, `crossedOut`)
* `hidden` - Prints the text, but makes it invisible (Alias: conceal)
* <span style="opacity: 0.5;">dim</span> - Decreased color intensity (Alias:
  `faint`)
* <span style="border-top: 1px">overlined</span> - Make text overlined
* blink - Hides and shows the text in an interval
* <span style="filter: invert(100%)">inverse</span> - Swap foreground and
  background colors (Alias: `swapcolors`, `swapColors`)
* <span style="border-bottom: 1px double;">doubleunderline</span> - Make text
  double underlined (Alias: `doubleUnderline`)
* <span style="border: 1px">framed</span> - Draw a frame around the text

#### Foreground colors

* `black`
* `red`
* `green`
* `yellow`
* `blue`
* `magenta`
* `cyan`
* `white`
* `gray` (alias: `grey`, `blackBright`)
* `redBright`
* `greenBright`
* `yellowBright`
* `blueBright`
* `magentaBright`
* `cyanBright`
* `whiteBright`

#### Background colors

* `bgBlack`
* `bgRed`
* `bgGreen`
* `bgYellow`
* `bgBlue`
* `bgMagenta`
* `bgCyan`
* `bgWhite`
* `bgGray` (alias: `bgGrey`, `bgBlackBright`)
* `bgRedBright`
* `bgGreenBright`
* `bgYellowBright`
* `bgBlueBright`
* `bgMagentaBright`
* `bgCyanBright`
* `bgWhiteBright`

### Custom inspection functions on Objects

<!-- type=misc -->

Objects may also define their own
[`[util.inspect.custom](depth, opts)`][util.inspect.custom] function,
which `util.inspect()` will invoke and use the result of when inspecting
the object:

```js
const util = require('util');

class Box {
  constructor(value) {
    this.value = value;
  }

  [util.inspect.custom](depth, options) {
    if (depth < 0) {
      return options.stylize('[Box]', 'special');
    }

    const newOptions = Object.assign({}, options, {
      depth: options.depth === null ? null : options.depth - 1
    });

    // Five space padding because that's the size of "Box< ".
    const padding = ' '.repeat(5);
    const inner = util.inspect(this.value, newOptions)
                      .replace(/\n/g, `\n${padding}`);
    return `${options.stylize('Box', 'special')}< ${inner} >`;
  }
}

const box = new Box(true);

util.inspect(box);
// Returns: "Box< true >"
```

Custom `[util.inspect.custom](depth, opts)` functions typically return a string
but may return a value of any type that will be formatted accordingly by
`util.inspect()`.

```js
const util = require('util');

const obj = { foo: 'this will not show up in the inspect() output' };
obj[util.inspect.custom] = (depth) => {
  return { bar: 'baz' };
};

util.inspect(obj);
// Returns: "{ bar: 'baz' }"
```

### `util.inspect.custom`
<!-- YAML
added: v6.6.0
changes:
  - version: v10.12.0
    pr-url: https://github.com/nodejs/node/pull/20857
    description: This is now defined as a shared symbol.
-->

* {symbol} that can be used to declare custom inspect functions.

In addition to being accessible through `util.inspect.custom`, this
symbol is [registered globally][global symbol registry] and can be
accessed in any environment as `Symbol.for('nodejs.util.inspect.custom')`.

```js
const inspect = Symbol.for('nodejs.util.inspect.custom');

class Password {
  constructor(value) {
    this.value = value;
  }

  toString() {
    return 'xxxxxxxx';
  }

  [inspect]() {
    return `Password <${this.toString()}>`;
  }
}

const password = new Password('r0sebud');
console.log(password);
// Prints Password <xxxxxxxx>
```

See [Custom inspection functions on Objects][] for more details.

### `util.inspect.defaultOptions`
<!-- YAML
added: v6.4.0
-->

The `defaultOptions` value allows customization of the default options used by
`util.inspect`. This is useful for functions like `console.log` or
`util.format` which implicitly call into `util.inspect`. It shall be set to an
object containing one or more valid [`util.inspect()`][] options. Setting
option properties directly is also supported.

```js
const util = require('util');
const arr = Array(101).fill(0);

console.log(arr); // Logs the truncated array
util.inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array
```

## `util.isDeepStrictEqual(val1, val2)`
<!-- YAML
added: v9.0.0
-->

* `val1` {any}
* `val2` {any}
* Returns: {boolean}

Returns `true` if there is deep strict equality between `val1` and `val2`.
Otherwise, returns `false`.

See [`assert.deepStrictEqual()`][] for more information about deep strict
equality.

## `util.promisify(original)`
<!-- YAML
added: v8.0.0
-->

* `original` {Function}
* Returns: {Function}

Takes a function following the common error-first callback style, i.e. taking
an `(err, value) => ...` callback as the last argument, and returns a version
that returns promises.

```js
const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
  // Do something with `stats`
}).catch((error) => {
  // Handle the error.
});
```

Or, equivalently using `async function`s:

```js
const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);

async function callStat() {
  const stats = await stat('.');
  console.log(`This directory is owned by ${stats.uid}`);
}
```

If there is an `original[util.promisify.custom]` property present, `promisify`
will return its value, see [Custom promisified functions][].

`promisify()` assumes that `original` is a function taking a callback as its
final argument in all cases. If `original` is not a function, `promisify()`
will throw an error. If `original` is a function but its last argument is not
an error-first callback, it will still be passed an error-first
callback as its last argument.

Using `promisify()` on class methods or other methods that use `this` may not
work as expected unless handled specially:

```js
const util = require('util');

class Foo {
  constructor() {
    this.a = 42;
  }

  bar(callback) {
    callback(null, this.a);
  }
}

const foo = new Foo();

const naiveBar = util.promisify(foo.bar);
// TypeError: Cannot read property 'a' of undefined
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'
```

### Custom promisified functions

Using the `util.promisify.custom` symbol one can override the return value of
[`util.promisify()`][]:

```js
const util = require('util');

function doSomething(foo, callback) {
  // ...
}

doSomething[util.promisify.custom] = (foo) => {
  return getPromiseSomehow();
};

const promisified = util.promisify(doSomething);
console.log(promisified === doSomething[util.promisify.custom]);
// prints 'true'
```

This can be useful for cases where the original function does not follow the
standard format of taking an error-first callback as the last argument.

For example, with a function that takes in
`(foo, onSuccessCallback, onErrorCallback)`:

```js
doSomething[util.promisify.custom] = (foo) => {
  return new Promise((resolve, reject) => {
    doSomething(foo, resolve, reject);
  });
};
```

If `promisify.custom` is defined but is not a function, `promisify()` will
throw an error.

### `util.promisify.custom`
<!-- YAML
added: v8.0.0
changes:
  - version: v13.12.0
    pr-url: https://github.com/nodejs/node/pull/31672
    description: This is now defined as a shared symbol.
-->

* {symbol} that can be used to declare custom promisified variants of functions,
see [Custom promisified functions][].

In addition to being accessible through `util.promisify.custom`, this
symbol is [registered globally][global symbol registry] and can be
accessed in any environment as `Symbol.for('nodejs.util.promisify.custom')`.

For example, with a function that takes in
`(foo, onSuccessCallback, onErrorCallback)`:

```js
const kCustomPromisifiedSymbol = Symbol.for('nodejs.util.promisify.custom');

doSomething[kCustomPromisifiedSymbol] = (foo) => {
  return new Promise((resolve, reject) => {
    doSomething(foo, resolve, reject);
  });
};
```

## Class: `util.TextDecoder`
<!-- YAML
added: v8.3.0
-->

An implementation of the [WHATWG Encoding Standard][] `TextDecoder` API.

```js
const decoder = new TextDecoder('shift_jis');
let string = '';
let buffer;
while (buffer = getNextChunkSomehow()) {
  string += decoder.decode(buffer, { stream: true });
}
string += decoder.decode(); // end-of-stream
```

### WHATWG Supported Encodings

Per the [WHATWG Encoding Standard][], the encodings supported by the
`TextDecoder` API are outlined in the tables below. For each encoding,
one or more aliases may be used.

Different Node.js build configurations support different sets of encodings.
(see [Internationalization][])

#### Encodings Supported by Default (With Full ICU Data)

| Encoding           | Aliases                          |
| -----------------  | -------------------------------- |
| `'ibm866'`         | `'866'`, `'cp866'`, `'csibm866'` |
| `'iso-8859-2'`     | `'csisolatin2'`, `'iso-ir-101'`, `'iso8859-2'`, `'iso88592'`, `'iso_8859-2'`, `'iso_8859-2:1987'`, `'l2'`, `'latin2'`  |
| `'iso-8859-3'`     | `'csisolatin3'`, `'iso-ir-109'`, `'iso8859-3'`, `'iso88593'`, `'iso_8859-3'`, `'iso_8859-3:1988'`, `'l3'`, `'latin3'`  |
| `'iso-8859-4'`     | `'csisolatin4'`, `'iso-ir-110'`, `'iso8859-4'`, `'iso88594'`, `'iso_8859-4'`, `'iso_8859-4:1988'`, `'l4'`, `'latin4'`  |
| `'iso-8859-5'`     | `'csisolatincyrillic'`, `'cyrillic'`, `'iso-ir-144'`, `'iso8859-5'`, `'iso88595'`, `'iso_8859-5'`, `'iso_8859-5:1988'` |
| `'iso-8859-6'`     | `'arabic'`, `'asmo-708'`, `'csiso88596e'`, `'csiso88596i'`, `'csisolatinarabic'`, `'ecma-114'`, `'iso-8859-6-e'`, `'iso-8859-6-i'`, `'iso-ir-127'`, `'iso8859-6'`, `'iso88596'`, `'iso_8859-6'`, `'iso_8859-6:1987'` |
| `'iso-8859-7'`     | `'csisolatingreek'`, `'ecma-118'`, `'elot_928'`, `'greek'`, `'greek8'`, `'iso-ir-126'`, `'iso8859-7'`, `'iso88597'`, `'iso_8859-7'`, `'iso_8859-7:1987'`, `'sun_eu_greek'` |
| `'iso-8859-8'`     | `'csiso88598e'`, `'csisolatinhebrew'`, `'hebrew'`, `'iso-8859-8-e'`, `'iso-ir-138'`, `'iso8859-8'`, `'iso88598'`, `'iso_8859-8'`, `'iso_8859-8:1988'`, `'visual'` |
| `'iso-8859-8-i'`   | `'csiso88598i'`, `'logical'` |
| `'iso-8859-10'`    | `'csisolatin6'`, `'iso-ir-157'`, `'iso8859-10'`, `'iso885910'`, `'l6'`, `'latin6'` |
| `'iso-8859-13'`    | `'iso8859-13'`, `'iso885913'` |
| `'iso-8859-14'`    | `'iso8859-14'`, `'iso885914'` |
| `'iso-8859-15'`    | `'csisolatin9'`, `'iso8859-15'`, `'iso885915'`, `'iso_8859-15'`, `'l9'` |
| `'koi8-r'`         | `'cskoi8r'`, `'koi'`, `'koi8'`, `'koi8_r'` |
| `'koi8-u'`         | `'koi8-ru'` |
| `'macintosh'`      | `'csmacintosh'`, `'mac'`, `'x-mac-roman'` |
| `'windows-874'`    | `'dos-874'`, `'iso-8859-11'`, `'iso8859-11'`, `'iso885911'`, `'tis-620'` |
| `'windows-1250'`   | `'cp1250'`, `'x-cp1250'` |
| `'windows-1251'`   | `'cp1251'`, `'x-cp1251'` |
| `'windows-1252'`   | `'ansi_x3.4-1968'`, `'ascii'`, `'cp1252'`, `'cp819'`, `'csisolatin1'`, `'ibm819'`, `'iso-8859-1'`, `'iso-ir-100'`, `'iso8859-1'`, `'iso88591'`, `'iso_8859-1'`, `'iso_8859-1:1987'`, `'l1'`, `'latin1'`, `'us-ascii'`, `'x-cp1252'` |
| `'windows-1253'`   | `'cp1253'`, `'x-cp1253'` |
| `'windows-1254'`   | `'cp1254'`, `'csisolatin5'`, `'iso-8859-9'`, `'iso-ir-148'`, `'iso8859-9'`, `'iso88599'`, `'iso_8859-9'`, `'iso_8859-9:1989'`, `'l5'`, `'latin5'`, `'x-cp1254'` |
| `'windows-1255'`   | `'cp1255'`, `'x-cp1255'` |
| `'windows-1256'`   | `'cp1256'`, `'x-cp1256'` |
| `'windows-1257'`   | `'cp1257'`, `'x-cp1257'` |
| `'windows-1258'`   | `'cp1258'`, `'x-cp1258'` |
| `'x-mac-cyrillic'` | `'x-mac-ukrainian'` |
| `'gbk'`            | `'chinese'`, `'csgb2312'`, `'csiso58gb231280'`, `'gb2312'`, `'gb_2312'`, `'gb_2312-80'`, `'iso-ir-58'`, `'x-gbk'` |
| `'gb18030'`        | |
| `'big5'`           | `'big5-hkscs'`, `'cn-big5'`, `'csbig5'`, `'x-x-big5'` |
| `'euc-jp'`         | `'cseucpkdfmtjapanese'`, `'x-euc-jp'` |
| `'iso-2022-jp'`    | `'csiso2022jp'` |
| `'shift_jis'`      | `'csshiftjis'`, `'ms932'`, `'ms_kanji'`, `'shift-jis'`, `'sjis'`, `'windows-31j'`, `'x-sjis'` |
| `'euc-kr'`         | `'cseuckr'`, `'csksc56011987'`, `'iso-ir-149'`, `'korean'`, `'ks_c_5601-1987'`, `'ks_c_5601-1989'`, `'ksc5601'`, `'ksc_5601'`, `'windows-949'` |

#### Encodings Supported when Node.js is built with the `small-icu` option

| Encoding     | Aliases                           |
| -----------  | --------------------------------- |
| `'utf-8'`    | `'unicode-1-1-utf-8'`, `'utf8'`   |
| `'utf-16le'` | `'utf-16'`                        |
| `'utf-16be'` |                                   |

#### Encodings Supported when ICU is disabled

| Encoding     | Aliases                           |
| -----------  | --------------------------------- |
| `'utf-8'`    | `'unicode-1-1-utf-8'`, `'utf8'`   |
| `'utf-16le'` | `'utf-16'`                        |

The `'iso-8859-16'` encoding listed in the [WHATWG Encoding Standard][]
is not supported.

### `new TextDecoder([encoding[, options]])`
<!-- YAML
added: v8.3.0
changes:
  - version: v11.0.0
    pr-url: v11.0.0
    description: The class is now available on the global object.
-->

* `encoding` {string} Identifies the `encoding` that this `TextDecoder` instance
  supports. **Default:** `'utf-8'`.
* `options` {Object}
  * `fatal` {boolean} `true` if decoding failures are fatal.
    This option is not supported when ICU is disabled
    (see [Internationalization][]). **Default:** `false`.
  * `ignoreBOM` {boolean} When `true`, the `TextDecoder` will include the byte
     order mark in the decoded result. When `false`, the byte order mark will
     be removed from the output. This option is only used when `encoding` is
     `'utf-8'`, `'utf-16be'` or `'utf-16le'`. **Default:** `false`.

Creates an new `TextDecoder` instance. The `encoding` may specify one of the
supported encodings or an alias.

The `TextDecoder` class is also available on the global object.

### `textDecoder.decode([input[, options]])`

* `input` {ArrayBuffer|DataView|TypedArray} An `ArrayBuffer`, `DataView` or
  `TypedArray` instance containing the encoded data.
* `options` {Object}
  * `stream` {boolean} `true` if additional chunks of data are expected.
    **Default:** `false`.
* Returns: {string}

Decodes the `input` and returns a string. If `options.stream` is `true`, any
incomplete byte sequences occurring at the end of the `input` are buffered
internally and emitted after the next call to `textDecoder.decode()`.

If `textDecoder.fatal` is `true`, decoding errors that occur will result in a
`TypeError` being thrown.

### `textDecoder.encoding`

* {string}

The encoding supported by the `TextDecoder` instance.

### `textDecoder.fatal`

* {boolean}

The value will be `true` if decoding errors result in a `TypeError` being
thrown.

### `textDecoder.ignoreBOM`

* {boolean}

The value will be `true` if the decoding result will include the byte order
mark.

## Class: `util.TextEncoder`
<!-- YAML
added: v8.3.0
changes:
  - version: v11.0.0
    pr-url: v11.0.0
    description: The class is now available on the global object.
-->

An implementation of the [WHATWG Encoding Standard][] `TextEncoder` API. All
instances of `TextEncoder` only support UTF-8 encoding.

```js
const encoder = new TextEncoder();
const uint8array = encoder.encode('this is some data');
```

The `TextEncoder` class is also available on the global object.

### `textEncoder.encode([input])`

* `input` {string} The text to encode. **Default:** an empty string.
* Returns: {Uint8Array}

UTF-8 encodes the `input` string and returns a `Uint8Array` containing the
encoded bytes.

### `textEncoder.encodeInto(src, dest)`

* `src` {string} The text to encode.
* `dest` {Uint8Array} The array to hold the encode result.
* Returns: {Object}
  * `read` {number} The read Unicode code units of src.
  * `written` {number} The written UTF-8 bytes of dest.

UTF-8 encodes the `src` string to the `dest` Uint8Array and returns an object
containing the read Unicode code units and written UTF-8 bytes.

```js
const encoder = new TextEncoder();
const src = 'this is some data';
const dest = new Uint8Array(10);
const { read, written } = encoder.encodeInto(src, dest);
```

### `textEncoder.encoding`

* {string}

The encoding supported by the `TextEncoder` instance. Always set to `'utf-8'`.

## `util.types`
<!-- YAML
added: v10.0.0
-->

`util.types` provides type checks for different kinds of built-in objects.
Unlike `instanceof` or `Object.prototype.toString.call(value)`, these checks do
not inspect properties of the object that are accessible from JavaScript (like
their prototype), and usually have the overhead of calling into C++.

The result generally does not make any guarantees about what kinds of
properties or behavior a value exposes in JavaScript. They are primarily
useful for addon developers who prefer to do type checking in JavaScript.

### `util.types.isAnyArrayBuffer(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`ArrayBuffer`][] or
[`SharedArrayBuffer`][] instance.

See also [`util.types.isArrayBuffer()`][] and
[`util.types.isSharedArrayBuffer()`][].

```js
util.types.isAnyArrayBuffer(new ArrayBuffer());  // Returns true
util.types.isAnyArrayBuffer(new SharedArrayBuffer());  // Returns true
```

### `util.types.isArgumentsObject(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is an `arguments` object.

<!-- eslint-disable prefer-rest-params -->
```js
function foo() {
  util.types.isArgumentsObject(arguments);  // Returns true
}
```

### `util.types.isArrayBuffer(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`ArrayBuffer`][] instance.
This does *not* include [`SharedArrayBuffer`][] instances. Usually, it is
desirable to test for both; See [`util.types.isAnyArrayBuffer()`][] for that.

```js
util.types.isArrayBuffer(new ArrayBuffer());  // Returns true
util.types.isArrayBuffer(new SharedArrayBuffer());  // Returns false
```

### `util.types.isAsyncFunction(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is an [async function][].
This only reports back what the JavaScript engine is seeing;
in particular, the return value may not match the original source code if
a transpilation tool was used.

```js
util.types.isAsyncFunction(function foo() {});  // Returns false
util.types.isAsyncFunction(async function foo() {});  // Returns true
```

### `util.types.isBigInt64Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a `BigInt64Array` instance.

```js
util.types.isBigInt64Array(new BigInt64Array());   // Returns true
util.types.isBigInt64Array(new BigUint64Array());  // Returns false
```

### `util.types.isBigUint64Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a `BigUint64Array` instance.

```js
util.types.isBigUint64Array(new BigInt64Array());   // Returns false
util.types.isBigUint64Array(new BigUint64Array());  // Returns true
```

### `util.types.isBooleanObject(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a boolean object, e.g. created
by `new Boolean()`.

```js
util.types.isBooleanObject(false);  // Returns false
util.types.isBooleanObject(true);   // Returns false
util.types.isBooleanObject(new Boolean(false)); // Returns true
util.types.isBooleanObject(new Boolean(true));  // Returns true
util.types.isBooleanObject(Boolean(false)); // Returns false
util.types.isBooleanObject(Boolean(true));  // Returns false
```

### `util.types.isBoxedPrimitive(value)`
<!-- YAML
added: v10.11.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is any boxed primitive object, e.g. created
by `new Boolean()`, `new String()` or `Object(Symbol())`.

For example:

```js
util.types.isBoxedPrimitive(false); // Returns false
util.types.isBoxedPrimitive(new Boolean(false)); // Returns true
util.types.isBoxedPrimitive(Symbol('foo')); // Returns false
util.types.isBoxedPrimitive(Object(Symbol('foo'))); // Returns true
util.types.isBoxedPrimitive(Object(BigInt(5))); // Returns true
```

### `util.types.isDataView(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`DataView`][] instance.

```js
const ab = new ArrayBuffer(20);
util.types.isDataView(new DataView(ab));  // Returns true
util.types.isDataView(new Float64Array());  // Returns false
```

See also [`ArrayBuffer.isView()`][].

### `util.types.isDate(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Date`][] instance.

```js
util.types.isDate(new Date());  // Returns true
```

### `util.types.isExternal(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a native `External` value.

A native `External` value is a special type of object that contains a
raw C++ pointer (`void*`) for access from native code, and has no other
properties. Such objects are created either by Node.js internals or native
addons. In JavaScript, they are [frozen][`Object.freeze()`] objects with a
`null` prototype.

```c
#include <js_native_api.h>
#include <stdlib.h>
napi_value result;
static napi_value MyNapi(napi_env env, napi_callback_info info) {
  int* raw = (int*) malloc(1024);
  napi_status status = napi_create_external(env, (void*) raw, NULL, NULL, &result);
  if (status != napi_ok) {
    napi_throw_error(env, NULL, "napi_create_external failed");
    return NULL;
  }
  return result;
}
...
DECLARE_NAPI_PROPERTY("myNapi", MyNapi)
...
```

```js
const native = require('napi_addon.node');
const data = native.myNapi();
util.types.isExternal(data); // returns true
util.types.isExternal(0); // returns false
util.types.isExternal(new String('foo')); // returns false
```

For further information on `napi_create_external`, refer to
[`napi_create_external()`][].

### `util.types.isFloat32Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Float32Array`][] instance.

```js
util.types.isFloat32Array(new ArrayBuffer());  // Returns false
util.types.isFloat32Array(new Float32Array());  // Returns true
util.types.isFloat32Array(new Float64Array());  // Returns false
```

### `util.types.isFloat64Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Float64Array`][] instance.

```js
util.types.isFloat64Array(new ArrayBuffer());  // Returns false
util.types.isFloat64Array(new Uint8Array());  // Returns false
util.types.isFloat64Array(new Float64Array());  // Returns true
```

### `util.types.isGeneratorFunction(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a generator function.
This only reports back what the JavaScript engine is seeing;
in particular, the return value may not match the original source code if
a transpilation tool was used.

```js
util.types.isGeneratorFunction(function foo() {});  // Returns false
util.types.isGeneratorFunction(function* foo() {});  // Returns true
```

### `util.types.isGeneratorObject(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a generator object as returned from a
built-in generator function.
This only reports back what the JavaScript engine is seeing;
in particular, the return value may not match the original source code if
a transpilation tool was used.

```js
function* foo() {}
const generator = foo();
util.types.isGeneratorObject(generator);  // Returns true
```

### `util.types.isInt8Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Int8Array`][] instance.

```js
util.types.isInt8Array(new ArrayBuffer());  // Returns false
util.types.isInt8Array(new Int8Array());  // Returns true
util.types.isInt8Array(new Float64Array());  // Returns false
```

### `util.types.isInt16Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Int16Array`][] instance.

```js
util.types.isInt16Array(new ArrayBuffer());  // Returns false
util.types.isInt16Array(new Int16Array());  // Returns true
util.types.isInt16Array(new Float64Array());  // Returns false
```

### `util.types.isInt32Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Int32Array`][] instance.

```js
util.types.isInt32Array(new ArrayBuffer());  // Returns false
util.types.isInt32Array(new Int32Array());  // Returns true
util.types.isInt32Array(new Float64Array());  // Returns false
```

### `util.types.isMap(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Map`][] instance.

```js
util.types.isMap(new Map());  // Returns true
```

### `util.types.isMapIterator(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is an iterator returned for a built-in
[`Map`][] instance.

```js
const map = new Map();
util.types.isMapIterator(map.keys());  // Returns true
util.types.isMapIterator(map.values());  // Returns true
util.types.isMapIterator(map.entries());  // Returns true
util.types.isMapIterator(map[Symbol.iterator]());  // Returns true
```

### `util.types.isModuleNamespaceObject(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is an instance of a [Module Namespace Object][].

<!-- eslint-skip -->
```js
import * as ns from './a.js';

util.types.isModuleNamespaceObject(ns);  // Returns true
```

### `util.types.isNativeError(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is an instance of a built-in [`Error`][] type.

```js
util.types.isNativeError(new Error());  // Returns true
util.types.isNativeError(new TypeError());  // Returns true
util.types.isNativeError(new RangeError());  // Returns true
```

### `util.types.isNumberObject(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a number object, e.g. created
by `new Number()`.

```js
util.types.isNumberObject(0);  // Returns false
util.types.isNumberObject(new Number(0));   // Returns true
```

### `util.types.isPromise(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Promise`][].

```js
util.types.isPromise(Promise.resolve(42));  // Returns true
```

### `util.types.isProxy(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a [`Proxy`][] instance.

```js
const target = {};
const proxy = new Proxy(target, {});
util.types.isProxy(target);  // Returns false
util.types.isProxy(proxy);  // Returns true
```

### `util.types.isRegExp(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a regular expression object.

```js
util.types.isRegExp(/abc/);  // Returns true
util.types.isRegExp(new RegExp('abc'));  // Returns true
```

### `util.types.isSet(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Set`][] instance.

```js
util.types.isSet(new Set());  // Returns true
```

### `util.types.isSetIterator(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is an iterator returned for a built-in
[`Set`][] instance.

```js
const set = new Set();
util.types.isSetIterator(set.keys());  // Returns true
util.types.isSetIterator(set.values());  // Returns true
util.types.isSetIterator(set.entries());  // Returns true
util.types.isSetIterator(set[Symbol.iterator]());  // Returns true
```

### `util.types.isSharedArrayBuffer(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`SharedArrayBuffer`][] instance.
This does *not* include [`ArrayBuffer`][] instances. Usually, it is
desirable to test for both; See [`util.types.isAnyArrayBuffer()`][] for that.

```js
util.types.isSharedArrayBuffer(new ArrayBuffer());  // Returns false
util.types.isSharedArrayBuffer(new SharedArrayBuffer());  // Returns true
```

### `util.types.isStringObject(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a string object, e.g. created
by `new String()`.

```js
util.types.isStringObject('foo');  // Returns false
util.types.isStringObject(new String('foo'));   // Returns true
```

### `util.types.isSymbolObject(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a symbol object, created
by calling `Object()` on a `Symbol` primitive.

```js
const symbol = Symbol('foo');
util.types.isSymbolObject(symbol);  // Returns false
util.types.isSymbolObject(Object(symbol));   // Returns true
```

### `util.types.isTypedArray(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`TypedArray`][] instance.

```js
util.types.isTypedArray(new ArrayBuffer());  // Returns false
util.types.isTypedArray(new Uint8Array());  // Returns true
util.types.isTypedArray(new Float64Array());  // Returns true
```

See also [`ArrayBuffer.isView()`][].

### `util.types.isUint8Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Uint8Array`][] instance.

```js
util.types.isUint8Array(new ArrayBuffer());  // Returns false
util.types.isUint8Array(new Uint8Array());  // Returns true
util.types.isUint8Array(new Float64Array());  // Returns false
```

### `util.types.isUint8ClampedArray(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Uint8ClampedArray`][] instance.

```js
util.types.isUint8ClampedArray(new ArrayBuffer());  // Returns false
util.types.isUint8ClampedArray(new Uint8ClampedArray());  // Returns true
util.types.isUint8ClampedArray(new Float64Array());  // Returns false
```

### `util.types.isUint16Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Uint16Array`][] instance.

```js
util.types.isUint16Array(new ArrayBuffer());  // Returns false
util.types.isUint16Array(new Uint16Array());  // Returns true
util.types.isUint16Array(new Float64Array());  // Returns false
```

### `util.types.isUint32Array(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`Uint32Array`][] instance.

```js
util.types.isUint32Array(new ArrayBuffer());  // Returns false
util.types.isUint32Array(new Uint32Array());  // Returns true
util.types.isUint32Array(new Float64Array());  // Returns false
```

### `util.types.isWeakMap(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`WeakMap`][] instance.

```js
util.types.isWeakMap(new WeakMap());  // Returns true
```

### `util.types.isWeakSet(value)`
<!-- YAML
added: v10.0.0
-->

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`WeakSet`][] instance.

```js
util.types.isWeakSet(new WeakSet());  // Returns true
```

### `util.types.isWebAssemblyCompiledModule(value)`
<!-- YAML
added: v10.0.0
deprecated: REPLACEME
-->

> Stability: 0 - Deprecated: Use `value instanceof WebAssembly.Module` instead.

* `value` {any}
* Returns: {boolean}

Returns `true` if the value is a built-in [`WebAssembly.Module`][] instance.

```js
const module = new WebAssembly.Module(wasmBuffer);
util.types.isWebAssemblyCompiledModule(module);  // Returns true
```

## Deprecated APIs

The following APIs are deprecated and should no longer be used. Existing
applications and modules should be updated to find alternative approaches.

### `util._extend(target, source)`
<!-- YAML
added: v0.7.5
deprecated: v6.0.0
-->

> Stability: 0 - Deprecated: Use [`Object.assign()`][] instead.

* `target` {Object}
* `source` {Object}

The `util._extend()` method was never intended to be used outside of internal
Node.js modules. The community found and used it anyway.

It is deprecated and should not be used in new code. JavaScript comes with very
similar built-in functionality through [`Object.assign()`][].

### `util.isArray(object)`
<!-- YAML
added: v0.6.0
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use [`Array.isArray()`][] instead.

* `object` {any}
* Returns: {boolean}

Alias for [`Array.isArray()`][].

Returns `true` if the given `object` is an `Array`. Otherwise, returns `false`.

```js
const util = require('util');

util.isArray([]);
// Returns: true
util.isArray(new Array());
// Returns: true
util.isArray({});
// Returns: false
```

### `util.isBoolean(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use `typeof value === 'boolean'` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `Boolean`. Otherwise, returns `false`.

```js
const util = require('util');

util.isBoolean(1);
// Returns: false
util.isBoolean(0);
// Returns: false
util.isBoolean(false);
// Returns: true
```

### `util.isBuffer(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use [`Buffer.isBuffer()`][] instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `Buffer`. Otherwise, returns `false`.

```js
const util = require('util');

util.isBuffer({ length: 0 });
// Returns: false
util.isBuffer([]);
// Returns: false
util.isBuffer(Buffer.from('hello world'));
// Returns: true
```

### `util.isDate(object)`
<!-- YAML
added: v0.6.0
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use [`util.types.isDate()`][] instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `Date`. Otherwise, returns `false`.

```js
const util = require('util');

util.isDate(new Date());
// Returns: true
util.isDate(Date());
// false (without 'new' returns a String)
util.isDate({});
// Returns: false
```

### `util.isError(object)`
<!-- YAML
added: v0.6.0
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use [`util.types.isNativeError()`][] instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is an [`Error`][]. Otherwise, returns
`false`.

```js
const util = require('util');

util.isError(new Error());
// Returns: true
util.isError(new TypeError());
// Returns: true
util.isError({ name: 'Error', message: 'an error occurred' });
// Returns: false
```

This method relies on `Object.prototype.toString()` behavior. It is
possible to obtain an incorrect result when the `object` argument manipulates
`@@toStringTag`.

```js
const util = require('util');
const obj = { name: 'Error', message: 'an error occurred' };

util.isError(obj);
// Returns: false
obj[Symbol.toStringTag] = 'Error';
util.isError(obj);
// Returns: true
```

### `util.isFunction(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use `typeof value === 'function'` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `Function`. Otherwise, returns
`false`.

```js
const util = require('util');

function Foo() {}
const Bar = () => {};

util.isFunction({});
// Returns: false
util.isFunction(Foo);
// Returns: true
util.isFunction(Bar);
// Returns: true
```

### `util.isNull(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use `value === null` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is strictly `null`. Otherwise, returns
`false`.

```js
const util = require('util');

util.isNull(0);
// Returns: false
util.isNull(undefined);
// Returns: false
util.isNull(null);
// Returns: true
```

### `util.isNullOrUndefined(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use
> `value === undefined || value === null` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is `null` or `undefined`. Otherwise,
returns `false`.

```js
const util = require('util');

util.isNullOrUndefined(0);
// Returns: false
util.isNullOrUndefined(undefined);
// Returns: true
util.isNullOrUndefined(null);
// Returns: true
```

### `util.isNumber(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use `typeof value === 'number'` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `Number`. Otherwise, returns `false`.

```js
const util = require('util');

util.isNumber(false);
// Returns: false
util.isNumber(Infinity);
// Returns: true
util.isNumber(0);
// Returns: true
util.isNumber(NaN);
// Returns: true
```

### `util.isObject(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated:
> Use `value !== null && typeof value === 'object'` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is strictly an `Object` **and** not a
`Function` (even though functions are objects in JavaScript).
Otherwise, returns `false`.

```js
const util = require('util');

util.isObject(5);
// Returns: false
util.isObject(null);
// Returns: false
util.isObject({});
// Returns: true
util.isObject(() => {});
// Returns: false
```

### `util.isPrimitive(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use
> `(typeof value !== 'object' && typeof value !== 'function') || value === null`
> instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a primitive type. Otherwise, returns
`false`.

```js
const util = require('util');

util.isPrimitive(5);
// Returns: true
util.isPrimitive('foo');
// Returns: true
util.isPrimitive(false);
// Returns: true
util.isPrimitive(null);
// Returns: true
util.isPrimitive(undefined);
// Returns: true
util.isPrimitive({});
// Returns: false
util.isPrimitive(() => {});
// Returns: false
util.isPrimitive(/^$/);
// Returns: false
util.isPrimitive(new Date());
// Returns: false
```

### `util.isRegExp(object)`
<!-- YAML
added: v0.6.0
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `RegExp`. Otherwise, returns `false`.

```js
const util = require('util');

util.isRegExp(/some regexp/);
// Returns: true
util.isRegExp(new RegExp('another regexp'));
// Returns: true
util.isRegExp({});
// Returns: false
```

### `util.isString(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use `typeof value === 'string'` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `string`. Otherwise, returns `false`.

```js
const util = require('util');

util.isString('');
// Returns: true
util.isString('foo');
// Returns: true
util.isString(String('foo'));
// Returns: true
util.isString(5);
// Returns: false
```

### `util.isSymbol(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use `typeof value === 'symbol'` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is a `Symbol`. Otherwise, returns `false`.

```js
const util = require('util');

util.isSymbol(5);
// Returns: false
util.isSymbol('foo');
// Returns: false
util.isSymbol(Symbol('foo'));
// Returns: true
```

### `util.isUndefined(object)`
<!-- YAML
added: v0.11.5
deprecated: v4.0.0
-->

> Stability: 0 - Deprecated: Use `value === undefined` instead.

* `object` {any}
* Returns: {boolean}

Returns `true` if the given `object` is `undefined`. Otherwise, returns `false`.

```js
const util = require('util');

const foo = undefined;
util.isUndefined(5);
// Returns: false
util.isUndefined(foo);
// Returns: true
util.isUndefined(null);
// Returns: false
```

### `util.log(string)`
<!-- YAML
added: v0.3.0
deprecated: v6.0.0
-->

> Stability: 0 - Deprecated: Use a third party module instead.

* `string` {string}

The `util.log()` method prints the given `string` to `stdout` with an included
timestamp.

```js
const util = require('util');

util.log('Timestamped message.');
```

[`'uncaughtException'`]: process.html#process_event_uncaughtexception
[`'warning'`]: process.html#process_event_warning
[`Array.isArray()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray
[`ArrayBuffer.isView()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer/isView
[`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer
[`Buffer.isBuffer()`]: buffer.html#buffer_class_method_buffer_isbuffer_obj
[`DataView`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView
[`Date`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date
[`Error`]: errors.html#errors_class_error
[`Float32Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float32Array
[`Float64Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float64Array
[`Int16Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int16Array
[`Int32Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int32Array
[`Int8Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int8Array
[`Map`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map
[`Object.assign()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
[`Object.freeze()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze
[`Promise`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
[`Proxy`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy
[`Set`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
[`SharedArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer
[`TypedArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
[`Uint16Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint16Array
[`Uint32Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint32Array
[`Uint8Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array
[`Uint8ClampedArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8ClampedArray
[`WeakMap`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap
[`WeakSet`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet
[`WebAssembly.Module`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Module
[`assert.deepStrictEqual()`]: assert.html#assert_assert_deepstrictequal_actual_expected_message
[`console.error()`]: console.html#console_console_error_data_args
[`target` and `handler`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy#Terminology
[`tty.hasColors()`]: tty.html#tty_writestream_hascolors_count_env
[`util.format()`]: #util_util_format_format_args
[`util.inspect()`]: #util_util_inspect_object_options
[`util.promisify()`]: #util_util_promisify_original
[`util.types.isAnyArrayBuffer()`]: #util_util_types_isanyarraybuffer_value
[`util.types.isArrayBuffer()`]: #util_util_types_isarraybuffer_value
[`util.types.isDate()`]: #util_util_types_isdate_value
[`util.types.isNativeError()`]: #util_util_types_isnativeerror_value
[`util.types.isSharedArrayBuffer()`]: #util_util_types_issharedarraybuffer_value
[Common System Errors]: errors.html#errors_common_system_errors
[Custom inspection functions on Objects]: #util_custom_inspection_functions_on_objects
[Custom promisified functions]: #util_custom_promisified_functions
[Customizing `util.inspect` colors]: #util_customizing_util_inspect_colors
[Internationalization]: intl.html
[Module Namespace Object]: https://tc39.github.io/ecma262/#sec-module-namespace-exotic-objects
[WHATWG Encoding Standard]: https://encoding.spec.whatwg.org/
[async function]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
[compare function]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort#Parameters
[constructor]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor
[default sort]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
[global symbol registry]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/for
[list of deprecated APIS]: deprecations.html#deprecations_list_of_deprecated_apis
[`napi_create_external()`]: n-api.html#n_api_napi_create_external
[semantically incompatible]: https://github.com/nodejs/node/issues/4179
[util.inspect.custom]: #util_util_inspect_custom