nodejs/doc/api/util.md

Util

Stability: 2 - Stable

The util module is primarily designed to support the needs of Node.js' own internal APIs. However, many of the utilities are useful for application and module developers as well. It can be accessed using:

const util = require('util');

util.callbackify(original)

• 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.

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:

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.

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)

• 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.

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:

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:

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:

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])

• 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.

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.

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])

• format {string} A printf-like format string.

The util.format() method returns a formatted string using the first argument as a printf-like format.

The first argument is a string containing zero or more placeholder tokens. Each placeholder token is replaced with the converted value from the corresponding argument. Supported placeholders are:

• %s - String.
• %d - Number (integer or floating point value).
• %i - Integer.
• %f - Floating point value.
• %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.
• %% - single percent sign ('%'). This does not consume an argument.
• Returns: {string} The formatted string

If the placeholder does not have a corresponding argument, the placeholder is not replaced.

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

If there are more arguments passed to the util.format() method than the number of placeholders, the extra arguments are coerced into strings then concatenated to the returned string, each delimited by a space. Excessive arguments whose typeof is 'object' or 'symbol' (except null) will be transformed by util.inspect().

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

If the first argument is not a string then util.format() returns a string that is the concatenation of all arguments separated by spaces. Each argument is converted to a string using util.inspect().

util.format(1, 2, 3); // '1 2 3'

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

util.format('%% %s'); // '%% %s'

Please note that util.format() is a synchronous method that is mainly 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])

• 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().

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)

• 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.

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

util.inherits(constructor, superConstructor)

• 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.

As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

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:

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])

• object {any} Any JavaScript primitive or Object.
• options {Object}
  • showHidden {boolean} If true, the object's non-enumerable symbols and properties will be included in the formatted result as well as WeakMap and WeakSet entries. Default: false.
  • depth {number} Specifies the number of times to recurse while formatting the object. This is useful for inspecting large complicated objects. To make it recurse up to the maximum call stack size pass Infinity or null. Default: 2.
  • colors {boolean} If true, the output will be styled with ANSI color codes. Colors are customizable, see Customizing util.inspect colors. Default: false.
  • customInspect {boolean} If false, then [util.inspect.custom](depth, opts) functions will not be called. Default: true.
  • showProxy {boolean} If true, then objects and functions that are Proxy objects will be introspected to show their target and handler objects. Default: false.
    • maxArrayLength {number} 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 {number} The length at which an object's keys are split across multiple lines. Set to Infinity to format an object as a single line. Default: 60 for legacy compatibility.
  • compact {boolean} Setting this to false changes the default indentation to use a line break for each object key instead of lining up multiple properties in one line. It will also break text that is above the breakLength size into smaller and better readable chunks and indents objects the same as arrays. Note that no text will be reduced below 16 characters, no matter the breakLength size. For more information, see the example below. Default: true.
• Returns: {string} The representation of passed 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 certain aspects of the formatted string. util.inspect() will use the constructor's name and/or @@toStringTag to make an identifiable tag for an inspected value.

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] {}'

The following example inspects all properties of the util object:

const util = require('util');

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

The following example highlights the difference with the compact option:

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 }));

// This will print

// { a:
//   [ 1,
//     2,
//     [ [ 'Lorem ipsum dolor sit amet, consectetur [...]', // A long line
//           'test',
//           'foo' ] ],
//     4 ],
//   b: Map { '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 {
//     '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.

Using the showHidden option allows to inspect WeakMap and WeakSet entries. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same WeakSet entries twice might actually result in a different output. Besides this any item might be collected at any point of time by the garbage collector if there is no strong reference left to that object. Therefore there is no guarantee to get a reliable output.

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 } }

Please note that util.inspect() is a synchronous method that is mainly 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.

Customizing util.inspect colors

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:

• number - yellow
• boolean - yellow
• string - green
• date - magenta
• regexp - red
• null - bold
• undefined - grey
• special - cyan (only applied to functions at this time)
• name - (no styling)

The predefined color codes are: white, grey, black, blue, cyan, green, magenta, red and yellow. There are also bold, italic, underline and inverse codes.

Color styling uses ANSI control codes that may not be supported on all terminals.

Custom inspection functions on Objects

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

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().

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

• {symbol} that can be used to declare custom inspect functions, see Custom inspection functions on Objects.

util.inspect.defaultOptions

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.

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)

• 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)

• 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.

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 functions:

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.

Custom promisified functions

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

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):

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

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

Class: util.TextDecoder

An implementation of the WHATWG Encoding Standard TextDecoder API.

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. While a very basic set of encodings is supported even on Node.js builds without ICU enabled, support for some encodings is provided only when Node.js is built with ICU and using the full ICU data (see Internationalization).

Encodings Supported Without ICU

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

Encodings Supported by Default (With ICU)

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

Encodings Requiring 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'

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

new TextDecoder([encoding[, options]])

• 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 only supported when ICU is enabled (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.

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

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

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

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.encoding

• {string}

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

util.types

util.types provides a number of 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)

• 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().

For example:

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

util.types.isArgumentsObject(value)

• value {any}
• Returns: {boolean}

Returns true if the value is an arguments object.

For example:

function foo() {
  util.types.isArgumentsObject(arguments);  // Returns true
}

util.types.isArrayBuffer(value)

• 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.

For example:

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

util.types.isAsyncFunction(value)

• value {any}
• Returns: {boolean}

Returns true if the value is an async function. Note that 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.

For example:

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

util.types.isBigInt64Array(value)

• value {any}
• Returns: {boolean}

Returns true if the value is a BigInt64Array instance. The --harmony-bigint command line flag is required in order to use the BigInt64Array type, but it is not required in order to use isBigInt64Array().

For example:

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

util.types.isBigUint64Array(value)

• value {any}
• Returns: {boolean}

Returns true if the value is a BigUint64Array instance. The --harmony-bigint command line flag is required in order to use the BigUint64Array type, but it is not required in order to use isBigUint64Array().

For example:

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

util.types.isBooleanObject(value)

• value {any}
• Returns: {boolean}

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

For example:

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.isDataView(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isExternal(value)

• value {any}
• Returns: {boolean}

Returns true if the value is a native External value.

util.types.isFloat32Array(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

Returns true if the value is a generator function. Note that 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.

For example:

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

util.types.isGeneratorObject(value)

• value {any}
• Returns: {boolean}

Returns true if the value is a generator object as returned from a built-in generator function. Note that 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.

For example:

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

util.types.isInt8Array(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isMapIterator(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

import * as ns from './a.js';

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

util.types.isNativeError(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isPromise(value)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isProxy(value)

• value {any}
• Returns: {boolean}

Returns true if the value is a Proxy instance.

For example:

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

util.types.isRegExp(value)

• value {any}
• Returns: {boolean}

Returns true if the value is a regular expression object.

For example:

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

util.types.isSet(value)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isSetIterator(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• 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.

For example:

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

util.types.isStringObject(value)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isSymbolObject(value)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isTypedArray(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isWeakSet(value)

• value {any}
• Returns: {boolean}

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

For example:

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

util.types.isWebAssemblyCompiledModule(value)

• value {any}
• Returns: {boolean}

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

For example:

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)

• target {Object}
• source {Object}

Stability: 0 - Deprecated: Use Object.assign() instead.

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.debug(string)

Stability: 0 - Deprecated: Use console.error() instead.

• string {string} The message to print to stderr

Deprecated predecessor of console.error.

util.error([...strings])

Stability: 0 - Deprecated: Use console.error() instead.

• ...strings {string} The message to print to stderr

Deprecated predecessor of console.error.

util.isArray(object)

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.

const util = require('util');

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

util.isBoolean(object)

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

• object {any}
• Returns: {boolean}

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

const util = require('util');

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

util.isBuffer(object)

Stability: 0 - Deprecated: Use Buffer.isBuffer() instead.

• object {any}
• Returns: {boolean}

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

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)

Stability: 0 - Deprecated: Use util.types.isDate() instead.

• object {any}
• Returns: {boolean}

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

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)

Stability: 0 - Deprecated: Use util.types.isNativeError() instead.

• object {any}
• Returns: {boolean}

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

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

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

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)

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

• object {any}
• Returns: {boolean}

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

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)

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

• object {any}
• Returns: {boolean}

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

const util = require('util');

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

util.isNullOrUndefined(object)

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.

const util = require('util');

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

util.isNumber(object)

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

• object {any}
• Returns: {boolean}

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

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)

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.

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)

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.

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)

Stability: 0 - Deprecated

• object {any}
• Returns: {boolean}

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

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)

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

• object {any}
• Returns: {boolean}

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

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)

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

• object {any}
• Returns: {boolean}

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

const util = require('util');

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

util.isUndefined(object)

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

• object {any}
• Returns: {boolean}

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

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)

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.

const util = require('util');

util.log('Timestamped message.');

util.print([...strings])

Stability: 0 - Deprecated: Use console.log() instead.

Deprecated predecessor of console.log.

util.puts([...strings])

Stability: 0 - Deprecated: Use console.log() instead.

Deprecated predecessor of console.log.