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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('node:util');

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

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

Will print:

hello world copy

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 && Object.hasOwn(err, 'reason') && err.reason === null;  // true
}); copy

util.debuglog(section[, callback])

• section <string> A string identifying the portion of the application for which the debuglog function is being created.
• callback <Function> A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.
• 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('node:util');
const debuglog = util.debuglog('foo');

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

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

FOO 3245: hello from foo [123] copy

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('node:util');
const debuglog = util.debuglog('foo-bar');

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

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] copy

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

The optional callback argument can be used to replace the logging function with a different function that doesn't have any initialization or unnecessary wrapping.

const util = require('node:util');
let debuglog = util.debuglog('internals', (debug) => {
  // Replace with a logging function that optimizes out
  // testing if the section is enabled
  debuglog = debug;
}); copy

debuglog().enabled

• <boolean>

The util.debuglog().enabled getter is used to create a test that can be used in conditionals 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 value will be true. If not, then the returned value will be false.

const util = require('node:util');
const enabled = util.debuglog('foo').enabled;
if (enabled) {
  console.log('hello from foo [%d]', 123);
} copy

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

hello from foo [123] copy

util.debug(section)

Alias for util.debuglog. Usage allows for readability of that doesn't imply logging when only using util.debuglog().enabled.

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('node:util');

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

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('node: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 copy

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 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 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:

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

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:

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

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:

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

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

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

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])

• 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. copy

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
}); copy

util.getSystemErrorMap()

• Returns: <Map>

Returns a Map of all system error codes available from the 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 errorMap = util.getSystemErrorMap();
  const name = errorMap.get(err.errno);
  console.error(name);  // ENOENT
}); copy

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.

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.

const util = require('node:util');
const EventEmitter = require('node: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!" copy

ES6 example using class and extends:

const EventEmitter = require('node: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'); copy

util.inspect(object[, options])

util.inspect(object[, showHidden[, depth[, colors]]])

• 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, inspect) 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, Map, Set, 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.
  • maxStringLength <integer> Specifies the maximum number of characters to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no characters. Default: 10000.
  • 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 break on new lines in 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. 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.
  • numericSeparator <boolean> If set to true, an underscore is used to separate every three digits in all bigints and numbers. 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.

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] {}' copy

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

const { inspect } = require('node: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] }
// } copy

The following example inspects all properties of the util object:

const util = require('node:util');

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

The following example highlights the effect of the compact option:

const util = require('node:util');

const o = {
  a: [1, 2, [[
    'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
      'eiusmod \ntempor 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,\nconsectetur [...]', // A long line
//           'test',
//           'foo' ] ],
//     4 ],
//   b: Map(2) { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false or an integer creates more reader friendly output.
console.log(util.inspect(o, { compact: false, depth: 5, breakLength: 80 }));

// {
//   a: [
//     1,
//     2,
//     [
//       [
//         'Lorem ipsum dolor sit amet,\n' +
//           'consectetur adipiscing elit, sed do eiusmod \n' +
//           '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. copy

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.

const { inspect } = require('node: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 } } copy

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

const { inspect } = require('node:util');
const assert = require('node: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 }),
); copy

The numericSeparator option adds an underscore every three digits to all numbers.

const { inspect } = require('node:util');

const thousand = 1_000;
const million = 1_000_000;
const bigNumber = 123_456_789n;
const bigDecimal = 1_234.123_45;

console.log(inspect(thousand, { numericSeparator: true }));
// 1_000
console.log(inspect(million, { numericSeparator: true }));
// 1_000_000
console.log(inspect(bigNumber, { numericSeparator: true }));
// 123_456_789n
console.log(inspect(bigDecimal, { numericSeparator: true }));
// 1_234.123_45 copy

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

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:

• 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
• underline - 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)
• dim - Decreased color intensity (Alias: faint)
• overlined - Make text overlined
• blink - Hides and shows the text in an interval
• inverse - Swap foreground and background colors (Alias: swapcolors, swapColors)
• doubleunderline - Make text double underlined (Alias: doubleUnderline)
• framed - 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

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

const util = require('node:util');

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

  [util.inspect.custom](depth, options, inspect) {
    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 = 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 >" copy

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

const util = require('node: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' }" copy

util.inspect.custom

• <symbol> that can be used to declare custom inspect functions.

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

Using this allows code to be written in a portable fashion, so that the custom inspect function is used in an Node.js environment and ignored in the browser. The util.inspect() function itself is passed as third argument to the custom inspect function to allow further portability.

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

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

  toString() {
    return 'xxxxxxxx';
  }

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

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

See Custom inspection functions on Objects for more details.

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('node: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 copy

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.

Class: util.MIMEType

An implementation of the MIMEType class.

In accordance with browser conventions, all properties of MIMEType objects are implemented as getters and setters on the class prototype, rather than as data properties on the object itself.

A MIME string is a structured string containing multiple meaningful components. When parsed, a MIMEType object is returned containing properties for each of these components.

Constructor: new MIMEType(input)

• input <string> The input MIME to parse

Creates a new MIMEType object by parsing the input.

MJS modules

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/plain');

CJS modules

const { MIMEType } = require('node:util');

const myMIME = new MIMEType('text/plain');

A TypeError will be thrown if the input is not a valid MIME. Note that an effort will be made to coerce the given values into strings. For instance:

MJS modules

import { MIMEType } from 'node:util';
const myMIME = new MIMEType({ toString: () => 'text/plain' });
console.log(String(myMIME));
// Prints: text/plain

CJS modules

const { MIMEType } = require('node:util');
const myMIME = new MIMEType({ toString: () => 'text/plain' });
console.log(String(myMIME));
// Prints: text/plain

mime.type

• <string>

Gets and sets the type portion of the MIME.

MJS modules

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/javascript');
console.log(myMIME.type);
// Prints: text
myMIME.type = 'application';
console.log(myMIME.type);
// Prints: application
console.log(String(myMIME));
// Prints: application/javascript

CJS modules

const { MIMEType } = require('node:util');

const myMIME = new MIMEType('text/javascript');
console.log(myMIME.type);
// Prints: text
myMIME.type = 'application';
console.log(myMIME.type);
// Prints: application
console.log(String(myMIME));
// Prints: application/javascript

mime.subtype

• <string>

Gets and sets the subtype portion of the MIME.

MJS modules

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/ecmascript');
console.log(myMIME.subtype);
// Prints: ecmascript
myMIME.subtype = 'javascript';
console.log(myMIME.subtype);
// Prints: javascript
console.log(String(myMIME));
// Prints: text/javascript

CJS modules

const { MIMEType } = require('node:util');

const myMIME = new MIMEType('text/ecmascript');
console.log(myMIME.subtype);
// Prints: ecmascript
myMIME.subtype = 'javascript';
console.log(myMIME.subtype);
// Prints: javascript
console.log(String(myMIME));
// Prints: text/javascript

mime.essence

• <string>

Gets the essence of the MIME. This property is read only. Use mime.type or mime.subtype to alter the MIME.

MJS modules

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/javascript;key=value');
console.log(myMIME.essence);
// Prints: text/javascript
myMIME.type = 'application';
console.log(myMIME.essence);
// Prints: application/javascript
console.log(String(myMIME));
// Prints: application/javascript;key=value

CJS modules

const { MIMEType } = require('node:util');

const myMIME = new MIMEType('text/javascript;key=value');
console.log(myMIME.essence);
// Prints: text/javascript
myMIME.type = 'application';
console.log(myMIME.essence);
// Prints: application/javascript
console.log(String(myMIME));
// Prints: application/javascript;key=value

mime.params

• <MIMEParams>

Gets the MIMEParams object representing the parameters of the MIME. This property is read-only. See MIMEParams documentation for details.

mime.toString()

• Returns: <string>

The toString() method on the MIMEType object returns the serialized MIME.

Because of the need for standard compliance, this method does not allow users to customize the serialization process of the MIME.

mime.toJSON()

• Returns: <string>

Alias for mime.toString().

This method is automatically called when an MIMEType object is serialized with JSON.stringify().

MJS modules

import { MIMEType } from 'node:util';

const myMIMES = [
  new MIMEType('image/png'),
  new MIMEType('image/gif'),
];
console.log(JSON.stringify(myMIMES));
// Prints: ["image/png", "image/gif"]

CJS modules

const { MIMEType } = require('node:util');

const myMIMES = [
  new MIMEType('image/png'),
  new MIMEType('image/gif'),
];
console.log(JSON.stringify(myMIMES));
// Prints: ["image/png", "image/gif"]

Class: util.MIMEParams

The MIMEParams API provides read and write access to the parameters of a MIMEType.

Constructor: new MIMEParams()

Creates a new MIMEParams object by with empty parameters

MJS modules

import { MIMEParams } from 'node:util';

const myParams = new MIMEParams();

CJS modules

const { MIMEParams } = require('node:util');

const myParams = new MIMEParams();

mimeParams.delete(name)

• name <string>

Remove all name-value pairs whose name is name.

mimeParams.entries()

• Returns: <Iterator>

Returns an iterator over each of the name-value pairs in the parameters. Each item of the iterator is a JavaScript Array. The first item of the array is the name, the second item of the array is the value.

mimeParams.get(name)

• name <string>
• Returns: <string> or null if there is no name-value pair with the given name.

Returns the value of the first name-value pair whose name is name. If there are no such pairs, null is returned.

mimeParams.has(name)

• name <string>
• Returns: <boolean>

Returns true if there is at least one name-value pair whose name is name.

mimeParams.keys()

• Returns: <Iterator>

Returns an iterator over the names of each name-value pair.

MJS modules

import { MIMEType } from 'node:util';

const { params } = new MIMEType('text/plain;foo=0;bar=1');
for (const name of params.keys()) {
  console.log(name);
}
// Prints:
//   foo
//   bar

CJS modules

const { MIMEType } = require('node:util');

const { params } = new MIMEType('text/plain;foo=0;bar=1');
for (const name of params.keys()) {
  console.log(name);
}
// Prints:
//   foo
//   bar

mimeParams.set(name, value)

• name <string>
• value <string>

Sets the value in the MIMEParams object associated with name to value. If there are any pre-existing name-value pairs whose names are name, set the first such pair's value to value.

MJS modules

import { MIMEType } from 'node:util';

const { params } = new MIMEType('text/plain;foo=0;bar=1');
params.set('foo', 'def');
params.set('baz', 'xyz');
console.log(params.toString());
// Prints: foo=def;bar=1;baz=xyz

CJS modules

const { MIMEType } = require('node:util');

const { params } = new MIMEType('text/plain;foo=0;bar=1');
params.set('foo', 'def');
params.set('baz', 'xyz');
console.log(params.toString());
// Prints: foo=def;bar=1;baz=xyz

mimeParams.values()

• Returns: <Iterator>

Returns an iterator over the values of each name-value pair.

mimeParams[@@iterator]()

• Returns: <Iterator>

Alias for mimeParams.entries().

MJS modules

import { MIMEType } from 'node:util';

const { params } = new MIMEType('text/plain;foo=bar;xyz=baz');
for (const [name, value] of params) {
  console.log(name, value);
}
// Prints:
//   foo bar
//   xyz baz

CJS modules

const { MIMEType } = require('node:util');

const { params } = new MIMEType('text/plain;foo=bar;xyz=baz');
for (const [name, value] of params) {
  console.log(name, value);
}
// Prints:
//   foo bar
//   xyz baz

util.parseArgs([config])

• config <Object> Used to provide arguments for parsing and to configure the parser. config supports the following properties:

  • args <string[]> array of argument strings. Default: process.argv with execPath and filename removed.
  • options <Object> Used to describe arguments known to the parser. Keys of options are the long names of options and values are an <Object> accepting the following properties:
    • type <string> Type of argument, which must be either boolean or string.
    • multiple <boolean> Whether this option can be provided multiple times. If true, all values will be collected in an array. If false, values for the option are last-wins. Default: false.
    • short <string> A single character alias for the option.
    • default <string> | <boolean> | <string[]> | <boolean[]> The default option value when it is not set by args. It must be of the same type as the type property. When multiple is true, it must be an array.
  • strict <boolean> Should an error be thrown when unknown arguments are encountered, or when arguments are passed that do not match the type configured in options. Default: true.
  • allowPositionals <boolean> Whether this command accepts positional arguments. Default: false if strict is true, otherwise true.
  • tokens <boolean> Return the parsed tokens. This is useful for extending the built-in behavior, from adding additional checks through to reprocessing the tokens in different ways. Default: false.

• Returns: <Object> The parsed command line arguments:

  • values <Object> A mapping of parsed option names with their <string> or <boolean> values.
  • positionals <string[]> Positional arguments.
  • tokens <Object[]> | <undefined> See parseArgs tokens section. Only returned if config includes tokens: true.

Provides a higher level API for command-line argument parsing than interacting with process.argv directly. Takes a specification for the expected arguments and returns a structured object with the parsed options and positionals.

MJS modules

import { parseArgs } from 'node:util';
const args = ['-f', '--bar', 'b'];
const options = {
  foo: {
    type: 'boolean',
    short: 'f',
  },
  bar: {
    type: 'string',
  },
};
const {
  values,
  positionals,
} = parseArgs({ args, options });
console.log(values, positionals);
// Prints: [Object: null prototype] { foo: true, bar: 'b' } []

CJS modules

const { parseArgs } = require('node:util');
const args = ['-f', '--bar', 'b'];
const options = {
  foo: {
    type: 'boolean',
    short: 'f',
  },
  bar: {
    type: 'string',
  },
};
const {
  values,
  positionals,
} = parseArgs({ args, options });
console.log(values, positionals);
// Prints: [Object: null prototype] { foo: true, bar: 'b' } []

parseArgs tokens

Detailed parse information is available for adding custom behaviors by specifying tokens: true in the configuration. The returned tokens have properties describing:

• all tokens
  • kind <string> One of 'option', 'positional', or 'option-terminator'.
  • index <number> Index of element in args containing token. So the source argument for a token is args[token.index].
• option tokens
  • name <string> Long name of option.
  • rawName <string> How option used in args, like -f of --foo.
  • value <string> | <undefined> Option value specified in args. Undefined for boolean options.
  • inlineValue <boolean> | <undefined> Whether option value specified inline, like --foo=bar.
• positional tokens
  • value <string> The value of the positional argument in args (i.e. args[index]).
• option-terminator token

The returned tokens are in the order encountered in the input args. Options that appear more than once in args produce a token for each use. Short option groups like -xy expand to a token for each option. So -xxx produces three tokens.

For example to use the returned tokens to add support for a negated option like --no-color, the tokens can be reprocessed to change the value stored for the negated option.

MJS modules

import { parseArgs } from 'node:util';

const options = {
  'color': { type: 'boolean' },
  'no-color': { type: 'boolean' },
  'logfile': { type: 'string' },
  'no-logfile': { type: 'boolean' },
};
const { values, tokens } = parseArgs({ options, tokens: true });

// Reprocess the option tokens and overwrite the returned values.
tokens
  .filter((token) => token.kind === 'option')
  .forEach((token) => {
    if (token.name.startsWith('no-')) {
      // Store foo:false for --no-foo
      const positiveName = token.name.slice(3);
      values[positiveName] = false;
      delete values[token.name];
    } else {
      // Resave value so last one wins if both --foo and --no-foo.
      values[token.name] = token.value ?? true;
    }
  });

const color = values.color;
const logfile = values.logfile ?? 'default.log';

console.log({ logfile, color });

CJS modules

const { parseArgs } = require('node:util');

const options = {
  'color': { type: 'boolean' },
  'no-color': { type: 'boolean' },
  'logfile': { type: 'string' },
  'no-logfile': { type: 'boolean' },
};
const { values, tokens } = parseArgs({ options, tokens: true });

// Reprocess the option tokens and overwrite the returned values.
tokens
  .filter((token) => token.kind === 'option')
  .forEach((token) => {
    if (token.name.startsWith('no-')) {
      // Store foo:false for --no-foo
      const positiveName = token.name.slice(3);
      values[positiveName] = false;
      delete values[token.name];
    } else {
      // Resave value so last one wins if both --foo and --no-foo.
      values[token.name] = token.value ?? true;
    }
  });

const color = values.color;
const logfile = values.logfile ?? 'default.log';

console.log({ logfile, color });

Example usage showing negated options, and when an option is used multiple ways then last one wins.

$ node negate.js
{ logfile: 'default.log', color: undefined }
$ node negate.js --no-logfile --no-color
{ logfile: false, color: false }
$ node negate.js --logfile=test.log --color
{ logfile: 'test.log', color: true }
$ node negate.js --no-logfile --logfile=test.log --color --no-color
{ logfile: 'test.log', color: false } copy

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('node:util');
const fs = require('node:fs');

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

Or, equivalently using async functions:

const util = require('node:util');
const fs = require('node:fs');

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

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

callStat(); copy

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:

const util = require('node: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' copy

Custom promisified functions

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

const util = require('node: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' copy

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);
  });
}; copy

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.

In addition to being accessible through util.promisify.custom, this symbol is registered globally 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):

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

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

util.stripVTControlCharacters(str)

• str <string>
• Returns: <string>

Returns str with any ANSI escape codes removed.

console.log(util.stripVTControlCharacters('\u001B[4mvalue\u001B[0m'));
// Prints "value" copy

Class: util.TextDecoder

An implementation of the WHATWG Encoding Standard TextDecoder API.

const decoder = new TextDecoder();
const u8arr = new Uint8Array([72, 101, 108, 108, 111]);
console.log(decoder.decode(u8arr)); // Hello copy

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)

Encodings supported when Node.js is built with the small-icu option

Encodings supported when ICU is disabled

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 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 a 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

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'); copy

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.

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

textEncoder.encoding

• <string>

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

util.toUSVString(string)

• string <string>

Returns the string after replacing any surrogate code points (or equivalently, any unpaired surrogate code units) with the Unicode "replacement character" U+FFFD.

util.transferableAbortController()

Creates and returns an <AbortController> instance whose <AbortSignal> is marked as transferable and can be used with structuredClone() or postMessage().

util.transferableAbortSignal(signal)

• signal <AbortSignal>
• Returns: <AbortSignal>

Marks the given <AbortSignal> as transferable so that it can be used with structuredClone() and postMessage().

const signal = transferableAbortSignal(AbortSignal.timeout(100));
const channel = new MessageChannel();
channel.port2.postMessage(signal, [signal]); copy

util.aborted(signal, resource)

• signal <AbortSignal>
• resource <Object> Any non-null entity, reference to which is held weakly.
• Returns: <Promise>

Listens to abort event on the provided signal and returns a promise that is fulfilled when the signal is aborted. If the passed resource is garbage collected before the signal is aborted, the returned promise shall remain pending indefinitely.

CJS modules

const { aborted } = require('node:util');

const dependent = obtainSomethingAbortable();

aborted(dependent.signal, dependent).then(() => {
  // Do something when dependent is aborted.
});

dependent.on('event', () => {
  dependent.abort();
});

MJS modules

import { aborted } from 'node:util';

const dependent = obtainSomethingAbortable();

aborted(dependent.signal, dependent).then(() => {
  // Do something when dependent is aborted.
});

dependent.on('event', () => {
  dependent.abort();
});

util.types

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.

The API is accessible via require('node:util').types or require('node:util/types').

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().

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

util.types.isArrayBufferView(value)

• value <any>
• Returns: <boolean>

Returns true if the value is an instance of one of the ArrayBuffer views, such as typed array objects or DataView. Equivalent to ArrayBuffer.isView().

util.types.isArrayBufferView(new Int8Array());  // true
util.types.isArrayBufferView(Buffer.from('hello world')); // true
util.types.isArrayBufferView(new DataView(new ArrayBuffer(16)));  // true
util.types.isArrayBufferView(new ArrayBuffer());  // false copy

util.types.isArgumentsObject(value)

• value <any>
• Returns: <boolean>

Returns true if the value is an arguments object.

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

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.

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

util.types.isAsyncFunction(value)

• 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.

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

util.types.isBigInt64Array(value)

• value <any>
• Returns: <boolean>

Returns true if the value is a BigInt64Array instance.

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

util.types.isBigUint64Array(value)

• value <any>
• Returns: <boolean>

Returns true if the value is a BigUint64Array instance.

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

util.types.isBooleanObject(value)

• value <any>
• Returns: <boolean>

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

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 copy

util.types.isBoxedPrimitive(value)

• 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:

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 copy

util.types.isCryptoKey(value)

• value <Object>
• Returns: <boolean>

Returns true if value is a <CryptoKey>, false otherwise.

util.types.isDataView(value)

• value <any>
• Returns: <boolean>

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

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

See also ArrayBuffer.isView().

util.types.isDate(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isExternal(value)

• 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 objects with a null prototype.

#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)
... copy

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 copy

For further information on napi_create_external, refer to napi_create_external().

util.types.isFloat32Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isFloat64Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isGeneratorFunction(value)

• 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.

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

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. 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.

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

util.types.isInt8Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isInt16Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isInt32Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isKeyObject(value)

• value <Object>
• Returns: <boolean>

Returns true if value is a <KeyObject>, false otherwise.

util.types.isMap(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isMapIterator(value)

• value <any>
• Returns: <boolean>

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

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 copy

util.types.isModuleNamespaceObject(value)

• value <any>
• Returns: <boolean>

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

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

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

util.types.isNativeError(value)

• value <any>
• Returns: <boolean>

Returns true if the value was returned by the constructor of a built-in Error type.

console.log(util.types.isNativeError(new Error()));  // true
console.log(util.types.isNativeError(new TypeError()));  // true
console.log(util.types.isNativeError(new RangeError()));  // true copy

Subclasses of the native error types are also native errors:

class MyError extends Error {}
console.log(util.types.isNativeError(new MyError()));  // true copy

A value being instanceof a native error class is not equivalent to isNativeError() returning true for that value. isNativeError() returns true for errors which come from a different realm while instanceof Error returns false for these errors:

const vm = require('node:vm');
const context = vm.createContext({});
const myError = vm.runInContext('new Error()', context);
console.log(util.types.isNativeError(myError)); // true
console.log(myError instanceof Error); // false copy

Conversely, isNativeError() returns false for all objects which were not returned by the constructor of a native error. That includes values which are instanceof native errors:

const myError = { __proto__: Error.prototype };
console.log(util.types.isNativeError(myError)); // false
console.log(myError instanceof Error); // true copy

util.types.isNumberObject(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isPromise(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isProxy(value)

• value <any>
• Returns: <boolean>

Returns true if the value is a Proxy instance.

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

util.types.isRegExp(value)

• value <any>
• Returns: <boolean>

Returns true if the value is a regular expression object.

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

util.types.isSet(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isSetIterator(value)

• value <any>
• Returns: <boolean>

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

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 copy

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.

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

util.types.isStringObject(value)

• value <any>
• Returns: <boolean>

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

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

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.

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

util.types.isTypedArray(value)

• value <any>
• Returns: <boolean>

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

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

See also ArrayBuffer.isView().

util.types.isUint8Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isUint8ClampedArray(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isUint16Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isUint32Array(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isWeakMap(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isWeakSet(value)

• value <any>
• Returns: <boolean>

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

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

util.types.isWebAssemblyCompiledModule(value)

• value <any>
• Returns: <boolean>

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

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

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>

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)

• object <any>
• Returns: <boolean>

Alias for Array.isArray().

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

const util = require('node:util');

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

util.isBoolean(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isBuffer(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isDate(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isError(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

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

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

util.isFunction(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

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

util.isNull(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isNullOrUndefined(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isNumber(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isObject(object)

• 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('node:util');

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

util.isPrimitive(object)

• object <any>
• Returns: <boolean>

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

const util = require('node: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 copy

util.isRegExp(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isString(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isSymbol(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.isUndefined(object)

• object <any>
• Returns: <boolean>

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

const util = require('node:util');

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

util.log(string)

• string <string>

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

const util = require('node:util');

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