Util

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

import util from 'node:util';

const util = require('node: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.

import { callbackify } from 'node:util';

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

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

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

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

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

Will print:

hello world
textcopy

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.

import util from 'node:util';

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
});

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

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
});

util.convertProcessSignalToExitCode(signal)

• signal <string> A signal name (e.g. 'SIGTERM')
• Returns: <number> The exit code corresponding to signal

The util.convertProcessSignalToExitCode() method converts a signal name to its corresponding POSIX exit code. Following the POSIX standard, the exit code for a process terminated by a signal is calculated as 128 + signal number.

If signal is not a valid signal name, then an error will be thrown. See signal(7) for a list of valid signals.

import { convertProcessSignalToExitCode } from 'node:util';

console.log(convertProcessSignalToExitCode('SIGTERM')); // 143 (128 + 15)
console.log(convertProcessSignalToExitCode('SIGKILL')); // 137 (128 + 9)

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

console.log(convertProcessSignalToExitCode('SIGTERM')); // 143 (128 + 15)
console.log(convertProcessSignalToExitCode('SIGKILL')); // 137 (128 + 9)

This is particularly useful when working with processes to determine the exit code based on the signal that terminated the process.

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.

import { debuglog } from 'node:util';
const log = debuglog('foo');

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

const { debuglog } = require('node:util');
const log = debuglog('foo');

log('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]
consolecopy

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:

import { debuglog } from 'node:util';
const log = debuglog('foo-bar');

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

const { debuglog } = require('node:util');
const log = debuglog('foo-bar');

log('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]
consolecopy

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.

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

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

debuglog().enabled

• Type: <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.

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

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

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

hello from foo [123]
consolecopy

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[, options]])

• 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.
• options <Object>
  • modifyPrototype <boolean> When false do not change the prototype of object while emitting the deprecation warning. Default: true.
• 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.

import { deprecate } from 'node:util';

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

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

exports.obsoleteFunction = 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.

import { deprecate } from 'node:util';

const fn1 = deprecate(
  () => 'a value',
  'deprecation message',
  'DEP0001',
);
const fn2 = deprecate(
  () => 'a  different value',
  'other dep message',
  'DEP0001',
);
fn1(); // Emits a deprecation warning with code DEP0001
fn2(); // Does not emit a deprecation warning because it has the same code

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

const fn1 = deprecate(
  function() {
    return 'a value';
  },
  'deprecation message',
  'DEP0001',
);
const fn2 = deprecate(
  function() {
    return 'a  different value';
  },
  'other dep message',
  '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.diff(actual, expected)

• actual <Array> | <string> The first value to compare

• expected <Array> | <string> The second value to compare

• Returns: <Array> An array of difference entries. Each entry is an array with two elements:

  • 0 <number> Operation code: -1 for delete, 0 for no-op/unchanged, 1 for insert
  • 1 <string> The value associated with the operation

• Algorithm complexity: O(N*D), where:

• N is the total length of the two sequences combined (N = actual.length + expected.length)

• D is the edit distance (the minimum number of operations required to transform one sequence into the other).

util.diff() compares two string or array values and returns an array of difference entries. It uses the Myers diff algorithm to compute minimal differences, which is the same algorithm used internally by assertion error messages.

If the values are equal, an empty array is returned.

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

// Comparing strings
const actualString = '12345678';
const expectedString = '12!!5!7!';
console.log(diff(actualString, expectedString));
// [
//   [0, '1'],
//   [0, '2'],
//   [1, '3'],
//   [1, '4'],
//   [-1, '!'],
//   [-1, '!'],
//   [0, '5'],
//   [1, '6'],
//   [-1, '!'],
//   [0, '7'],
//   [1, '8'],
//   [-1, '!'],
// ]
// Comparing arrays
const actualArray = ['1', '2', '3'];
const expectedArray = ['1', '3', '4'];
console.log(diff(actualArray, expectedArray));
// [
//   [0, '1'],
//   [1, '2'],
//   [0, '3'],
//   [-1, '4'],
// ]
// Equal values return empty array
console.log(diff('same', 'same'));
// []
jscopy

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 neither a user defined toString function nor Symbol.toPrimitive 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'
jscopy

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'
jscopy

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'
jscopy

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

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

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.
jscopy

util.getCallSites([frameCount][, options])

• frameCount <integer> Optional number of frames to capture as call site objects. Default: 10. Allowable range is between 1 and 200.
• options <Object> Optional
  • sourceMap <boolean> Reconstruct the original location in the stacktrace from the source-map. Enabled by default with the flag --enable-source-maps.
• Returns: <Object[]> An array of call site objects
  • functionName <string> Returns the name of the function associated with this call site.
  • scriptName <string> Returns the name of the resource that contains the script for the function for this call site.
  • scriptId <string> Returns the unique id of the script, as in Chrome DevTools protocol Runtime.ScriptId.
  • lineNumber <number> Returns the JavaScript script line number (1-based).
  • columnNumber <number> Returns the JavaScript script column number (1-based).

Returns an array of call site objects containing the stack of the caller function.

Unlike accessing an error.stack, the result returned from this API is not interfered with Error.prepareStackTrace.

import { getCallSites } from 'node:util';

function exampleFunction() {
  const callSites = getCallSites();

  console.log('Call Sites:');
  callSites.forEach((callSite, index) => {
    console.log(`CallSite ${index + 1}:`);
    console.log(`Function Name: ${callSite.functionName}`);
    console.log(`Script Name: ${callSite.scriptName}`);
    console.log(`Line Number: ${callSite.lineNumber}`);
    console.log(`Column Number: ${callSite.columnNumber}`);
  });
  // CallSite 1:
  // Function Name: exampleFunction
  // Script Name: /home/example.js
  // Line Number: 5
  // Column Number: 26

  // CallSite 2:
  // Function Name: anotherFunction
  // Script Name: /home/example.js
  // Line Number: 22
  // Column Number: 3

  // ...
}

// A function to simulate another stack layer
function anotherFunction() {
  exampleFunction();
}

anotherFunction();

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

function exampleFunction() {
  const callSites = getCallSites();

  console.log('Call Sites:');
  callSites.forEach((callSite, index) => {
    console.log(`CallSite ${index + 1}:`);
    console.log(`Function Name: ${callSite.functionName}`);
    console.log(`Script Name: ${callSite.scriptName}`);
    console.log(`Line Number: ${callSite.lineNumber}`);
    console.log(`Column Number: ${callSite.columnNumber}`);
  });
  // CallSite 1:
  // Function Name: exampleFunction
  // Script Name: /home/example.js
  // Line Number: 5
  // Column Number: 26

  // CallSite 2:
  // Function Name: anotherFunction
  // Script Name: /home/example.js
  // Line Number: 22
  // Column Number: 3

  // ...
}

// A function to simulate another stack layer
function anotherFunction() {
  exampleFunction();
}

anotherFunction();

It is possible to reconstruct the original locations by setting the option sourceMap to true. If the source map is not available, the original location will be the same as the current location. When the --enable-source-maps flag is enabled,sourceMap will be true by default.

import { getCallSites } from 'node:util';

interface Foo {
  foo: string;
}

const callSites = getCallSites({ sourceMap: true });

// With sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 7
// Column Number: 26

// Without sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 2
// Column Number: 26
tscopy

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

const callSites = getCallSites({ sourceMap: true });

// With sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 7
// Column Number: 26

// Without sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 2
// Column Number: 26
cjscopy

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
});
jscopy

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
});
jscopy

util.getSystemErrorMessage(err)

• err <number>
• Returns: <string>

Returns the string message for a numeric error code that comes from a Node.js API. The mapping between error codes and string messages is platform-dependent.

fs.access('file/that/does/not/exist', (err) => {
  const message = util.getSystemErrorMessage(err.errno);
  console.error(message);  // No such file or directory
});
jscopy

util.setTraceSigInt(enable)

• enable <boolean>

Enable or disable printing a stack trace on SIGINT. The API is only available on the main thread.

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!"
jscopy

ES6 example using class and extends:

import EventEmitter from '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');

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');

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>, <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 Symbol.toStringTag property 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] {}'
jscopy

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

import { inspect } from '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] }
// }

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] }
// }

The following example inspects all properties of the util object:

import util from 'node:util';

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

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

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

The following example highlights the effect of the compact option:

import { inspect } from '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(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(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.

const { inspect } = 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(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(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.

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.

import { inspect } from '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 } }

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 } }

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

import { inspect } from 'node:util';
import assert from '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 }),
);

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 }),
);

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

import { inspect } from 'node:util';

const thousand = 1000;
const million = 1000000;
const bigNumber = 123456789n;
const bigDecimal = 1234.12345;

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

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

const thousand = 1000;
const million = 1000000;
const bigNumber = 123456789n;
const bigDecimal = 1234.12345;

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

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: A method that colors character classes, groups, assertions, and other parts for improved readability. To customize the coloring, change the colors property. It is set to ['red', 'green', 'yellow', 'cyan', 'magenta'] by default and may be adjusted as needed. The array is repetitively iterated through depending on the "depth".
• 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").

Complex custom coloring

It is possible to define a method as style. It receives the stringified value of the input. It is invoked in case coloring is active and the type is inspected.

Example: util.inspect.styles.regexp(value)

• value <string> The string representation of the input type.
• Returns: <string> The adjusted representation of object.

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.

import { inspect } from 'node:util';

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

  [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);

console.log(inspect(box));
// "Box< true >"

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

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

  [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);

console.log(inspect(box));
// "Box< true >"

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().

import { inspect } from 'node:util';

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

console.log(inspect(obj));
// "{ bar: 'baz' }"

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

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

console.log(inspect(obj));
// "{ bar: 'baz' }"

util.inspect.custom

• Type: <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>
jscopy

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.

import { inspect } from 'node:util';
const arr = Array(156).fill(0);

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

const { inspect } = require('node:util');
const arr = Array(156).fill(0);

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

util.isDeepStrictEqual(val1, val2[, options])

• val1 <any>
• val2 <any>
• skipPrototype <boolean> If true, prototype and constructor comparison is skipped during deep strict equality check. Default: false.
• Returns: <boolean>

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

By default, deep strict equality includes comparison of object prototypes and constructors. When skipPrototype is true, objects with different prototypes or constructors can still be considered equal if their enumerable properties are deeply strictly equal.

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

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

class Bar {
  constructor(a) {
    this.a = a;
  }
}

const foo = new Foo(1);
const bar = new Bar(1);

// Different constructors, same properties
console.log(util.isDeepStrictEqual(foo, bar));
// false

console.log(util.isDeepStrictEqual(foo, bar, true));
// true
jscopy

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.

new MIMEType(input)

• input <string> The input MIME to parse

Creates a new MIMEType object by parsing the input.

import { MIMEType } from 'node:util';

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

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:

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

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

mime.type

• Type: <string>

Gets and sets the type portion of the MIME.

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

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

• Type: <string>

Gets and sets the subtype portion of the MIME.

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

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

• Type: <string>

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

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

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

• Type: <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().

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"]

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.

new MIMEParams()

Creates a new MIMEParams object by with empty parameters

import { MIMEParams } from 'node:util';

const myParams = new MIMEParams();

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> | <null> A 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.

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

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.

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

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[Symbol.iterator]()

• Returns: <Iterator>

Alias for mimeParams.entries().

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

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 value to assign to the option if it does not appear in the arguments to be parsed. The value must match the type specified by the type property. If multiple is true, it must be an array. No default value is applied when the option does appear in the arguments to be parsed, even if the provided value is falsy.
  • 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.
  • allowNegative <boolean> If true, allows explicitly setting boolean options to false by prefixing the option name with --no-. Default: false.
  • 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.

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' } []

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 add support for a negated option like --no-color (which allowNegative supports when the option is of boolean type), the returned tokens can be reprocessed to change the value stored for the negated option.

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 });

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 }
consolecopy

util.parseEnv(content)

• content <string>

The raw contents of a .env file.

• Returns: <Object>

Given an example .env file:

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

parseEnv('HELLO=world\nHELLO=oh my\n');
// Returns: { HELLO: 'oh my' }

import { parseEnv } from 'node:util';

parseEnv('HELLO=world\nHELLO=oh my\n');
// Returns: { HELLO: 'oh my' }

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.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

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

const { promisify } = require('node:util');
const { stat } = require('node:fs');

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

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

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

callStat();

const { promisify } = require('node:util');
const { stat } = require('node:fs');

const promisifiedStat = promisify(stat);

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

callStat();

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:

import { promisify } from 'node:util';

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

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

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// 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'

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

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

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

const foo = new Foo();

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

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

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

Custom promisified functions

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

import { promisify } from 'node:util';

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

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

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

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

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

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

const promisified = promisify(doSomething);
console.log(promisified === doSomething[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);
  });
};
jscopy

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

util.promisify.custom

• Type: <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);
  });
};
jscopy

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"
jscopy

util.styleText(format, text[, options])

• format <string> | <Array> A text format or an Array of text formats defined in util.inspect.colors, or a hex color in #RGB or #RRGGBB form.
• text <string> The text to be formatted.
• options <Object>
  • validateStream <boolean> When true, stream is checked to see if it can handle colors. Default: true.
  • stream <Stream> A stream that will be validated if it can be colored. Default: process.stdout.

This function returns a formatted text considering the format passed for printing in a terminal. It is aware of the terminal's capabilities and acts according to the configuration set via NO_COLOR, NODE_DISABLE_COLORS and FORCE_COLOR environment variables.

import { styleText } from 'node:util';
import { stderr } from 'node:process';

const successMessage = styleText('green', 'Success!');
console.log(successMessage);

const errorMessage = styleText(
  'red',
  'Error! Error!',
  // Validate if process.stderr has TTY
  { stream: stderr },
);
console.error(errorMessage);

const { styleText } = require('node:util');
const { stderr } = require('node:process');

const successMessage = styleText('green', 'Success!');
console.log(successMessage);

const errorMessage = styleText(
  'red',
  'Error! Error!',
  // Validate if process.stderr has TTY
  { stream: stderr },
);
console.error(errorMessage);

util.inspect.colors also provides text formats such as italic, and underline and you can combine both:

console.log(
  util.styleText(['underline', 'italic'], 'My italic underlined message'),
);
cjscopy

When passing an array of formats, the order of the format applied is left to right so the following style might overwrite the previous one.

console.log(
  util.styleText(['red', 'green'], 'text'), // green
);
cjscopy

The special format value none applies no additional styling to the text.

In addition to predefined color names, util.styleText() supports hex color strings using ANSI TrueColor (24-bit) escape sequences. Hex colors can be specified in either 3-digit (#RGB) or 6-digit (#RRGGBB) format:

import { styleText } from 'node:util';

// 6-digit hex color
console.log(styleText('#ff5733', 'Orange text'));

// 3-digit hex color (shorthand)
console.log(styleText('#f00', 'Red text'));

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

// 6-digit hex color
console.log(styleText('#ff5733', 'Orange text'));

// 3-digit hex color (shorthand)
console.log(styleText('#f00', 'Red text'));

The full list of formats can be found in modifiers.

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
jscopy

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

• Type: <string>

The encoding supported by the TextDecoder instance.

textDecoder.fatal

• Type: <boolean>

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

textDecoder.ignoreBOM

• Type: <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');
jscopy

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);
jscopy

textEncoder.encoding

• Type: <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]);
jscopy

util.aborted(signal, resource)

• signal <AbortSignal>
• resource <Object> Any non-null object tied to the abortable operation and held weakly. If resource is garbage collected before the signal aborts, the promise remains pending, allowing Node.js to stop tracking it. This helps prevent memory leaks in long-running or non-cancelable operations.
• Returns: <Promise>

Listens to abort event on the provided signal and returns a promise that resolves when the signal is aborted. If resource is provided, it weakly references the operation's associated object, so if resource is garbage collected before the signal aborts, then returned promise shall remain pending. This prevents memory leaks in long-running or non-cancelable operations.

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

// Obtain an object with an abortable signal, like a custom resource or operation.
const dependent = obtainSomethingAbortable();

// Pass `dependent` as the resource, indicating the promise should only resolve
// if `dependent` is still in memory when the signal is aborted.
aborted(dependent.signal, dependent).then(() => {

  // This code runs when `dependent` is aborted.
  console.log('Dependent resource was aborted.');
});

// Simulate an event that triggers the abort.
dependent.on('event', () => {
  dependent.abort(); // This will cause the `aborted` promise to resolve.
});

import { aborted } from 'node:util';

// Obtain an object with an abortable signal, like a custom resource or operation.
const dependent = obtainSomethingAbortable();

// Pass `dependent` as the resource, indicating the promise should only resolve
// if `dependent` is still in memory when the signal is aborted.
aborted(dependent.signal, dependent).then(() => {

  // This code runs when `dependent` is aborted.
  console.log('Dependent resource was aborted.');
});

// Simulate an event that triggers the abort.
dependent.on('event', () => {
  dependent.abort(); // This will cause the `aborted` promise to resolve.
});

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
jscopy

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
jscopy

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
}
jscopy

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
jscopy

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
jscopy

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
jscopy

util.types.isBigIntObject(value)

• value <any>
• Returns: <boolean>

Returns true if the value is a BigInt object, e.g. created by Object(BigInt(123)).

util.types.isBigIntObject(Object(BigInt(123)));   // Returns true
util.types.isBigIntObject(BigInt(123));   // Returns false
util.types.isBigIntObject(123);  // Returns false
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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.

import native from 'napi_addon.node';
import { types } from 'node:util';

const data = native.myNapi();
types.isExternal(data); // returns true
types.isExternal(0); // returns false
types.isExternal(new String('foo')); // returns false

const native = require('napi_addon.node');
const { types } = require('node:util');

const data = native.myNapi();
types.isExternal(data); // returns true
types.isExternal(0); // returns false
types.isExternal(new String('foo')); // returns false

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

util.types.isFloat16Array(value)

• value <any>
• Returns: <boolean>

Returns true if the value is a built-in <Float16Array> instance.

util.types.isFloat16Array(new ArrayBuffer());  // Returns false
util.types.isFloat16Array(new Float16Array());  // Returns true
util.types.isFloat16Array(new Float32Array());  // Returns false
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
mjscopy

util.types.isNativeError(value)

Note: As of Node.js 24, Error.isError() is currently slower than util.types.isNativeError(). If performance is critical, consider benchmarking both in your environment.

• 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
jscopy

Subclasses of the native error types are also native errors:

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

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:

import { createContext, runInContext } from 'node:vm';
import { types } from 'node:util';

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

const { createContext, runInContext } = require('node:vm');
const { types } = require('node:util');

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

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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
jscopy

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().

An automated migration is available (source):

npx codemod@latest @nodejs/util-extend-to-object-assign
bashcopy

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
jscopy

An automated migration is available (source):

npx codemod@latest @nodejs/util-is
bashcopy