Util | Node.js v23.11.0 Documentation (original) (raw)

Source Code: lib/util.js

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)#

Added in: v8.2.0

• original An async function
• Returns: 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 Promiseresolved), 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

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 namedreason.

`` 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.debuglog(section[, callback])#

Added in: v0.11.3

• section A string identifying the portion of the application for which the debuglog function is being created.
• callback A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.
• Returns: 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_DEBUGenvironment variable. If the section name appears within the value of that environment variable, then the returned function operates similar toconsole.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]

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');

log('hi there, it's foo-bar [%d]', 2333); const { debuglog } = require('node:util'); const log = debuglog('foo');

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]

Multiple comma-separated section names may be specified in the NODE_DEBUGenvironment 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#

Added in: v14.9.0

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 befalse.

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]

util.debug(section)#

Added in: v14.9.0

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 The function that is being deprecated.
• msg A warning message to display when the deprecated function is invoked.
• code A deprecation code. See the list of deprecated APIs for a list of codes.
• Returns: 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 aDeprecationWarning 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 theprocess.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.throwDeprecationproperty take precedence over --trace-deprecation andprocess.traceDeprecation.

util.diff(actual, expected)#

Added in: v23.11.0

• actual | The first value to compare
• expected | The second value to compare
• Returns: An array of difference entries. Each entry is an array with two elements:
  • Index 0: Operation code: -1 for delete, 0 for no-op/unchanged, 1 for insert
  • Index 1: 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')); // []`

util.format(format[, ...args])#

• format 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, Objectand -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 andSymbol.
• %i: parseInt(value, 10) is used for all values except BigInt andSymbol.
• %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: The formatted string

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

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

Values that are not part of the format string are formatted usingutil.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'

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'

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

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

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

util.formatWithOptions(inspectOptions, format[, ...args])#

Added in: v10.0.0

• inspectOptions
• format

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

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

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

• frameCount Optional number of frames to capture as call site objects.Default: 10. Allowable range is between 1 and 200.
• options Optional
  • sourceMap 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 Returns the name of the function associated with this call site.
  • scriptName Returns the name of the resource that contains the script for the function for this call site.
  • scriptId Returns the unique id of the script, as in Chrome DevTools protocol Runtime.ScriptId.
  • lineNumber Returns the JavaScript script line number (1-based).
  • columnNumber Returns the JavaScript script column number (1-based).

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

`` 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.column}); }); // 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.column}); }); // 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, for example when using --experimental-transform-types,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`

`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`

util.getSystemErrorName(err)#

Added in: v9.7.0

• err
• Returns:

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

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

util.getSystemErrorMap()#

Added in: v16.0.0, v14.17.0

• Returns:

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 });

util.getSystemErrorMessage(err)#

Added in: v23.1.0

• err
• Returns:

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 });

util.inherits(constructor, superConstructor)#

Stability: 3 - Legacy: Use ES2015 class syntax and extends keyword instead.

• constructor
• superConstructor

Usage of util.inherits() is discouraged. Please use the ES6 class andextends 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 fromsuperConstructor.

This mainly adds some input validation on top ofObject.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!" ``

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 JavaScript primitive or Object.
• options
  • showHidden If true, object's non-enumerable symbols and properties are included in the formatted result. and entries are also included as well as user defined prototype properties (excluding method properties). Default: false.
  • depth Specifies the number of times to recurse while formattingobject. This is useful for inspecting large objects. To recurse up to the maximum call stack size pass Infinity or null.Default: 2.
  • colors If true, the output is styled with ANSI color codes. Colors are customizable. See Customizing util.inspect colors.Default: false.
  • customInspect If false,[util.inspect.custom](depth, opts, inspect) functions are not invoked.Default: true.
  • showProxy If true, Proxy inspection includes the target and handler objects. Default: false.
  • maxArrayLength Specifies the maximum number of Array,, , , and 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 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 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 | 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 intobreakLength. Short array elements are also grouped together. For more information, see the example below. Default: 3.
  • sorted | 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 | 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 If set to true, an underscore is used to separate every three digits in all bigints and numbers.Default: false.
• Returns: 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] {}'`

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 and entries to be inspected. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same 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 fromutil.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.

`` 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#

• 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 ``

See Custom inspection functions on Objects for more details.

util.inspect.defaultOptions#

Added in: v6.4.0

The defaultOptions value allows customization of the default options used byutil.inspect. This is useful for functions like console.log orutil.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)#

Added in: v9.0.0

• val1
• val2
• Returns:

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 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#

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#

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#

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#

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

mime.toString()#

• Returns:

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:

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#

Added in: v19.1.0, v18.13.0

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

Constructor: 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

Remove all name-value pairs whose name is name.

mimeParams.entries()#

• Returns:

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
• Returns: | 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
• Returns:

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

mimeParams.keys()#

• Returns:

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
• value

Sets the value in the MIMEParams object associated with name tovalue. 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:

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

mimeParams[@@iterator]()#

• Returns:

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 Used to provide arguments for parsing and to configure the parser. config supports the following properties:
  • args <string[]> array of argument strings. Default: process.argvwith execPath and filename removed.
  • options Used to describe arguments known to the parser. Keys of options are the long names of options and values are an accepting the following properties:
    * type Type of argument, which must be either boolean or string.
    * multiple Whether this option can be provided multiple times. If true, all values will be collected in an array. Iffalse, values for the option are last-wins. Default: false.
    * short A single character alias for the option.
    * default | | <string[]> | <boolean[]> The default value to be used if (and only if) the option does not appear in the arguments to be parsed. It must be of the same type as the type property. When multipleis true, it must be an array.
  • strict Should an error be thrown when unknown arguments are encountered, or when arguments are passed that do not match thetype configured in options.Default: true.
  • allowPositionals Whether this command accepts positional arguments.Default: false if strict is true, otherwise true.
  • allowNegative If true, allows explicitly setting boolean options to false by prefixing the option name with --no-.Default: false.
  • tokens 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: The parsed command line arguments:
  • values A mapping of parsed option names with their or values.
  • positionals <string[]> Positional arguments.
  • tokens <Object[]> | See parseArgs tokenssection. 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 One of 'option', 'positional', or 'option-terminator'.
  • index Index of element in args containing token. So the source argument for a token is args[token.index].
• option tokens
  • name Long name of option.
  • rawName How option used in args, like -f of --foo.
  • value | Option value specified in args. Undefined for boolean options.
  • inlineValue | Whether option value specified inline, like --foo=bar.
• positional tokens
  • value 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 (whichallowNegative 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 }

util.parseEnv(content)#

Added in: v21.7.0, v20.12.0

• content

The raw contents of a .env file.

• Returns:

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
• Returns:

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, promisifywill 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 ofutil.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); }); };

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

util.promisify.custom#

• 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); }); };`

util.stripVTControlCharacters(str)#

Added in: v16.11.0

• str
• Returns:

Returns str with any ANSI escape codes removed.

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

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

• format | A text format or an Array of text formats defined in util.inspect.colors.
• text The text to to be formatted.
• options
  • validateStream When true, stream is checked to see if it can handle colors. Default: true.
  • 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_COLORS,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, andunderline and you can combine both:

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

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 );

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

WHATWG supported encodings#

Per the WHATWG Encoding Standard, the encodings supported by theTextDecoder 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 Standardis not supported.

new TextDecoder([encoding[, options]])#

• encoding Identifies the encoding that this TextDecoder instance supports. Default: 'utf-8'.
• options
  • fatal true if decoding failures are fatal. This option is not supported when ICU is disabled (see Internationalization). Default: false.
  • ignoreBOM 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 | | An ArrayBuffer, DataView, orTypedArray instance containing the encoded data.
• options
  • stream true if additional chunks of data are expected.Default: false.
• Returns:

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 aTypeError being thrown.

textDecoder.encoding#

The encoding supported by the TextDecoder instance.

textDecoder.fatal#

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

textDecoder.ignoreBOM#

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');

The TextEncoder class is also available on the global object.

textEncoder.encode([input])#

• input The text to encode. Default: an empty string.
• Returns:

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

textEncoder.encodeInto(src, dest)#

Added in: v12.11.0

• src The text to encode.
• dest The array to hold the encode result.
• Returns:
  • read The read Unicode code units of src.
  • written 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);

textEncoder.encoding#

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

util.toUSVString(string)#

Added in: v16.8.0, v14.18.0

• 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 instance whose is marked as transferable and can be used with structuredClone() or postMessage().

util.transferableAbortSignal(signal)#

• signal
• Returns:

Marks the given as transferable so that it can be used withstructuredClone() and postMessage().

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

util.aborted(signal, resource)#

Added in: v19.7.0, v18.16.0

• signal
• resource 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:

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)#

Added in: v10.0.0

• value
• Returns:

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

See also util.types.isArrayBuffer() andutil.types.isSharedArrayBuffer().

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

util.types.isArrayBufferView(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is an instance of one of the views, such as typed array objects or . Equivalent toArrayBuffer.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

util.types.isArgumentsObject(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is an arguments object.

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

util.types.isArrayBuffer(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is a built-in instance. This does not include 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

util.types.isAsyncFunction(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isBigInt64Array(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is a BigInt64Array instance.

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

util.types.isBigIntObject(value)#

Added in: v10.4.0

• value
• Returns:

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

util.types.isBigUint64Array(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is a BigUint64Array instance.

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

util.types.isBooleanObject(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isBoxedPrimitive(value)#

Added in: v10.11.0

• value
• Returns:

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

util.types.isDataView(value)#

Added in: v10.0.0

• value
• Returns:

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

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

See also ArrayBuffer.isView().

util.types.isDate(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isExternal(value)#

Added in: v10.0.0

• value
• Returns:

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 anull 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) ...

`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 tonapi_create_external().

util.types.isFloat32Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isFloat64Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isGeneratorFunction(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isGeneratorObject(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isInt8Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isInt16Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isInt32Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isMap(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isMapIterator(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isModuleNamespaceObject(value)#

Added in: v10.0.0

• value
• Returns:

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`

util.types.isNativeError(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value was returned by the constructor of abuilt-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

Subclasses of the native error types are also native errors:

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

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 falsefor 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

util.types.isNumberObject(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isPromise(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is a built-in .

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

util.types.isProxy(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is a instance.

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

util.types.isRegExp(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is a regular expression object.

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

util.types.isSet(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isSetIterator(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isSharedArrayBuffer(value)#

Added in: v10.0.0

• value
• Returns:

Returns true if the value is a built-in instance. This does not include 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

util.types.isStringObject(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isSymbolObject(value)#

Added in: v10.0.0

• value
• Returns:

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

util.types.isTypedArray(value)#

Added in: v10.0.0

• value
• Returns:

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

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

See also ArrayBuffer.isView().

util.types.isUint8Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isUint8ClampedArray(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isUint16Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isUint32Array(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isWeakMap(value)#

Added in: v10.0.0

• value
• Returns:

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

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

util.types.isWeakSet(value)#

Added in: v10.0.0

• value
• Returns:

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

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

Deprecated APIs#

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

util._extend(target, source)#

Added in: v0.7.5Deprecated since: v6.0.0

• target
• source

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)#

Added in: v0.6.0Deprecated since: v4.0.0

• object
• Returns:

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`