Util | Node.js v24.14.1 Documentation

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Table of contents

- Util
  - `util.callbackify(original)`
  - `util.convertProcessSignalToExitCode(signalCode)`
  - `util.debuglog(section[, callback])`
    - `debuglog().enabled`
  - `util.debug(section)`
  - `util.deprecate(fn, msg[, code[, options]])`
  - `util.diff(actual, expected)`
  - `util.format(format[, ...args])`
  - `util.formatWithOptions(inspectOptions, format[, ...args])`
  - `util.getCallSites([frameCount][, options])`
  - `util.getSystemErrorName(err)`
  - `util.getSystemErrorMap()`
  - `util.getSystemErrorMessage(err)`
  - `util.setTraceSigInt(enable)`
  - `util.inherits(constructor, superConstructor)`
  - `util.inspect(object[, options])`
  - `util.inspect(object[, showHidden[, depth[, colors]]])`
    - Customizing `util.inspect` colors
      - Modifiers
      - Foreground colors
      - Background colors
    - Custom inspection functions on objects
    - `util.inspect.custom`
    - `util.inspect.defaultOptions`
  - `util.isDeepStrictEqual(val1, val2[, options])`
  - Class: `util.MIMEType`
    - `new MIMEType(input)`
    - `mime.type`
    - `mime.subtype`
    - `mime.essence`
    - `mime.params`
    - `mime.toString()`
    - `mime.toJSON()`
  - Class: `util.MIMEParams`
    - `new MIMEParams()`
    - `mimeParams.delete(name)`
    - `mimeParams.entries()`
    - `mimeParams.get(name)`
    - `mimeParams.has(name)`
    - `mimeParams.keys()`
    - `mimeParams.set(name, value)`
    - `mimeParams.values()`
    - `mimeParams[Symbol.iterator]()`
  - `util.parseArgs([config])`
    - `parseArgs` `tokens`
  - `util.parseEnv(content)`
  - `util.promisify(original)`
    - Custom promisified functions
    - `util.promisify.custom`
  - `util.stripVTControlCharacters(str)`
  - `util.styleText(format, text[, options])`
  - Class: `util.TextDecoder`
    - WHATWG supported encodings
      - 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
    - `new TextDecoder([encoding[, options]])`
    - `textDecoder.decode([input[, options]])`
    - `textDecoder.encoding`
    - `textDecoder.fatal`
    - `textDecoder.ignoreBOM`
  - Class: `util.TextEncoder`
    - `textEncoder.encode([input])`
    - `textEncoder.encodeInto(src, dest)`
    - `textEncoder.encoding`
  - `util.toUSVString(string)`
  - `util.transferableAbortController()`
  - `util.transferableAbortSignal(signal)`
  - `util.aborted(signal, resource)`
  - `util.types`
    - `util.types.isAnyArrayBuffer(value)`
    - `util.types.isArrayBufferView(value)`
    - `util.types.isArgumentsObject(value)`
    - `util.types.isArrayBuffer(value)`
    - `util.types.isAsyncFunction(value)`
    - `util.types.isBigInt64Array(value)`
    - `util.types.isBigIntObject(value)`
    - `util.types.isBigUint64Array(value)`
    - `util.types.isBooleanObject(value)`
    - `util.types.isBoxedPrimitive(value)`
    - `util.types.isCryptoKey(value)`
    - `util.types.isDataView(value)`
    - `util.types.isDate(value)`
    - `util.types.isExternal(value)`
    - `util.types.isFloat16Array(value)`
    - `util.types.isFloat32Array(value)`
    - `util.types.isFloat64Array(value)`
    - `util.types.isGeneratorFunction(value)`
    - `util.types.isGeneratorObject(value)`
    - `util.types.isInt8Array(value)`
    - `util.types.isInt16Array(value)`
    - `util.types.isInt32Array(value)`
    - `util.types.isKeyObject(value)`
    - `util.types.isMap(value)`
    - `util.types.isMapIterator(value)`
    - `util.types.isModuleNamespaceObject(value)`
    - `util.types.isNativeError(value)`
    - `util.types.isNumberObject(value)`
    - `util.types.isPromise(value)`
    - `util.types.isProxy(value)`
    - `util.types.isRegExp(value)`
    - `util.types.isSet(value)`
    - `util.types.isSetIterator(value)`
    - `util.types.isSharedArrayBuffer(value)`
    - `util.types.isStringObject(value)`
    - `util.types.isSymbolObject(value)`
    - `util.types.isTypedArray(value)`
    - `util.types.isUint8Array(value)`
    - `util.types.isUint8ClampedArray(value)`
    - `util.types.isUint16Array(value)`
    - `util.types.isUint32Array(value)`
    - `util.types.isWeakMap(value)`
    - `util.types.isWeakSet(value)`
  - Deprecated APIs
    - `util._extend(target, source)`
    - `util.isArray(object)`

## Util#

Stability: 2 - Stable

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` <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

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

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

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

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

### `util.convertProcessSignalToExitCode(signalCode)`#

Added in: v24.14.0

- `signalCode` <string> A signal name (e.g., `'SIGTERM'`, `'SIGKILL'`).
- Returns: <number> | <null> The exit code, or `null` if the signal is invalid.

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`.

    import { convertProcessSignalToExitCode } from 'node:util';

    console.log(convertProcessSignalToExitCode('SIGTERM')); // 143 (128 + 15)
    console.log(convertProcessSignalToExitCode('SIGKILL')); // 137 (128 + 9)
    console.log(convertProcessSignalToExitCode('INVALID')); // nullconst { convertProcessSignalToExitCode } = require('node:util');

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

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])`#

Added in: v0.11.3

- `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]

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]

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`#

Added in: v14.9.0

- 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]

### `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[, options]])`#

History

Version | Changes
--- | ---
v24.12.0 | Add options object with modifyPrototype to conditionally modify the prototype of the deprecated object.
v10.0.0 | Deprecation warnings are only emitted once for each code.
v0.8.0 | Added in: v0.8.0

- `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 codeconst { 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)`#

Added in: v23.11.0, v22.15.0

Stability: 1 - Experimental

- `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'));
    // []

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

History

Version | Changes
--- | ---
v12.11.0 | The `%c` specifier is ignored now.
v12.0.0 | The `format` argument is now only taken as such if it actually contains format specifiers.
v12.0.0 | If the `format` argument is not a format string, the output string's formatting is no longer dependent on the type of the first argument. This change removes previously present quotes from strings that were being output when the first argument was not a string.
v11.4.0 | The `%d`, `%f`, and `%i` specifiers now support Symbols properly.
v11.4.0 | The `%o` specifier's `depth` has default depth of 4 again.
v11.0.0 | The `%o` specifier's `depth` option will now fall back to the default depth.
v10.12.0 | The `%d` and `%i` specifiers now support BigInt.
v8.4.0 | The `%o` and `%O` specifiers are supported now.
v0.5.3 | Added in: v0.5.3

- `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'

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'

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` <Object>
- `format` <string>

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

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

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

History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | Property `column` is deprecated in favor of `columnNumber`.
v23.7.0, v22.14.0 | Property `CallSite.scriptId` is exposed.
v23.3.0, v22.12.0 | The API is renamed from `util.getCallSite` to `util.getCallSites()`.
v22.9.0 | Added in: v22.9.0

Stability: 1.1 - Active development

- `frameCount` <number> 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, 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` <number>
- Returns: <string>

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

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

### `util.getSystemErrorMap()`#

Added in: v16.0.0, v14.17.0

- 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
    });

### `util.getSystemErrorMessage(err)`#

Added in: v23.1.0, v22.12.0

- `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
    });

### `util.setTraceSigInt(enable)`#

Added in: v24.6.0

- `enable` <boolean>

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

### `util.inherits(constructor, superConstructor)`#

History

Version | Changes
--- | ---
v5.0.0 | The `constructor` parameter can refer to an ES6 class now.
v0.3.0 | Added in: v0.3.0

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

- `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!"

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]]])`#

History

Version | Changes
--- | ---
v16.18.0 | add support for `maxArrayLength` when inspecting `Set` and `Map`.
v17.3.0, v16.14.0 | The `numericSeparator` option is supported now.
v13.0.0 | Circular references now include a marker to the reference.
v14.6.0, v12.19.0 | If `object` is from a different `vm.Context` now, a custom inspection function on it will not receive context-specific arguments anymore.
v13.13.0, v12.17.0 | The `maxStringLength` option is supported now.
v13.5.0, v12.16.0 | User defined prototype properties are inspected in case `showHidden` is `true`.
v12.0.0 | The `compact` options default is changed to `3` and the `breakLength` options default is changed to `80`.
v12.0.0 | Internal properties no longer appear in the context argument of a custom inspection function.
v11.11.0 | The `compact` option accepts numbers for a new output mode.
v11.7.0 | ArrayBuffers now also show their binary contents.
v11.5.0 | The `getters` option is supported now.
v11.4.0 | The `depth` default changed back to `2`.
v11.0.0 | The `depth` default changed to `20`.
v11.0.0 | The inspection output is now limited to about 128 MiB. Data above that size will not be fully inspected.
v10.12.0 | The `sorted` option is supported now.
v10.6.0 | Inspecting linked lists and similar objects is now possible up to the maximum call stack size.
v10.0.0 | The `WeakMap` and `WeakSet` entries can now be inspected as well.
v9.9.0 | The `compact` option is supported now.
v6.6.0 | Custom inspection functions can now return `this`.
v6.3.0 | The `breakLength` option is supported now.
v6.1.0 | The `maxArrayLength` option is supported now; in particular, long arrays are truncated by default.
v6.1.0 | The `showProxy` option is supported now.
v0.3.0 | Added in: v0.3.0

- `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] {}'

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_45const { 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`: `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#

History

Version | Changes
--- | ---
v17.3.0, v16.14.0 | The inspect argument is added for more interoperability.
v0.1.97 | Added in: v0.1.97

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`#

History

Version | Changes
--- | ---
v10.12.0 | This is now defined as a shared symbol.
v6.6.0 | Added in: v6.6.0

- 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>

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 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 arrayconst { 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])`#

History

Version | Changes
--- | ---
v24.9.0 | Added `options` parameter to allow skipping prototype comparison.
v9.0.0 | Added in: v9.0.0

- `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

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

### Class: `util.MIMEType`#

History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.
v19.1.0, v18.13.0 | Added in: v19.1.0, v18.13.0

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/plainconst { 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/javascriptconst { 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/javascriptconst { 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=valueconst { 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`#

Added in: v19.1.0, v18.13.0

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
    //   barconst { 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=xyzconst { 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 bazconst { 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])`#

History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | add support for allowing negative options in input `config`.
v20.0.0 | The API is no longer experimental.
v18.11.0, v16.19.0 | Add support for default values in input `config`.
v18.7.0, v16.17.0 | add support for returning detailed parse information using `tokens` in input `config` and returned properties.
v18.3.0, v16.17.0 | Added in: v18.3.0, v16.17.0

- `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 }

### `util.parseEnv(content)`#

History

Version | Changes
--- | ---
v24.10.0 | This API is no longer experimental.
v21.7.0, v20.12.0 | Added in: v21.7.0, v20.12.0

- `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)`#

History

Version | Changes
--- | ---
v20.8.0 | Calling `promisify` on a function that returns a `Promise` is deprecated.
v8.0.0 | Added in: v8.0.0

- `original` <Function>
- Returns: <Function>

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

    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 function`s:

    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);
      });
    };

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

#### `util.promisify.custom`#

History

Version | Changes
--- | ---
v13.12.0, v12.16.2 | This is now defined as a shared symbol.
v8.0.0 | Added in: v8.0.0

- 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);
      });
    };

### `util.stripVTControlCharacters(str)`#

Added in: v16.11.0

- `str` <string>
- Returns: <string>

Returns `str` with any ANSI escape codes removed.

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

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

History

Version | Changes
--- | ---
v24.2.0 | Added the `'none'` format as a non-op format.
v23.5.0, v22.13.0 | styleText is now stable.
v22.8.0, v20.18.0 | Respect isTTY and environment variables such as NO_COLOR, NODE_DISABLE_COLORS, and FORCE_COLOR.
v21.7.0, v20.12.0 | Added in: v21.7.0, v20.12.0

- `format` <string> | <Array> A text format or an Array of text formats defined in `util.inspect.colors`.
- `text` <string> The text to 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'),
    );

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

The full list of formats can be found in modifiers.

### Class: `util.TextDecoder`#

History

Version | Changes
--- | ---
v11.0.0 | The class is now available on the global object.
v8.3.0 | Added in: v8.3.0

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 the `TextDecoder` API are outlined in the tables below. For each encoding, one or more aliases may be used.

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

##### Encodings supported by default (with full ICU data)#

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

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

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

##### Encodings supported when ICU is disabled#

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

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

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

- `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`#

History

Version | Changes
--- | ---
v11.0.0 | The class is now available on the global object.
v8.3.0 | Added in: v8.3.0

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` <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)`#

Added in: v12.11.0

- `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);

#### `textEncoder.encoding`#

- Type: <string>

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

### `util.toUSVString(string)`#

Added in: v16.8.0, v14.18.0

- `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()`#

History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.
v18.11.0 | Added in: v18.11.0

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

### `util.transferableAbortSignal(signal)`#

History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.
v18.11.0 | Added in: v18.11.0

- `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]);

### `util.aborted(signal, resource)`#

History

Version | Changes
--- | ---
v24.0.0 | Change stability index for this feature from Experimental to Stable.
v19.7.0, v18.16.0 | Added in: v19.7.0, v18.16.0

- `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`#

History

Version | Changes
--- | ---
v15.3.0 | Exposed as `require('util/types')`.
v10.0.0 | Added in: v10.0.0

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

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

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

#### `util.types.isAnyArrayBuffer(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

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

#### `util.types.isArrayBufferView(value)`#

Added in: v10.0.0

- `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

#### `util.types.isArgumentsObject(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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` <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

#### `util.types.isAsyncFunction(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

    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` <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

#### `util.types.isBigIntObject(value)`#

Added in: v10.4.0

- `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

#### `util.types.isBigUint64Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isBooleanObject(value)`#

Added in: v10.0.0

- `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

#### `util.types.isBoxedPrimitive(value)`#

Added in: v10.11.0

- `value` <any>
- Returns: <boolean>

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

For example:

    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.isCryptoKey(value)`#

Added in: v16.2.0

- `value` <Object>
- Returns: <boolean>

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

#### `util.types.isDataView(value)`#

Added in: v10.0.0

- `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

See also `ArrayBuffer.isView()`.

#### `util.types.isDate(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isExternal(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

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

    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 falseconst 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)`#

Added in: v24.0.0

- `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

#### `util.types.isFloat32Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isFloat64Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isGeneratorFunction(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isGeneratorObject(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isInt8Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isInt16Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isInt32Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isKeyObject(value)`#

Added in: v16.2.0

- `value` <Object>
- Returns: <boolean>

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

#### `util.types.isMap(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isMapIterator(value)`#

Added in: v10.0.0

- `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

#### `util.types.isModuleNamespaceObject(value)`#

Added in: v10.0.0

- `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

#### `util.types.isNativeError(value)`#

Added in: v10.0.0Deprecated since: v24.2.0

Stability: 0 - Deprecated: Use `Error.isError` instead.

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

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 `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); // falseconst { 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` <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

#### `util.types.isPromise(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isProxy(value)`#

Added in: v10.0.0

- `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

#### `util.types.isRegExp(value)`#

Added in: v10.0.0

- `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

#### `util.types.isSet(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isSetIterator(value)`#

Added in: v10.0.0

- `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

#### `util.types.isSharedArrayBuffer(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isStringObject(value)`#

Added in: v10.0.0

- `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

#### `util.types.isSymbolObject(value)`#

Added in: v10.0.0

- `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

#### `util.types.isTypedArray(value)`#

Added in: v10.0.0

- `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

See also `ArrayBuffer.isView()`.

#### `util.types.isUint8Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isUint8ClampedArray(value)`#

Added in: v10.0.0

- `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

#### `util.types.isUint16Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isUint32Array(value)`#

Added in: v10.0.0

- `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

#### `util.types.isWeakMap(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

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

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

#### `util.types.isWeakSet(value)`#

Added in: v10.0.0

- `value` <any>
- Returns: <boolean>

Returns `true` if the value is a built-in <WeakSet> 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

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

- `target` <Object>
- `source` <Object>

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

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

An automated migration is available (source):

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

#### `util.isArray(object)`#

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

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

- `object` <any>
- Returns: <boolean>

Alias for `Array.isArray()`.

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

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

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

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is