GeezerVison

Util#

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

import util from 'node:util';
const util = require('node:util');
javascript

util.callbackify(original)#

• original <Function> An async function
• Returns: <Function> a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

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

callbackFunction((err, ret) => {
  if (err) throw err;
  console.log(ret);
});
const { callbackify } = require('node:util');

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

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

Will print:

hello world
text

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

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

import util from 'node:util';

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

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

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

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

util.convertProcessSignalToExitCode(signal)#

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

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

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

import { convertProcessSignalToExitCode } from 'node:util';

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

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

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

util.debuglog(section[, callback])#

• section <string> A string identifying the portion of the application for which the debuglog function is being created.
• callback <Function> A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.
• Returns: <Function> The logging function

The util.debuglog() method is used to create a function that conditionally writes debug messages to stderr based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned function operates similar to console.error(). If not, then the returned function is a no-op.

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

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

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

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

FOO 3245: hello from foo [123]
console

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);
javascript

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]
console

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;
});
javascript

debuglog().enabled#

• Type: <boolean>

The util.debuglog().enabled getter is used to create a test that can be used in conditionals based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned value will be true. If not, then the returned value will be false.

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

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

hello from foo [123]
console

util.debug(section)#

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

util.deprecate(fn, msg[, code[, options]])#

• fn <Function> The function that is being deprecated.
• msg <string> A warning message to display when the deprecated function is invoked.
• code <string> A deprecation code. See the list of deprecated APIs for a list of codes.
• options <Object>
  • modifyPrototype <boolean> When false do not change the prototype of object while emitting the deprecation warning. Default: true.
• Returns: <Function> The deprecated function wrapped to emit a warning.

The util.deprecate() method wraps fn (which may be a function or class) in such a way that it is marked as deprecated.

import { deprecate } from 'node:util';

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

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

When called, util.deprecate() will return a function that will emit a DeprecationWarning using the 'warning' event. The warning will be emitted and printed to stderr the first time the returned function is called. After the warning is emitted, the wrapped function is called without emitting a warning.

If the same optional code is supplied in multiple calls to util.deprecate(), the warning will be emitted only once for that code.

import { deprecate } from 'node:util';

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

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

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.debounce(fn, wait[, options])#

• fn <Function> The function to debounce.
• wait <integer> The number of milliseconds to delay fn.
• options <Object>
  • leading <boolean> When true, invokes fn immediately when a new debounce window begins. Default: false.
  • rejectOnCancel <boolean> When true, a call superseded by a later call rejects with an AbortError. Default: false.
  • signal <AbortSignal> An AbortSignal that cancels pending calls and prevents future calls when aborted.
• Returns: <Function> The debounced function.

Creates a function that delays calling fn until wait milliseconds have elapsed since the most recent invocation. The debounced function returns a <Promise> for the value returned by fn. If fn throws or returns a rejected promise, the returned promise is rejected with the same reason.

When the debounced function is called more than once before the delay expires, fn receives the arguments from the most recent call. By default, the promises from all calls resolve or reject with the result of that invocation. If options.rejectOnCancel is true, the promises from superseded calls reject with an AbortError instead.

When options.leading is true, the first call in a debounce window invokes fn immediately. Calls made during that window are delayed until wait milliseconds have elapsed since the most recent call. A trailing invocation only occurs if the debounced function was called again during the window. The window begins before fn is invoked, so recursive calls and calls made while an asynchronous fn is pending are part of the same window if they occur before the delay expires. This also applies to calls made after a synchronous fn returns but before the delay expires.

If options.signal is aborted, pending and future calls reject with an AbortError, with the signal's reason set as the error's cause, and fn is not invoked by those calls. If the signal is already aborted, debounce() throws an AbortError.

The returned function has the following properties:

• cancel([reason]) cancels the current debounce window. Its pending promises reject with an AbortError. If provided, reason is set as the error's cause.
• flush() cancels the delay and invokes fn immediately. It has no effect if no invocation is pending.
• pending <Promise> | <null> is the promise returned by the most recent call in the current debounce window, or null if no invocation is pending.
• pendingCount <integer> is the number of calls awaiting the invocation in the current debounce window.
• ref() makes the pending and future timeout keep the Node.js event loop active. Returns the debounced function.
• unref() allows the event loop to exit while a timeout is pending. This also applies to future timeouts. Returns the debounced function.

When invoked, fn has the debounced function as its this value. After a trailing invocation, a new debounce window can begin even if a promise returned by fn is still pending. The debounced function preserves the name and length of fn.

import { setTimeout as wait } from 'node:timers/promises';
import { debounce } from 'node:util';

const fn = debounce(async (value) => {
  await wait(100);
  return value;
}, 50);

const first = fn(1);
const second = fn(2);

console.log(await first);  // 2
console.log(await second); // 2
mjs

A debounced function can be used to trigger an action after a period of inactivity. Each call resets the timeout:

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

const onInactivity = debounce(() => {
  console.log('No activity for 5 seconds');
}, 5_000).unref();

process.stdin.on('data', (data) => {
  console.log(`Received ${data.length} bytes`);
  onInactivity();
});

// Start the initial inactivity timeout.
onInactivity();
cjs

util.throttle(fn, limit, interval[, options])#

• fn <Function> The function to throttle.
• limit <integer> The maximum number of times to invoke fn during an interval. Must be greater than 0.
• interval <integer> The length of each interval in milliseconds.
• options <Object>
  • concurrency <number> The maximum number of invocations of fn whose return values may be unsettled at once. Must be a positive integer or Infinity. Default: Infinity.
  • maxPending <number> The maximum number of calls that may be queued when overflow is 'queue'. Must be a non-negative integer or Infinity. Default: Infinity.
  • overflow <string> Determines how calls exceeding the limit are handled. Default: 'queue'.
    • 'queue': Queue calls in the order received.
    • 'drop': Reject calls immediately without queueing them.
  • signal <AbortSignal> An AbortSignal that cancels pending calls and prevents future calls when aborted.
  • strict <boolean> When true, ensures that limit is not exceeded during any rolling interval. Default: false.
• Returns: <Function> The throttled function.

Creates a function that limits how often fn is invoked. By default, calls that exceed the limit are queued in the order received rather than discarded. The throttled function returns a <Promise> for the value returned by fn. If fn throws or returns a rejected promise, the returned promise is rejected with the same reason.

An invocation starts only when both rate and concurrency capacity are available. Rate capacity is consumed when fn starts, not when a call enters the queue. Concurrency capacity is released when the value returned by fn settles. Non-promise values settle during the next microtask.

When options.overflow is 'drop', calls made without available rate or concurrency capacity are rejected immediately. When options.overflow is 'queue' and options.maxPending calls are already queued, additional calls are also rejected immediately. maxPending has no effect when overflow is 'drop'.

In both cases, rejected calls return a promise rejected with an ERR_THROTTLED error. The rejected promise is marked as handled, so ignoring it does not emit an 'unhandledRejection' event. Awaiting or explicitly handling the promise still observes the rejection. Rejected calls do not consume rate or concurrency capacity, enter the queue, or schedule a timeout.

By default, the interval begins when the first call in a new window invokes fn. Up to limit calls can invoke fn during that window. Queued calls are processed in groups of up to limit as each subsequent window begins. This windowed behavior can result in calls occurring close together at a window boundary.

When options.strict is true, invocation times are tracked individually. This ensures that no more than limit calls begin during any rolling interval, at the cost of additional bookkeeping.

If options.signal is aborted, pending and future calls reject with an AbortError, with the signal's reason set as the error's cause, and fn is not invoked by those calls. If the signal is already aborted, throttle() throws an AbortError.

The returned function has the following properties:

• cancel([reason]) cancels all queued calls and resets the current throttle window. The queued promises reject with an AbortError. If provided, reason is set as the error's cause. Does not cancel invocations that have already started.
• hasImmediateCapacity() returns true if a call made at that moment could invoke fn without being queued or rejected. The check does not reserve capacity, and the throttled function always checks again when called. It returns false while calls are queued to preserve their order. Callers can avoid creating a timeout by only calling the throttled function when this method returns true.
• pending <Promise> | <null> is the promise returned by the most recently queued call, or null if no invocation is queued.
• pendingCount <integer> is the number of calls awaiting invocation.
• activeCount <integer> is the number of invocations whose return values have not settled.
• ref() makes the pending and future timeout keep the Node.js event loop active. Returns the throttled function.
• unref() allows the event loop to exit while a timeout is pending. This also applies to future timeouts. Returns the throttled function.

Calls that have already invoked fn are not affected by cancel() or by an aborted signal. When invoked, fn has the throttled function as its this value. The throttled function preserves the name and length of fn.

import { throttle } from 'node:util';

const request = throttle(async (id) => {
  const response = await fetch(`https://example.com/items/${id}`);
  return response.json();
}, 2, 1_000);

// At most two requests begin during each one-second interval. All other calls
// remain queued and retain their original arguments.
const results = await Promise.all([
  request(1),
  request(2),
  request(3),
  request(4),
]);
mjs

util.diff(actual, expected)#

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

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

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

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

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

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

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

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

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

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

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

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

• format <string> A printf-like format string.

The util.format() method returns a formatted string using the first argument as a printf-like format string which can contain zero or more format specifiers. Each specifier is replaced with the converted value from the corresponding argument. Supported specifiers are:

• %s: String will be used to convert all values except BigInt, Object and -0. BigInt values will be represented with an n and Objects that have neither a user defined toString function nor Symbol.toPrimitive function are inspected using util.inspect() with options { depth: 0, colors: false, compact: 3 }.
• %d: Number will be used to convert all values except BigInt and Symbol.
• %i: parseInt(value, 10) is used for all values except BigInt and Symbol.
• %f: parseFloat(value) is used for all values except 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'
js

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'
js

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'
js

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

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

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

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

• inspectOptions <Object>
• format <string>

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

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

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

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

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

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

import { getCallSites } from 'node:util';

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

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

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

  // ...
}

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

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

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

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

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

  // ...
}

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

anotherFunction();
javascript

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

import { getCallSites } from 'node:util';

interface Foo {
  foo: string;
}

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

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

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

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
cjs

util.getSystemErrorName(err)#

• err <number>
• Returns: <string>

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

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

util.getSystemErrorMap()#

• Returns: <Map>

Returns a Map of all system error codes available from the Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

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

util.getSystemErrorMessage(err)#

• err <number>
• Returns: <string>

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

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

util.setTraceSigInt(enable)#

• enable <boolean>

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

util.inherits(constructor, superConstructor)#

• constructor <Function>
• superConstructor <Function>

Usage of util.inherits() is discouraged. Please use the ES6 class and extends keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

Inherit the prototype methods from one constructor into another. The prototype of constructor will be set to a new object created from superConstructor.

This mainly adds some input validation on top of Object.setPrototypeOf(constructor.prototype, superConstructor.prototype). As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

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

function MyStream() {
  EventEmitter.call(this);
}

util.inherits(MyStream, EventEmitter);

MyStream.prototype.write = function(data) {
  this.emit('data', data);
};

const stream = new MyStream();

console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"
js

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');
javascript

util.inspect(object[, options])#

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

• object <any> Any JavaScript primitive or Object.
• options <Object>
  • showHidden <boolean> If true, object's non-enumerable symbols and properties are included in the formatted result. <WeakMap> and <WeakSet> entries are also included as well as user defined prototype properties (excluding method properties). Default: false.
  • depth <number> Specifies the number of times to recurse while formatting object. This is useful for inspecting large objects. To recurse up to the maximum call stack size pass Infinity or null. Default: 2.
  • colors <boolean> If true, the output is styled with ANSI color codes. Colors are customizable. See Customizing util.inspect colors. Default: false.
  • customInspect <boolean> If false, [util.inspect.custom](depth, opts, inspect) functions are not invoked. Default: true.
  • showProxy <boolean> If true, Proxy inspection includes the target and handler objects. Default: false.
  • maxArrayLength <integer> Specifies the maximum number of Array, <TypedArray>, <Map>, <WeakMap>, and <WeakSet> elements to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no elements. Default: 100.
  • maxStringLength <integer> Specifies the maximum number of characters to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no characters. Default: 10000.
  • breakLength <integer> The length at which input values are split across multiple lines. Set to Infinity to format the input as a single line (in combination with compact set to true or any number >= 1). Default: 80.
  • compact <boolean> | <integer> Setting this to false causes each object key to be displayed on a new line. It will break on new lines in text that is longer than breakLength. If set to a number, the most n inner elements are united on a single line as long as all properties fit into breakLength. Short array elements are also grouped together. For more information, see the example below. Default: 3.
  • sorted <boolean> | <Function> If set to true or a function, all properties of an object, and Set and Map entries are sorted in the resulting string. If set to true the default sort is used. If set to a function, it is used as a compare function.
  • getters <boolean> | <string> If set to true, getters are inspected. If set to 'get', only getters without a corresponding setter are inspected. If set to 'set', only getters with a corresponding setter are inspected. This might cause side effects depending on the getter function. Default: false.
  • numericSeparator <boolean> If set to true, an underscore is used to separate every three digits in all bigints and numbers. Default: false.
• Returns: <string> The representation of object.

The util.inspect() method returns a string representation of object that is intended for debugging. The output of util.inspect may change at any time and should not be depended upon programmatically. Additional options may be passed that alter the result. util.inspect() will use the constructor's name and/or Symbol.toStringTag property to make an identifiable tag for an inspected value.

class Foo {
  get [Symbol.toStringTag]() {
    return 'bar';
  }
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz);       // '[foo] {}'
js

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] }
// }
javascript

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 }));
javascript

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.
javascript

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 } }
javascript

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 }),
);
javascript

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

import { inspect } from 'node:util';

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

console.log(inspect(thousand, { numericSeparator: true }));
// 1_000
console.log(inspect(million, { numericSeparator: true }));
// 1_000_000
console.log(inspect(bigNumber, { numericSeparator: true }));
// 123_456_789n
console.log(inspect(bigDecimal, { numericSeparator: true }));
// 1_234.123_45
const { inspect } = require('node:util');

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

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

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

Customizing util.inspect colors#

Color output (if enabled) of util.inspect is customizable globally via the util.inspect.styles and util.inspect.colors properties.

util.inspect.styles is a map associating a style name to a color from util.inspect.colors.

The default styles and associated colors are:

• bigint: yellow
• boolean: yellow
• date: magenta
• module: underline
• name: (no styling)
• null: bold
• number: yellow
• regexp: A method that colors character classes, groups, assertions, and other parts for improved readability. To customize the coloring, change the colors property. It is set to ['red', 'green', 'yellow', 'cyan', 'magenta'] by default and may be adjusted as needed. The array is repetitively iterated through depending on the "depth".
• special: cyan (e.g., Proxies)
• string: green
• symbol: green
• undefined: grey

Color styling uses ANSI control codes that may not be supported on all terminals. To verify color support use tty.hasColors().

Predefined control codes are listed below (grouped as "Modifiers", "Foreground colors", and "Background colors").

Complex custom coloring#

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

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

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

Modifiers#

Modifier support varies throughout different terminals. They will mostly be ignored, if not supported.

• reset - Resets all (color) modifiers to their defaults
• bold - Make text bold
• italic - Make text italic
• underline - Make text underlined
• strikethrough - Puts a horizontal line through the center of the text (Alias: strikeThrough, crossedout, crossedOut)
• hidden - Prints the text, but makes it invisible (Alias: conceal)
• dim - Decreased color intensity (Alias: faint)
• overlined - Make text overlined
• blink - Hides and shows the text in an interval
• inverse - Swap foreground and background colors (Alias: swapcolors, swapColors)
• doubleunderline - Make text double underlined (Alias: doubleUnderline)
• framed - Draw a frame around the text

Foreground colors#

• black
• red
• green
• yellow
• blue
• magenta
• cyan
• white
• gray (alias: grey, blackBright)
• redBright
• greenBright
• yellowBright
• blueBright
• magentaBright
• cyanBright
• whiteBright

Background colors#

• bgBlack
• bgRed
• bgGreen
• bgYellow
• bgBlue
• bgMagenta
• bgCyan
• bgWhite
• bgGray (alias: bgGrey, bgBlackBright)
• bgRedBright
• bgGreenBright
• bgYellowBright
• bgBlueBright
• bgMagentaBright
• bgCyanBright
• bgWhiteBright

Custom inspection functions on objects#

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

import { inspect } from 'node:util';

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

  [inspect.custom](depth, options, inspect) {
    if (depth < 0) {
      return options.stylize('[Box]', 'special');
    }

    const newOptions = Object.assign({}, options, {
      depth: options.depth === null ? null : options.depth - 1,
    });

    // Five space padding because that's the size of "Box< ".
    const padding = ' '.repeat(5);
    const inner = inspect(this.value, newOptions)
                  .replace(/\n/g, `\n${padding}`);
    return `${options.stylize('Box', 'special')}< ${inner} >`;
  }
}

const box = new Box(true);

console.log(inspect(box));
// "Box< true >"
const { inspect } = require('node:util');

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

  [inspect.custom](depth, options, inspect) {
    if (depth < 0) {
      return options.stylize('[Box]', 'special');
    }

    const newOptions = Object.assign({}, options, {
      depth: options.depth === null ? null : options.depth - 1,
    });

    // Five space padding because that's the size of "Box< ".
    const padding = ' '.repeat(5);
    const inner = inspect(this.value, newOptions)
                  .replace(/\n/g, `\n${padding}`);
    return `${options.stylize('Box', 'special')}< ${inner} >`;
  }
}

const box = new Box(true);

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

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' }"
javascript

util.inspect.custom#

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

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

Using this allows code to be written in a portable fashion, so that the custom inspect function is used in a 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>
js

See Custom inspection functions on Objects for more details.

util.inspect.defaultOptions#

The defaultOptions value allows customization of the default options used by util.inspect. This is useful for functions like console.log or util.format which implicitly call into util.inspect. It shall be set to an object containing one or more valid util.inspect() options. Setting option properties directly is also supported.

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

console.log(arr); // Logs the truncated array
inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array
const { inspect } = require('node:util');
const arr = Array(156).fill(0);

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

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

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

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

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

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

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

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

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

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

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

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

util.isPartialDeepStrictEqual(val1, val2)#

• val1 <any>
• val2 <any>
• Returns: <boolean>

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

"Partial" equality means that only properties that exist on val2 are going to be compared.

See assert.partialDeepStrictEqual() for more information about partial deep strict equality.

util.markPromiseAsHandled(promise)#

• promise <Promise> The promise to mark as handled

Marks a promise as handled so that unhandled rejections are ignored and are not reported to the 'unhandledrejection' event.

Class: util.MIMEType#

An implementation of the MIMEType class.

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

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

new MIMEType(input)#

• input <string> The input MIME to parse

Creates a new MIMEType object by parsing the input.

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/plain');
const { MIMEType } = require('node:util');

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

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

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

mime.type#

• Type: <string>

Gets and sets the type portion of the MIME.

import { MIMEType } from 'node:util';

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

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

mime.subtype#

• Type: <string>

Gets and sets the subtype portion of the MIME.

import { MIMEType } from 'node:util';

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

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

mime.essence#

• Type: <string>

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

import { MIMEType } from 'node:util';

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

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

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"]
javascript

MIMEType.parse(string)#

• string <string> The input MIME to parse
• Returns: <MIMEType> | <null>

Attempts to parse the given string as a MIMEType. If the string cannot be parsed, null is returned.

Class: util.MIMEParams#

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

new MIMEParams()#

Creates a new MIMEParams object by with empty parameters

import { MIMEParams } from 'node:util';

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

const myParams = new MIMEParams();
javascript

mimeParams.delete(name)#

• name <string>

Remove all name-value pairs whose name is name.

mimeParams.entries()#

• Returns: <Iterator>

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

mimeParams.get(name)#

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

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

mimeParams.has(name)#

• name <string>
• Returns: <boolean>

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

mimeParams.keys()#

• Returns: <Iterator>

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

import { MIMEType } from 'node:util';

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

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

mimeParams.set(name, value)#

• name <string>
• value <string>

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

import { MIMEType } from 'node:util';

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

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

mimeParams.values()#

• Returns: <Iterator>

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

mimeParams[Symbol.iterator]()#

• Returns: <Iterator>

Alias for mimeParams.entries().

import { MIMEType } from 'node:util';

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

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

util.parseArgs([config])#

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

  • args <string>[] array of argument strings. Default: process.argv with execPath and filename removed.
  • options <Object> Used to describe arguments known to the parser. Keys of options are the long names of options and values are an <Object> accepting the following properties:
    • type <string> Type of argument, which must be either boolean or string.
    • multiple <boolean> Whether this option can be provided multiple times. If true, all values will be collected in an array. If false, values for the option are last-wins. Default: false.
    • short <string> A single character alias for the option.
    • default <string> | <boolean> | <string>[] | <boolean>[] The value to assign to the option if it does not appear in the arguments to be parsed. The value must match the type specified by the type property. If multiple is true, it must be an array. No default value is applied when the option does appear in the arguments to be parsed, even if the provided value is falsy.
  • strict <boolean> Should an error be thrown when unknown arguments are encountered, or when arguments are passed that do not match the type configured in options. Default: true.
  • allowPositionals <boolean> Whether this command accepts positional arguments. Default: false if strict is true, otherwise true.
  • allowNegative <boolean> If true, allows explicitly setting boolean options to false by prefixing the option name with --no-. Default: false.
  • tokens <boolean> Return the parsed tokens. This is useful for extending the built-in behavior, from adding additional checks through to reprocessing the tokens in different ways. Default: false.

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

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

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

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

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 });
javascript