Util

Util

Stability: 2 - Stable

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

import util from 'node:util';

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

`util.callbackify(original)`

original {Function} An async function
Returns: {Function} a callback style function

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

import { callbackify } from 'node:util';

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

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

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

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

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

Will print:

hello world

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

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

import util from 'node:util';

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

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

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

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

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

`util.convertProcessSignalToExitCode(signal)`

signal {string} A signal name (e.g. 'SIGTERM')
Returns: {number} The exit code corresponding to signal

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

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

import { convertProcessSignalToExitCode } from 'node:util';

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

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

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

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

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

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

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

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

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

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

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

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

FOO 3245: hello from foo [123]

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`

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)`

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

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

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

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

import { deprecate } from 'node:util';

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

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

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

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

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

import { deprecate } from 'node:util';

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

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

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

If either the --no-deprecation or --no-warnings command-line flags are used, or if the process.noDeprecation property is set to true prior to the first deprecation warning, the util.deprecate() method does nothing.

If the --trace-deprecation or --trace-warnings command-line flags are set, or the process.traceDeprecation property is set to true, a warning and a stack trace are printed to stderr the first time the deprecated function is called.

If the --throw-deprecation command-line flag is set, or the process.throwDeprecation property is set to true, then an exception will be thrown when the deprecated function is called.

The --throw-deprecation command-line flag and process.throwDeprecation property take precedence over --trace-deprecation and process.traceDeprecation.

`util.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

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();

`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.

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),
]);

`util.diff(actual, expected)`

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])`

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'

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])`

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])`

Stability: 1.1 - Active development

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

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

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

import { getCallSites } from 'node:util';

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

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

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

  // ...
}

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

anotherFunction();

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

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

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

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

  // ...
}

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

anotherFunction();

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

import { getCallSites } from 'node:util';

interface Foo {
  foo: string;
}

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

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

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

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)`

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()`

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)`

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)`

enable {boolean}

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

`util.inherits(constructor, superConstructor)`

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]]])`

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.

```cjs 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

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