Node.js module

cluster

The 'node:cluster' module provides a way to create a small network of processes to share server ports and distribute workload across CPU cores. It uses the fork() mechanism to spawn worker processes that share the same server handles.

Clustered applications can handle more concurrent connections by distributing requests among workers. The module includes utilities for worker management, messaging, and graceful shutdown.

Works in Bun

Implemented but not battle-tested. Handles and file descriptors cannot be passed between workers, limiting TCP server load-balancing across processes to Linux (via SO_REUSEPORT).

class Worker
A Worker object contains all public information and method about a worker. In the primary it can be obtained using cluster.workers. In a worker it can be obtained using cluster.worker.
- exitedAfterDisconnect: boolean
  This property is true if the worker exited due to .disconnect(). If the worker exited any other way, it is false. If the worker has not exited, it is undefined.
  The boolean worker.exitedAfterDisconnect allows distinguishing between voluntary and accidental exit, the primary may choose not to respawn a worker based on this value.
  cluster.on('exit', (worker, code, signal) => { if (worker.exitedAfterDisconnect === true) { console.log('Oh, it was just voluntary – no need to worry'); } }); // kill worker worker.kill();
- id: number
  Each new worker is given its own unique id, this id is stored in the id.
  While a worker is alive, this is the key that indexes it in cluster.workers.
- process: ChildProcess
  All workers are created using child_process.fork(), the returned object from this function is stored as .process. In a worker, the global process is stored.
  See: Child Process module.
  Workers will call process.exit(0) if the 'disconnect' event occurs on process and .exitedAfterDisconnect is not true. This protects against accidental disconnection.
- static captureRejections: boolean
  Value: boolean
  Change the default captureRejections option on all new EventEmitter objects.
- readonly static captureRejectionSymbol: typeof captureRejectionSymbol
  Value: Symbol.for('nodejs.rejection')
  See how to write a custom rejection handler.
- static defaultMaxListeners: number
  By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.
  Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.
  This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:
  import { EventEmitter } from 'node:events'; const emitter = new EventEmitter(); emitter.setMaxListeners(emitter.getMaxListeners() + 1); emitter.once('event', () => { // do stuff emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0)); });
  The --trace-warnings command-line flag can be used to display the stack trace for such warnings.
  The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.
- readonly static errorMonitor: typeof errorMonitor
  This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.
  Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.
- [events.captureRejectionSymbol]<K>(
  error: Error,
  event: string | symbol,
  ...args: AnyRest
  ): void;
- addListener(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  events.EventEmitter
  disconnect
  error
  exit
  listening
  message
  online
  addListener(
  event: 'disconnect',
  listener: () => void
  ): this;
  events.EventEmitter
  disconnect
  error
  exit
  listening
  message
  online
  addListener(
  event: 'error',
  listener: (error: Error) => void
  ): this;
  events.EventEmitter
  disconnect
  error
  exit
  listening
  message
  online
  addListener(
  event: 'exit',
  listener: (code: number, signal: string) => void
  ): this;
  events.EventEmitter
  disconnect
  error
  exit
  listening
  message
  online
  addListener(
  event: 'listening',
  listener: (address: Address) => void
  ): this;
  events.EventEmitter
  disconnect
  error
  exit
  listening
  message
  online
  addListener(
  event: 'message',
  listener: (message: any, handle: Socket | Server) => void
  ): this;
  events.EventEmitter
  disconnect
  error
  exit
  listening
  message
  online
  addListener(
  event: 'online',
  listener: () => void
  ): this;
  events.EventEmitter
  disconnect
  error
  exit
  listening
  message
  online
- destroy(
  signal?: string
  ): void;
- disconnect(): this;
  In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.
  In the primary, an internal message is sent to the worker causing it to call .disconnect() on itself.
  Causes .exitedAfterDisconnect to be set.
  After a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close(), the IPC channel to the worker will close allowing it to die gracefully.
  The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.
  In a worker, process.disconnect exists, but it is not this function; it is disconnect().
  Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time.
  import net from 'node:net'; if (cluster.isPrimary) { const worker = cluster.fork(); let timeout; worker.on('listening', (address) => { worker.send('shutdown'); worker.disconnect(); timeout = setTimeout(() => { worker.kill(); }, 2000); }); worker.on('disconnect', () => { clearTimeout(timeout); }); } else if (cluster.isWorker) { const server = net.createServer((socket) => { // Connections never end }); server.listen(8000); process.on('message', (msg) => { if (msg === 'shutdown') { // Initiate graceful close of any connections to server } }); }
  @returns
  A reference to worker.
- emit(
  event: string | symbol,
  ...args: any[]
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'disconnect'
  ): boolean;
  emit(
  event: 'error',
  error: Error
  ): boolean;
  emit(
  event: 'exit',
  code: number,
  signal: string
  ): boolean;
  emit(
  event: 'listening',
  address: Address
  ): boolean;
  emit(
  event: 'message',
  message: any,
  handle: Socket | Server
  ): boolean;
  emit(
  event: 'online'
  ): boolean;
- eventNames(): string | symbol[];
  Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => {}); myEE.on('bar', () => {}); const sym = Symbol('symbol'); myEE.on(sym, () => {}); console.log(myEE.eventNames()); // Prints: [ 'foo', 'bar', Symbol(symbol) ]
- getMaxListeners(): number;
  Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.
- isConnected(): boolean;
  This function returns true if the worker is connected to its primary via its IPC channel, false otherwise. A worker is connected to its primary after it has been created. It is disconnected after the 'disconnect' event is emitted.
- isDead(): boolean;
  This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false.
  import cluster from 'node:cluster'; import http from 'node:http'; import { availableParallelism } from 'node:os'; import process from 'node:process'; const numCPUs = availableParallelism(); if (cluster.isPrimary) { console.log(`Primary ${process.pid} is running`); // Fork workers. for (let i = 0; i < numCPUs; i++) { cluster.fork(); } cluster.on('fork', (worker) => { console.log('worker is dead:', worker.isDead()); }); cluster.on('exit', (worker, code, signal) => { console.log('worker is dead:', worker.isDead()); }); } else { // Workers can share any TCP connection. In this case, it is an HTTP server. http.createServer((req, res) => { res.writeHead(200); res.end(`Current process\n ${process.pid}`); process.kill(process.pid); }).listen(8000); }
- kill(
  signal?: string
  ): void;
  This function will kill the worker. In the primary worker, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by killing the process with signal.
  The kill() function kills the worker process without waiting for a graceful disconnect, it has the same behavior as worker.process.kill().
  This method is aliased as worker.destroy() for backwards compatibility.
  In a worker, process.kill() exists, but it is not this function; it is kill().
  @param signal
  Name of the kill signal to send to the worker process.
- listenerCount<K>(
  eventName: string | symbol,
  listener?: Function
  ): number;
  Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.
  @param eventName
  The name of the event being listened for
  @param listener
  The event handler function
- listeners<K>(
  eventName: string | symbol
  ): Function[];
  Returns a copy of the array of listeners for the event named eventName.
  server.on('connection', (stream) => { console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ]
- off<K>(
  eventName: string | symbol,
  listener: (...args: any[]) => void
  ): this;
  Alias for emitter.removeListener().
- on(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'disconnect',
  listener: () => void
  ): this;
  on(
  event: 'error',
  listener: (error: Error) => void
  ): this;
  on(
  event: 'exit',
  listener: (code: number, signal: string) => void
  ): this;
  on(
  event: 'listening',
  listener: (address: Address) => void
  ): this;
  on(
  event: 'message',
  listener: (message: any, handle: Socket | Server) => void
  ): this;
  on(
  event: 'online',
  listener: () => void
  ): this;
- once(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'disconnect',
  listener: () => void
  ): this;
  once(
  event: 'error',
  listener: (error: Error) => void
  ): this;
  once(
  event: 'exit',
  listener: (code: number, signal: string) => void
  ): this;
  once(
  event: 'listening',
  listener: (address: Address) => void
  ): this;
  once(
  event: 'message',
  listener: (message: any, handle: Socket | Server) => void
  ): this;
  once(
  event: 'online',
  listener: () => void
  ): this;
- prependListener(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'disconnect',
  listener: () => void
  ): this;
  prependListener(
  event: 'error',
  listener: (error: Error) => void
  ): this;
  prependListener(
  event: 'exit',
  listener: (code: number, signal: string) => void
  ): this;
  prependListener(
  event: 'listening',
  listener: (address: Address) => void
  ): this;
  prependListener(
  event: 'message',
  listener: (message: any, handle: Socket | Server) => void
  ): this;
  prependListener(
  event: 'online',
  listener: () => void
  ): this;
- prependOnceListener(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'disconnect',
  listener: () => void
  ): this;
  prependOnceListener(
  event: 'error',
  listener: (error: Error) => void
  ): this;
  prependOnceListener(
  event: 'exit',
  listener: (code: number, signal: string) => void
  ): this;
  prependOnceListener(
  event: 'listening',
  listener: (address: Address) => void
  ): this;
  prependOnceListener(
  event: 'message',
  listener: (message: any, handle: Socket | Server) => void
  ): this;
  prependOnceListener(
  event: 'online',
  listener: () => void
  ): this;
- rawListeners<K>(
  eventName: string | symbol
  ): Function[];
  Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).
  import { EventEmitter } from 'node:events'; const emitter = new EventEmitter(); emitter.once('log', () => console.log('log once')); // Returns a new Array with a function `onceWrapper` which has a property // `listener` which contains the original listener bound above const listeners = emitter.rawListeners('log'); const logFnWrapper = listeners[0]; // Logs "log once" to the console and does not unbind the `once` event logFnWrapper.listener(); // Logs "log once" to the console and removes the listener logFnWrapper(); emitter.on('log', () => console.log('log persistently')); // Will return a new Array with a single function bound by `.on()` above const newListeners = emitter.rawListeners('log'); // Logs "log persistently" twice newListeners[0](); emitter.emit('log');
- removeAllListeners(
  eventName?: string | symbol
  ): this;
  Removes all listeners, or those of the specified eventName.
  It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).
  Returns a reference to the EventEmitter, so that calls can be chained.
- removeListener<K>(
  eventName: string | symbol,
  listener: (...args: any[]) => void
  ): this;
  Removes the specified listener from the listener array for the event named eventName.
  const callback = (stream) => { console.log('someone connected!'); }; server.on('connection', callback); // ... server.removeListener('connection', callback);
  removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.
  Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.
  import { EventEmitter } from 'node:events'; class MyEmitter extends EventEmitter {} const myEmitter = new MyEmitter(); const callbackA = () => { console.log('A'); myEmitter.removeListener('event', callbackB); }; const callbackB = () => { console.log('B'); }; myEmitter.on('event', callbackA); myEmitter.on('event', callbackB); // callbackA removes listener callbackB but it will still be called. // Internal listener array at time of emit [callbackA, callbackB] myEmitter.emit('event'); // Prints: // A // B // callbackB is now removed. // Internal listener array [callbackA] myEmitter.emit('event'); // Prints: // A
  Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.
  When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:
  import { EventEmitter } from 'node:events'; const ee = new EventEmitter(); function pong() { console.log('pong'); } ee.on('ping', pong); ee.once('ping', pong); ee.removeListener('ping', pong); ee.emit('ping'); ee.emit('ping');
  Returns a reference to the EventEmitter, so that calls can be chained.
- send(
  message: Serializable,
  callback?: (error: null | Error) => void
  ): boolean;
  Send a message to a worker or primary, optionally with a handle.
  In the primary, this sends a message to a specific worker. It is identical to ChildProcess.send().
  In a worker, this sends a message to the primary. It is identical to process.send().
  This example will echo back all messages from the primary:
  if (cluster.isPrimary) { const worker = cluster.fork(); worker.send('hi there'); } else if (cluster.isWorker) { process.on('message', (msg) => { process.send(msg); }); }
  send(
  message: Serializable,
  sendHandle: SendHandle,
  callback?: (error: null | Error) => void
  ): boolean;
  Send a message to a worker or primary, optionally with a handle.
  In the primary, this sends a message to a specific worker. It is identical to ChildProcess.send().
  In a worker, this sends a message to the primary. It is identical to process.send().
  This example will echo back all messages from the primary:
  if (cluster.isPrimary) { const worker = cluster.fork(); worker.send('hi there'); } else if (cluster.isWorker) { process.on('message', (msg) => { process.send(msg); }); }
  send(
  message: Serializable,
  sendHandle: SendHandle,
  options?: MessageOptions,
  callback?: (error: null | Error) => void
  ): boolean;
  Send a message to a worker or primary, optionally with a handle.
  In the primary, this sends a message to a specific worker. It is identical to ChildProcess.send().
  In a worker, this sends a message to the primary. It is identical to process.send().
  This example will echo back all messages from the primary:
  if (cluster.isPrimary) { const worker = cluster.fork(); worker.send('hi there'); } else if (cluster.isWorker) { process.on('message', (msg) => { process.send(msg); }); }
  @param options
  The options argument, if present, is an object used to parameterize the sending of certain types of handles.
- setMaxListeners(
  n: number
  ): this;
  By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.
  Returns a reference to the EventEmitter, so that calls can be chained.
- static addAbortListener(
  signal: AbortSignal,
  resource: (event: Event) => void
  ): Disposable;
  Listens once to the abort event on the provided signal.
  Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.
  This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.
  Returns a disposable so that it may be unsubscribed from more easily.
  import { addAbortListener } from 'node:events'; function example(signal) { let disposable; try { signal.addEventListener('abort', (e) => e.stopImmediatePropagation()); disposable = addAbortListener(signal, (e) => { // Do something when signal is aborted. }); } finally { disposable?.[Symbol.dispose](); } }
  @returns
  Disposable that removes the abort listener.
- static getEventListeners(
  emitter: EventEmitter<DefaultEventMap> | EventTarget,
  name: string | symbol
  ): Function[];
  Returns a copy of the array of listeners for the event named eventName.
  For EventEmitters this behaves exactly the same as calling .listeners on the emitter.
  For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.
  import { getEventListeners, EventEmitter } from 'node:events'; { const ee = new EventEmitter(); const listener = () => console.log('Events are fun'); ee.on('foo', listener); console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ] } { const et = new EventTarget(); const listener = () => console.log('Events are fun'); et.addEventListener('foo', listener); console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ] }
- static getMaxListeners(
  emitter: EventEmitter<DefaultEventMap> | EventTarget
  ): number;
  Returns the currently set max amount of listeners.
  For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.
  For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.
  import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events'; { const ee = new EventEmitter(); console.log(getMaxListeners(ee)); // 10 setMaxListeners(11, ee); console.log(getMaxListeners(ee)); // 11 } { const et = new EventTarget(); console.log(getMaxListeners(et)); // 10 setMaxListeners(11, et); console.log(getMaxListeners(et)); // 11 }
- static on(
  emitter: EventEmitter,
  eventName: string | symbol,
  options?: StaticEventEmitterIteratorOptions
  ): AsyncIterator<any[]>;
  import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here
  Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.
  An AbortSignal can be used to cancel waiting on events:
  import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ac = new AbortController(); (async () => { const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo', { signal: ac.signal })) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here })(); process.nextTick(() => ac.abort());
  Use the close option to specify an array of event names that will end the iteration:
  import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); ee.emit('close'); }); for await (const event of on(ee, 'foo', { close: ['close'] })) { console.log(event); // prints ['bar'] [42] } // the loop will exit after 'close' is emitted console.log('done'); // prints 'done'
  @returns
  An AsyncIterator that iterates eventName events emitted by the emitter
  static on(
  emitter: EventTarget,
  eventName: string,
  options?: StaticEventEmitterIteratorOptions
  ): AsyncIterator<any[]>;
  import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here
  Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.
  An AbortSignal can be used to cancel waiting on events:
  import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ac = new AbortController(); (async () => { const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo', { signal: ac.signal })) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here })(); process.nextTick(() => ac.abort());
  Use the close option to specify an array of event names that will end the iteration:
  import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); ee.emit('close'); }); for await (const event of on(ee, 'foo', { close: ['close'] })) { console.log(event); // prints ['bar'] [42] } // the loop will exit after 'close' is emitted console.log('done'); // prints 'done'
  @returns
  An AsyncIterator that iterates eventName events emitted by the emitter
- static once(
  emitter: EventEmitter,
  eventName: string | symbol,
  options?: StaticEventEmitterOptions
  ): Promise<any[]>;
  Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.
  This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.
  import { once, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); process.nextTick(() => { ee.emit('myevent', 42); }); const [value] = await once(ee, 'myevent'); console.log(value); const err = new Error('kaboom'); process.nextTick(() => { ee.emit('error', err); }); try { await once(ee, 'myevent'); } catch (err) { console.error('error happened', err); }
  The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:
  import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); once(ee, 'error') .then(([err]) => console.log('ok', err.message)) .catch((err) => console.error('error', err.message)); ee.emit('error', new Error('boom')); // Prints: ok boom
  An AbortSignal can be used to cancel waiting for the event:
  import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); const ac = new AbortController(); async function foo(emitter, event, signal) { try { await once(emitter, event, { signal }); console.log('event emitted!'); } catch (error) { if (error.name === 'AbortError') { console.error('Waiting for the event was canceled!'); } else { console.error('There was an error', error.message); } } } foo(ee, 'foo', ac.signal); ac.abort(); // Abort waiting for the event ee.emit('foo'); // Prints: Waiting for the event was canceled!
  static once(
  emitter: EventTarget,
  eventName: string,
  options?: StaticEventEmitterOptions
  ): Promise<any[]>;
  Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.
  This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.
  import { once, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); process.nextTick(() => { ee.emit('myevent', 42); }); const [value] = await once(ee, 'myevent'); console.log(value); const err = new Error('kaboom'); process.nextTick(() => { ee.emit('error', err); }); try { await once(ee, 'myevent'); } catch (err) { console.error('error happened', err); }
  The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:
  import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); once(ee, 'error') .then(([err]) => console.log('ok', err.message)) .catch((err) => console.error('error', err.message)); ee.emit('error', new Error('boom')); // Prints: ok boom
  An AbortSignal can be used to cancel waiting for the event:
  import { EventEmitter, once } from 'node:events'; const ee = new EventEmitter(); const ac = new AbortController(); async function foo(emitter, event, signal) { try { await once(emitter, event, { signal }); console.log('event emitted!'); } catch (error) { if (error.name === 'AbortError') { console.error('Waiting for the event was canceled!'); } else { console.error('There was an error', error.message); } } } foo(ee, 'foo', ac.signal); ac.abort(); // Abort waiting for the event ee.emit('foo'); // Prints: Waiting for the event was canceled!
- static setMaxListeners(
  n?: number,
  ...eventTargets: EventEmitter<DefaultEventMap> | EventTarget[]
  ): void;
  import { setMaxListeners, EventEmitter } from 'node:events'; const target = new EventTarget(); const emitter = new EventEmitter(); setMaxListeners(5, target, emitter);
  @param n
  A non-negative number. The maximum number of listeners per EventTarget event.
  @param eventTargets
  Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, n is set as the default max for all newly created {EventTarget} and {EventEmitter} objects.

Type definitions

interface Address
- address: string
- addressType: 4 | 6 | -1 | 'udp4' | 'udp6'
  The addressType is one of:
  4 (TCPv4)
  6 (TCPv6)
  -1 (Unix domain socket)
  'udp4' or 'udp6' (UDPv4 or UDPv6)
- port: number
interface Cluster
The EventEmitter class is defined and exposed by the node:events module:
import { EventEmitter } from 'node:events';
All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.
It supports the following option:
- readonly isPrimary: boolean
  True if the process is a primary. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isPrimary is true.
- readonly isWorker: boolean
  True if the process is not a primary (it is the negation of cluster.isPrimary).
- readonly SCHED_NONE: number
- readonly SCHED_RR: number
- schedulingPolicy: number
  The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once either the first worker is spawned, or .setupPrimary() is called, whichever comes first.
  SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.
  cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are 'rr' and 'none'.
- readonly settings: ClusterSettings
  After calling .setupPrimary() (or .fork()) this settings object will contain the settings, including the default values.
  This object is not intended to be changed or set manually.
- readonly worker?: Worker
  A reference to the current worker object. Not available in the primary process.
  import cluster from 'node:cluster'; if (cluster.isPrimary) { console.log('I am primary'); cluster.fork(); cluster.fork(); } else if (cluster.isWorker) { console.log(`I am worker #${cluster.worker.id}`); }
- readonly workers?: Dict<Worker>
  A hash that stores the active worker objects, keyed by id field. This makes it easy to loop through all the workers. It is only available in the primary process.
  A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before the last 'disconnect' or 'exit' event is emitted.
  import cluster from 'node:cluster'; for (const worker of Object.values(cluster.workers)) { worker.send('big announcement to all workers'); }
- [events.captureRejectionSymbol]<K>(
  error: Error,
  event: string | symbol,
  ...args: AnyRest
  ): void;
- addListener(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  events.EventEmitter
  disconnect
  exit
  fork
  listening
  message
  online
  setup
  addListener(
  event: 'disconnect',
  listener: (worker: Worker) => void
  ): this;
  Alias for emitter.on(eventName, listener).
  addListener(
  event: 'exit',
  listener: (worker: Worker, code: number, signal: string) => void
  ): this;
  Alias for emitter.on(eventName, listener).
  addListener(
  event: 'fork',
  listener: (worker: Worker) => void
  ): this;
  Alias for emitter.on(eventName, listener).
  addListener(
  event: 'listening',
  listener: (worker: Worker, address: Address) => void
  ): this;
  Alias for emitter.on(eventName, listener).
  addListener(
  event: 'message',
  listener: (worker: Worker, message: any, handle: Socket | Server) => void
  ): this;
  Alias for emitter.on(eventName, listener).
  addListener(
  event: 'online',
  listener: (worker: Worker) => void
  ): this;
  Alias for emitter.on(eventName, listener).
  addListener(
  event: 'setup',
  listener: (settings: ClusterSettings) => void
  ): this;
  Alias for emitter.on(eventName, listener).
- disconnect(
  callback?: () => void
  ): void;
- emit(
  event: string | symbol,
  ...args: any[]
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'disconnect',
  worker: Worker
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'exit',
  worker: Worker,
  code: number,
  signal: string
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'fork',
  worker: Worker
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'listening',
  worker: Worker,
  address: Address
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'message',
  worker: Worker,
  message: any,
  handle: Socket | Server
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'online',
  worker: Worker
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
  emit(
  event: 'setup',
  settings: ClusterSettings
  ): boolean;
  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.
  Returns true if the event had listeners, false otherwise.
  import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
- eventNames(): string | symbol[];
  Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => {}); myEE.on('bar', () => {}); const sym = Symbol('symbol'); myEE.on(sym, () => {}); console.log(myEE.eventNames()); // Prints: [ 'foo', 'bar', Symbol(symbol) ]
- fork(
  env?: any
  ): Worker;
  Spawn a new worker process.
  This can only be called from the primary process.
  @param env
  Key/value pairs to add to worker process environment.
- getMaxListeners(): number;
  Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.
- listenerCount<K>(
  eventName: string | symbol,
  listener?: Function
  ): number;
  Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.
  @param eventName
  The name of the event being listened for
  @param listener
  The event handler function
- listeners<K>(
  eventName: string | symbol
  ): Function[];
  Returns a copy of the array of listeners for the event named eventName.
  server.on('connection', (stream) => { console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ]
- off<K>(
  eventName: string | symbol,
  listener: (...args: any[]) => void
  ): this;
  Alias for emitter.removeListener().
- on(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'disconnect',
  listener: (worker: Worker) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'exit',
  listener: (worker: Worker, code: number, signal: string) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'fork',
  listener: (worker: Worker) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'listening',
  listener: (worker: Worker, address: Address) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'message',
  listener: (worker: Worker, message: any, handle: Socket | Server) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'online',
  listener: (worker: Worker) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  on(
  event: 'setup',
  listener: (settings: ClusterSettings) => void
  ): this;
  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.on('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
- once(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'disconnect',
  listener: (worker: Worker) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'exit',
  listener: (worker: Worker, code: number, signal: string) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'fork',
  listener: (worker: Worker) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'listening',
  listener: (worker: Worker, address: Address) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'message',
  listener: (worker: Worker, message: any, handle: Socket | Server) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'online',
  listener: (worker: Worker) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
  once(
  event: 'setup',
  listener: (settings: ClusterSettings) => void
  ): this;
  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
  server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.
  import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a
  @param listener
  The callback function
- prependListener(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'disconnect',
  listener: (worker: Worker) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'exit',
  listener: (worker: Worker, code: number, signal: string) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'fork',
  listener: (worker: Worker) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'listening',
  listener: (worker: Worker, address: Address) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'message',
  listener: (worker: Worker, message: any, handle: Socket | Server) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'online',
  listener: (worker: Worker) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependListener(
  event: 'setup',
  listener: (settings: ClusterSettings) => void
  ): this;
  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
  server.prependListener('connection', (stream) => { console.log('someone connected!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
- prependOnceListener(
  event: string,
  listener: (...args: any[]) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'disconnect',
  listener: (worker: Worker) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'exit',
  listener: (worker: Worker, code: number, signal: string) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'fork',
  listener: (worker: Worker) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'listening',
  listener: (worker: Worker, address: Address) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'message',
  listener: (worker: Worker, message: any, handle: Socket | Server) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'online',
  listener: (worker: Worker) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
  prependOnceListener(
  event: 'setup',
  listener: (settings: ClusterSettings) => void
  ): this;
  Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
  server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); });
  Returns a reference to the EventEmitter, so that calls can be chained.
  @param listener
  The callback function
- rawListeners<K>(
  eventName: string | symbol
  ): Function[];
  Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).
  import { EventEmitter } from 'node:events'; const emitter = new EventEmitter(); emitter.once('log', () => console.log('log once')); // Returns a new Array with a function `onceWrapper` which has a property // `listener` which contains the original listener bound above const listeners = emitter.rawListeners('log'); const logFnWrapper = listeners[0]; // Logs "log once" to the console and does not unbind the `once` event logFnWrapper.listener(); // Logs "log once" to the console and removes the listener logFnWrapper(); emitter.on('log', () => console.log('log persistently')); // Will return a new Array with a single function bound by `.on()` above const newListeners = emitter.rawListeners('log'); // Logs "log persistently" twice newListeners[0](); emitter.emit('log');
- removeAllListeners(
  eventName?: string | symbol
  ): this;
  Removes all listeners, or those of the specified eventName.
  It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).
  Returns a reference to the EventEmitter, so that calls can be chained.
- removeListener<K>(
  eventName: string | symbol,
  listener: (...args: any[]) => void
  ): this;
  Removes the specified listener from the listener array for the event named eventName.
  const callback = (stream) => { console.log('someone connected!'); }; server.on('connection', callback); // ... server.removeListener('connection', callback);
  removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.
  Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.
  import { EventEmitter } from 'node:events'; class MyEmitter extends EventEmitter {} const myEmitter = new MyEmitter(); const callbackA = () => { console.log('A'); myEmitter.removeListener('event', callbackB); }; const callbackB = () => { console.log('B'); }; myEmitter.on('event', callbackA); myEmitter.on('event', callbackB); // callbackA removes listener callbackB but it will still be called. // Internal listener array at time of emit [callbackA, callbackB] myEmitter.emit('event'); // Prints: // A // B // callbackB is now removed. // Internal listener array [callbackA] myEmitter.emit('event'); // Prints: // A
  Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.
  When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:
  import { EventEmitter } from 'node:events'; const ee = new EventEmitter(); function pong() { console.log('pong'); } ee.on('ping', pong); ee.once('ping', pong); ee.removeListener('ping', pong); ee.emit('ping'); ee.emit('ping');
  Returns a reference to the EventEmitter, so that calls can be chained.
- setMaxListeners(
  n: number
  ): this;
  By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.
  Returns a reference to the EventEmitter, so that calls can be chained.
- setupPrimary(
  settings?: ClusterSettings
  ): void;
  setupPrimary is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.
  Any settings changes only affect future calls to .fork() and have no effect on workers that are already running.
  The only attribute of a worker that cannot be set via .setupPrimary() is the env passed to .fork().
  The defaults above apply to the first call only; the defaults for later calls are the current values at the time of cluster.setupPrimary() is called.
  import cluster from 'node:cluster'; cluster.setupPrimary({ exec: 'worker.js', args: ['--use', 'https'], silent: true, }); cluster.fork(); // https worker cluster.setupPrimary({ exec: 'worker.js', args: ['--use', 'http'], }); cluster.fork(); // http worker
  This can only be called from the primary process.
interface ClusterSettings
- args?: readonly string[]
  String arguments passed to worker.
- cwd?: string
  Current working directory of the worker process.
- exec?: string
  File path to worker file.
- execArgv?: string[]
  List of string arguments passed to the Node.js executable.
- gid?: number
  Sets the group identity of the process. (See setgid(2).)
- inspectPort?: number | () => number
  Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the primary's process.debugPort.
- serialization?: SerializationType
  Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced serialization for child_process for more details.
- silent?: boolean
  Whether or not to send output to parent's stdio.
- stdio?: any[]
  Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent. See child_prcess.spawn()'s stdio.
- uid?: number
  Sets the user identity of the process. (See setuid(2).)
- windowsHide?: boolean
  Hide the forked processes console window that would normally be created on Windows systems.

cluster

class Worker

Type definitions

interface Address

interface Cluster

interface ClusterSettings