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LiveEventScope class

  • Reference
Package:
@microsoft/live-share

Object responsible for sending and receiving live share events.

Extends

TypedEventEmitter<IErrorEvent>

Remarks

Live objects send and receive events using an event scope. Event scopes can be restricted to only receive events from clients with specific roles. Any events that are received from clients without an allowed role type will be ignored.

Event scopes are isolated on a per Fluid object basis. That means that two different Fluid objects using the same event names don't have to worry about collisions. Two event scopes within the same Fluid object, however, don't have any isolation. You can use multiple event scopes within the same FLuid object, you just need to be careful that they send different events.

Constructors

LiveEventScope(IRuntimeSignaler, LiveShareRuntime, UserMeetingRole[])

Creates a new LiveEventScope instance.

Properties

allowedRoles

List of roles allowed to send events through this scope.
clientId

The runtimes current client ID. This will be undefined if the client is disconnected.

Inherited Properties

addListener
captureRejections

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.
captureRejectionSymbol

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.
defaultMaxListeners

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'.
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.
off
on
once
prependListener
prependOnceListener
removeListener

Methods

offEvent<TEvent>(string, LiveEventListener<TEvent>)

Un-registers a listener for a named event.
onEvent<TEvent>(string, LiveEventListener<TEvent>)

Registers a listener for a named event.
sendEvent<TEvent>(string, TEvent)

Sends an event to other event scope instances for the Fluid object.
sendLocalEvent<TEvent>(string, TEvent)

Sends a local only event to local event scope instance for the Fluid object.

Inherited Methods

addAbortListener(AbortSignal, (event: Event) => void)

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]();
  }
}
emit<K>(string | symbol, AnyRest)

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()

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) ]
getEventListeners(EventEmitter<DefaultEventMap> | EventTarget, string | symbol)

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] ]
}
getMaxListeners()

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to defaultMaxListeners.
getMaxListeners(EventEmitter<DefaultEventMap> | EventTarget)

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
}
listenerCount(EventEmitter<DefaultEventMap>, string | symbol)

A class method that returns the number of listeners for the given eventName registered on the given emitter.

import { EventEmitter, listenerCount } from 'node:events';

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2
listenerCount<K>(string | symbol, Function)

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.
listeners<K>(string | symbol)

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] ]
on(EventEmitter<DefaultEventMap>, string | symbol, StaticEventEmitterIteratorOptions) 
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'
on(EventTarget, string, StaticEventEmitterIteratorOptions)
once(EventEmitter<DefaultEventMap>, string | symbol, StaticEventEmitterOptions)

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!
once(EventTarget, string, StaticEventEmitterOptions)
rawListeners<K>(string | symbol)

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(string | symbol)

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.
setMaxListeners(number)

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.
setMaxListeners(number, (EventEmitter<DefaultEventMap> | EventTarget)[]) 
import { setMaxListeners, EventEmitter } from 'node:events';

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);
[captureRejectionSymbol]<K>(Error, string | symbol, AnyRest)

Constructor Details

LiveEventScope(IRuntimeSignaler, LiveShareRuntime, UserMeetingRole[])

Creates a new LiveEventScope instance.

new LiveEventScope(runtime: IRuntimeSignaler, _liveRuntime: LiveShareRuntime, allowedRoles?: UserMeetingRole[])

Parameters

runtime
IRuntimeSignaler

A Fluid objects runtime instance, typically this.runtime.

_liveRuntime
LiveShareRuntime
allowedRoles

UserMeetingRole[]

Optional. List of roles allowed to send events using this scope. You should use a second scope if you need mixed permission support.

Property Details

allowedRoles

List of roles allowed to send events through this scope.

UserMeetingRole[] allowedRoles

Property Value

UserMeetingRole[]

clientId

The runtimes current client ID. This will be undefined if the client is disconnected.

undefined | string clientId

Property Value

undefined | string

Inherited Property Details

addListener 

addListener: TypedEventTransform<LiveEventScope, IErrorEvent>

Property Value

TypedEventTransform<LiveEventScope, IErrorEvent>

Inherited From TypedEventEmitter.addListener

captureRejections

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

static captureRejections: boolean

Property Value

boolean

Inherited From TypedEventEmitter.captureRejections

captureRejectionSymbol

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.

static captureRejectionSymbol: typeof captureRejectionSymbol

Property Value

typeof captureRejectionSymbol

Inherited From TypedEventEmitter.captureRejectionSymbol

defaultMaxListeners

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'.

static defaultMaxListeners: number

Property Value

number

Inherited From TypedEventEmitter.defaultMaxListeners

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.

static errorMonitor: typeof errorMonitor

Property Value

typeof errorMonitor

Inherited From TypedEventEmitter.errorMonitor

off 

off: TypedEventTransform<LiveEventScope, IErrorEvent>

Property Value

TypedEventTransform<LiveEventScope, IErrorEvent>

Inherited From TypedEventEmitter.off

on 

on: TypedEventTransform<LiveEventScope, IErrorEvent>

Property Value

TypedEventTransform<LiveEventScope, IErrorEvent>

Inherited From TypedEventEmitter.on

once 

once: TypedEventTransform<LiveEventScope, IErrorEvent>

Property Value

TypedEventTransform<LiveEventScope, IErrorEvent>

Inherited From TypedEventEmitter.once

prependListener 

prependListener: TypedEventTransform<LiveEventScope, IErrorEvent>

Property Value

TypedEventTransform<LiveEventScope, IErrorEvent>

Inherited From TypedEventEmitter.prependListener

prependOnceListener 

prependOnceListener: TypedEventTransform<LiveEventScope, IErrorEvent>

Property Value

TypedEventTransform<LiveEventScope, IErrorEvent>

Inherited From TypedEventEmitter.prependOnceListener

removeListener 

removeListener: TypedEventTransform<LiveEventScope, IErrorEvent>

Property Value

TypedEventTransform<LiveEventScope, IErrorEvent>

Inherited From TypedEventEmitter.removeListener

Method Details

offEvent<TEvent>(string, LiveEventListener<TEvent>)

Un-registers a listener for a named event.

function offEvent<TEvent>(eventName: string, listener: LiveEventListener<TEvent>): LiveEventScope

Parameters

eventName

string

Name of event to un-register.

listener

LiveEventListener<TEvent>

Function that was originally passed to onEvent().

Returns

LiveEventScope

onEvent<TEvent>(string, LiveEventListener<TEvent>)

Registers a listener for a named event.

function onEvent<TEvent>(eventName: string, listener: LiveEventListener<TEvent>): LiveEventScope

Parameters

eventName

string

Name of event to listen for.

listener

LiveEventListener<TEvent>

Function to call when the named event is sent or received.

Returns

LiveEventScope

sendEvent<TEvent>(string, TEvent)

Sends an event to other event scope instances for the Fluid object.

function sendEvent<TEvent>(eventName: string, evt: TEvent): Promise<ILiveEvent<TEvent>>

Parameters

eventName

string

Name of the event to send.

evt

TEvent

Optional. Partial event object to send. The ILiveEvent.name, ILiveEvent.timestamp, and ILiveEvent.clientId fields will be automatically populated prior to sending.

Returns

Promise<ILiveEvent<TEvent>>

The full event, including ILiveEvent.name, ILiveEvent.timestamp, and ILiveEvent.clientId fields if known.

sendLocalEvent<TEvent>(string, TEvent)

Sends a local only event to local event scope instance for the Fluid object.

function sendLocalEvent<TEvent>(eventName: string, evt: TEvent): Promise<ILiveEvent<TEvent>>

Parameters

eventName

string

Name of the event to send.

evt

TEvent

Optional. Partial event object to send. The ILiveEvent.name, ILiveEvent.timestamp, and ILiveEvent.clientId fields will be automatically populated prior to sending.

Returns

Promise<ILiveEvent<TEvent>>

The full event, including ILiveEvent.name, ILiveEvent.timestamp, and ILiveEvent.clientId fields if known.

Inherited Method Details

addAbortListener(AbortSignal, (event: Event) => void)

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]();
  }
}

static function addAbortListener(signal: AbortSignal, resource: (event: Event) => void): Disposable

Parameters

signal

AbortSignal

resource

(event: Event) => void

Returns

Disposable

Disposable that removes the abort listener.

Inherited From TypedEventEmitter.addAbortListener

emit<K>(string | symbol, AnyRest)

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

function emit<K>(eventName: string | symbol, args: AnyRest): boolean

Parameters

eventName

string | symbol

args

AnyRest

Returns

boolean

Inherited From TypedEventEmitter.emit

eventNames()

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) ]

function eventNames(): (string | symbol)[]

Returns

(string | symbol)[]

Inherited From TypedEventEmitter.eventNames

getEventListeners(EventEmitter<DefaultEventMap> | EventTarget, string | symbol)

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 function getEventListeners(emitter: EventEmitter<DefaultEventMap> | EventTarget, name: string | symbol): Function[]

Parameters

emitter

EventEmitter<DefaultEventMap> | EventTarget

name

string | symbol

Returns

Function[]

Inherited From TypedEventEmitter.getEventListeners

getMaxListeners()

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to defaultMaxListeners.

function getMaxListeners(): number

Returns

number

Inherited From TypedEventEmitter.getMaxListeners

getMaxListeners(EventEmitter<DefaultEventMap> | EventTarget)

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 function getMaxListeners(emitter: EventEmitter<DefaultEventMap> | EventTarget): number

Parameters

emitter

EventEmitter<DefaultEventMap> | EventTarget

Returns

number

Inherited From TypedEventEmitter.getMaxListeners

listenerCount(EventEmitter<DefaultEventMap>, string | symbol)

Warning

This API is now deprecated.

Since v3.2.0 - Use listenerCount instead.

A class method that returns the number of listeners for the given eventName registered on the given emitter.

import { EventEmitter, listenerCount } from 'node:events';

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2

static function listenerCount(emitter: EventEmitter<DefaultEventMap>, eventName: string | symbol): number

Parameters

emitter

EventEmitter<DefaultEventMap>

The emitter to query

eventName

string | symbol

The event name

Returns

number

Inherited From TypedEventEmitter.listenerCount

listenerCount<K>(string | symbol, Function)

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.

function listenerCount<K>(eventName: string | symbol, listener?: Function): number

Parameters

eventName

string | symbol

The name of the event being listened for

listener

Function

The event handler function

Returns

number

Inherited From TypedEventEmitter.listenerCount

listeners<K>(string | symbol)

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] ]

function listeners<K>(eventName: string | symbol): Function[]

Parameters

eventName

string | symbol

Returns

Function[]

Inherited From TypedEventEmitter.listeners

on(EventEmitter<DefaultEventMap>, string | symbol, StaticEventEmitterIteratorOptions) 

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'

static function on(emitter: EventEmitter<DefaultEventMap>, eventName: string | symbol, options?: StaticEventEmitterIteratorOptions): AsyncIterator<any[], any, any>

Parameters

emitter

EventEmitter<DefaultEventMap>

eventName

string | symbol

options

StaticEventEmitterIteratorOptions

Returns

AsyncIterator<any[], any, any>

An AsyncIterator that iterates eventName events emitted by the emitter

Inherited From TypedEventEmitter.on

on(EventTarget, string, StaticEventEmitterIteratorOptions) 

static function on(emitter: EventTarget, eventName: string, options?: StaticEventEmitterIteratorOptions): AsyncIterator<any[], any, any>

Parameters

emitter

EventTarget

eventName

string

options

StaticEventEmitterIteratorOptions

Returns

AsyncIterator<any[], any, any>

Inherited From TypedEventEmitter.on

once(EventEmitter<DefaultEventMap>, string | symbol, StaticEventEmitterOptions)

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 function once(emitter: EventEmitter<DefaultEventMap>, eventName: string | symbol, options?: StaticEventEmitterOptions): Promise<any[]>

Parameters

emitter

EventEmitter<DefaultEventMap>

eventName

string | symbol

options

StaticEventEmitterOptions

Returns

Promise<any[]>

Inherited From TypedEventEmitter.once

once(EventTarget, string, StaticEventEmitterOptions) 

static function once(emitter: EventTarget, eventName: string, options?: StaticEventEmitterOptions): Promise<any[]>

Parameters

emitter

EventTarget

eventName

string

options

StaticEventEmitterOptions

Returns

Promise<any[]>

Inherited From TypedEventEmitter.once

rawListeners<K>(string | symbol)

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');

function rawListeners<K>(eventName: string | symbol): Function[]

Parameters

eventName

string | symbol

Returns

Function[]

Inherited From TypedEventEmitter.rawListeners

removeAllListeners(string | symbol)

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.

function removeAllListeners(eventName?: string | symbol): LiveEventScope

Parameters

eventName

string | symbol

Returns

LiveEventScope

Inherited From TypedEventEmitter.removeAllListeners

setMaxListeners(number)

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.

function setMaxListeners(n: number): LiveEventScope

Parameters

n

number

Returns

LiveEventScope

Inherited From TypedEventEmitter.setMaxListeners

setMaxListeners(number, (EventEmitter<DefaultEventMap> | EventTarget)[]) 

import { setMaxListeners, EventEmitter } from 'node:events';

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);

static function setMaxListeners(n?: number, eventTargets: (EventEmitter<DefaultEventMap> | EventTarget)[])

Parameters

n

number

A non-negative number. The maximum number of listeners per EventTarget event.

eventTargets

(EventEmitter<DefaultEventMap> | EventTarget)[]

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.

Inherited From TypedEventEmitter.setMaxListeners

[captureRejectionSymbol]<K>(Error, string | symbol, AnyRest) 

function [captureRejectionSymbol]<K>(error: Error, event: string | symbol, args: AnyRest)

Parameters

error

Error

event

string | symbol

args

AnyRest

Inherited From TypedEventEmitter.__@captureRejectionSymbol@283