RuntimeEventEmitter
The EventEmitter
class is defined and exposed by the node:events
module:
import { EventEmitter } from 'node:events';
All EventEmitter
s emit the event 'newListener'
when new listeners are
added and 'removeListener'
when existing listeners are removed.
It supports the following option:
v0.1.26
Extends
Section titled “Extends”EventEmitter
<T
>
Extended by
Section titled “Extended by”Type Parameters
Section titled “Type Parameters”Type Parameter | Default type |
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Constructors
Section titled “Constructors”Constructor
Section titled “Constructor”new RuntimeEventEmitter<T>(options?): EventEmitter<T>
Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”EventEmitter
<T
>
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter<T>.constructor
Properties
Section titled “Properties”captureRejections
Section titled “captureRejections”static captureRejections: boolean;
Value: boolean
Change the default captureRejections
option on all new EventEmitter
objects.
v13.4.0, v12.16.0
captureRejectionSymbol
Section titled “captureRejectionSymbol”readonly static captureRejectionSymbol: typeof captureRejectionSymbol;
Value: Symbol.for('nodejs.rejection')
See how to write a custom rejection handler
.
v13.4.0, v12.16.0
defaultMaxListeners
Section titled “defaultMaxListeners”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'
.
v0.11.2
errorMonitor
Section titled “errorMonitor”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.
v13.6.0, v12.17.0
Methods
Section titled “Methods”[captureRejectionSymbol]()?
Section titled “[captureRejectionSymbol]()?”optional [captureRejectionSymbol]<K>( error, event, ... args): void
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”void
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.[captureRejectionSymbol]
addListener()
Section titled “addListener()”addListener<K>(eventName, listener): this
Alias for emitter.on(eventName, listener)
.
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”this
v0.1.26
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.addListener
emit()
Section titled “emit()”emit<K>(eventName, ...args): 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 listenermyEmitter.on('event', function firstListener() { console.log('Helloooo! first listener');});// Second listenermyEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`);});// Third listenermyEmitter.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
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type |
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… |
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Returns
Section titled “Returns”boolean
v0.1.26
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.emit
eventNames()
Section titled “eventNames()”eventNames(): string | symbol & Key2<unknown, T>[]
Returns an array listing the events for which the emitter has registered
listeners. The values in the array are strings or Symbol
s.
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) ]
Returns
Section titled “Returns”string
| symbol
& Key2
<unknown
, T
>[]
v6.0.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.eventNames
getMaxListeners()
Section titled “getMaxListeners()”getMaxListeners(): number
Returns the current max listener value for the EventEmitter
which is either
set by emitter.setMaxListeners(n)
or defaults to EventEmitter.defaultMaxListeners.
Returns
Section titled “Returns”number
v1.0.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.getMaxListeners
listenerCount()
Section titled “listenerCount()”listenerCount<K>(eventName, listener?): 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.
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type | Description |
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The name of the event being listened for |
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The event handler function |
Returns
Section titled “Returns”number
v3.2.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.listenerCount
listeners()
Section titled “listeners()”listeners<K>(eventName): Listener<K, T, 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] ]
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”Listener
<K
, T
, Function
>[]
v0.1.26
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.listeners
off<K>(eventName, listener): this
Alias for emitter.removeListener()
.
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”this
v10.0.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.off
on<K>(eventName, listener): 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
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type | Description |
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The name of the event. |
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The callback function |
Returns
Section titled “Returns”this
v0.1.101
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.on
once()
Section titled “once()”once<K>(eventName, listener): 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
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type | Description |
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The name of the event. |
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The callback function |
Returns
Section titled “Returns”this
v0.3.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.once
prependListener()
Section titled “prependListener()”prependListener<K>(eventName, listener): 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.
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type | Description |
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The name of the event. |
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The callback function |
Returns
Section titled “Returns”this
v6.0.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.prependListener
prependOnceListener()
Section titled “prependOnceListener()”prependOnceListener<K>(eventName, listener): 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.
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type | Description |
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The name of the event. |
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The callback function |
Returns
Section titled “Returns”this
v6.0.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.prependOnceListener
rawListeners()
Section titled “rawListeners()”rawListeners<K>(eventName): Listener<K, T, 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 aboveconst listeners = emitter.rawListeners('log');const logFnWrapper = listeners[0];
// Logs "log once" to the console and does not unbind the `once` eventlogFnWrapper.listener();
// Logs "log once" to the console and removes the listenerlogFnWrapper();
emitter.on('log', () => console.log('log persistently'));// Will return a new Array with a single function bound by `.on()` aboveconst newListeners = emitter.rawListeners('log');
// Logs "log persistently" twicenewListeners[0]();emitter.emit('log');
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”Listener
<K
, T
, Function
>[]
v9.4.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.rawListeners
removeAllListeners()
Section titled “removeAllListeners()”removeAllListeners(eventName?): 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.
Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”this
v0.1.26
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.removeAllListeners
removeListener()
Section titled “removeListener()”removeListener<K>(eventName, listener): 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.
Type Parameters
Section titled “Type Parameters”Type Parameter |
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Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”this
v0.1.26
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.removeListener
setMaxListeners()
Section titled “setMaxListeners()”setMaxListeners(n): this
By default EventEmitter
s 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.
Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”this
v0.3.5
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.setMaxListeners
addAbortListener()
Section titled “addAbortListener()”static addAbortListener(signal, resource): 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 AbortSignal
s 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](); }}
Parameters
Section titled “Parameters”Parameter | Type |
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( |
Returns
Section titled “Returns”Disposable
Disposable that removes the abort
listener.
v20.5.0
getEventListeners()
Section titled “getEventListeners()”static getEventListeners(emitter, name): Function[]
Returns a copy of the array of listeners for the event named eventName
.
For EventEmitter
s this behaves exactly the same as calling .listeners
on
the emitter.
For EventTarget
s 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] ]}
Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”Function
[]
v15.2.0, v14.17.0
getMaxListeners()
Section titled “getMaxListeners()”static getMaxListeners(emitter): number
Returns the currently set max amount of listeners.
For EventEmitter
s this behaves exactly the same as calling .getMaxListeners
on
the emitter.
For EventTarget
s 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}
Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”number
v19.9.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.getMaxListeners
listenerCount()
Section titled “listenerCount()”static listenerCount(emitter, eventName): number
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
Parameters
Section titled “Parameters”Parameter | Type | Description |
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The emitter to query |
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The event name |
Returns
Section titled “Returns”number
v0.9.12
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.listenerCount
Call Signature
Section titled “Call Signature”static on( emitter, eventName,options?): AsyncIterator<any[]>
import { on, EventEmitter } from 'node:events';import process from 'node:process';
const ee = new EventEmitter();
// Emit later onprocess.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 onprocess.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 emittedconsole.log('done'); // prints 'done'
Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”AsyncIterator
<any
[]>
An AsyncIterator
that iterates eventName
events emitted by the emitter
v13.6.0, v12.16.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.on
Call Signature
Section titled “Call Signature”static on( emitter, eventName,options?): AsyncIterator<any[]>
import { on, EventEmitter } from 'node:events';import process from 'node:process';
const ee = new EventEmitter();
// Emit later onprocess.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 onprocess.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 emittedconsole.log('done'); // prints 'done'
Parameters
Section titled “Parameters”Parameter | Type |
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Returns
Section titled “Returns”AsyncIterator
<any
[]>
An AsyncIterator
that iterates eventName
events emitted by the emitter
v13.6.0, v12.16.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.on
once()
Section titled “once()”Call Signature
Section titled “Call Signature”static once( emitter, eventName,options?): 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 eventee.emit('foo'); // Prints: Waiting for the event was canceled!
Parameters
Section titled “Parameters”Parameter | Type |
---|---|
|
|
|
|
|
|
Returns
Section titled “Returns”Promise
<any
[]>
v11.13.0, v10.16.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.once
Call Signature
Section titled “Call Signature”static once( emitter, eventName,options?): 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 eventee.emit('foo'); // Prints: Waiting for the event was canceled!
Parameters
Section titled “Parameters”Parameter | Type |
---|---|
|
|
|
|
|
|
Returns
Section titled “Returns”Promise
<any
[]>
v11.13.0, v10.16.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.once
setMaxListeners()
Section titled “setMaxListeners()”static setMaxListeners(n?, ...eventTargets?): void
import { setMaxListeners, EventEmitter } from 'node:events';
const target = new EventTarget();const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
Parameters
Section titled “Parameters”Parameter | Type | Description |
---|---|---|
|
|
A non-negative number. The maximum number of listeners per |
… |
( |
Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, |
Returns
Section titled “Returns”void
v15.4.0
Inherited from
Section titled “Inherited from”NodeJS.EventEmitter.setMaxListeners