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RealtimeSession

A RealtimeSession is the corner piece of building Voice Agents. It’s the equivalent of a Runner in text-based agents except that it automatically handles multiple turns by maintaining a connection with the underlying transport layer.

The session handles managing the local history copy, executes tools, runs output guardrails, and facilities handoffs.

The actual audio handling and generation of model responses is handled by the underlying transport layer. By default if you are using a browser with WebRTC support, the session will automatically use the WebRTC version of the OpenAI Realtime API. On the server or if you pass websocket as the transport layer, the session will establish a connection using WebSockets.

In the case of WebRTC, in the browser, the transport layer will also automatically configure the microphone and audio output to be used by the session.

You can also create a transport layer instance yourself and pass it in to have more control over the configuration or even extend the existing ones. Check out the TwilioRealtimeTransportLayer for an example of how to create a custom transport layer.

const agent = new RealtimeAgent({
name: 'my-agent',
instructions: 'You are a helpful assistant that can answer questions and help with tasks.',
})
const session = new RealtimeSession(agent);
session.connect({
apiKey: 'your-api-key',
});
Type Parameter Default type

TBaseContext

unknown

new RealtimeSession<TBaseContext>(initialAgent, options): RealtimeSession<TBaseContext>
Parameter Type

initialAgent

| RealtimeAgent<TBaseContext> | RealtimeAgent<RealtimeContextData<TBaseContext>>

options

Partial<RealtimeSessionOptions<TBaseContext>>

RealtimeSession<TBaseContext>

RuntimeEventEmitter<RealtimeSessionEventTypes<TBaseContext>>.constructor
readonly initialAgent:
| RealtimeAgent<TBaseContext>
| RealtimeAgent<RealtimeContextData<TBaseContext>>;

readonly options: Partial<RealtimeSessionOptions<TBaseContext>> = {};

static captureRejections: boolean;

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

v13.4.0, v12.16.0

RuntimeEventEmitter.captureRejections

readonly static captureRejectionSymbol: typeof captureRejectionSymbol;

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

See how to write a custom rejection handler.

v13.4.0, v12.16.0

RuntimeEventEmitter.captureRejectionSymbol

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

RuntimeEventEmitter.defaultMaxListeners

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

RuntimeEventEmitter.errorMonitor
get context(): RunContext<RealtimeContextData<TBaseContext>>

The current context of the session.

RunContext<RealtimeContextData<TBaseContext>>


get currentAgent():
| RealtimeAgent<TBaseContext>
| RealtimeAgent<RealtimeContextData<TBaseContext>>

The current agent in the session.

| RealtimeAgent<TBaseContext> | RealtimeAgent<RealtimeContextData<TBaseContext>>


get history(): RealtimeItem[]

The history of the session.

RealtimeItem[]


get muted(): null | boolean

Whether the session is muted. Might be null if the underlying transport layer does not support muting.

null | boolean


get transport(): RealtimeTransportLayer

The transport layer used by the session.

RealtimeTransportLayer


get usage(): Usage

The current usage of the session.

Usage

optional [captureRejectionSymbol]<K>(
error,
event, ...
args): void
Type Parameter

K

Parameter Type

error

Error

event

keyof RealtimeSessionEventTypes<TBaseContext> | K

args

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] : never

void

RuntimeEventEmitter.[captureRejectionSymbol]

addListener<K>(eventName, listener): this

Alias for emitter.on(eventName, listener).

Type Parameter

K

Parameter Type

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

listener

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

this

v0.1.26

RuntimeEventEmitter.addListener

approve(approvalItem, options): Promise<void>

Approve a tool call. This will also trigger the tool call to the agent.

Parameter Type Description

approvalItem

RunToolApprovalItem

The approval item to approve.

options

{ alwaysApprove: boolean; }

Additional options.

options.alwaysApprove?

boolean

Whether to always approve the tool call.

Promise<void>


close(): void

Disconnect from the session.

void


connect(options): Promise<void>

Connect to the session. This will establish the connection to the underlying transport layer and start the session.

After connecting, the session will also emit a history_updated event with an empty history.

Parameter Type Description

options

RealtimeSessionConnectOptions

The options for the connection.

Promise<void>


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 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
Type Parameter

K

Parameter Type

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

args

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] : never

boolean

v0.1.26

RuntimeEventEmitter.emit

eventNames(): (
| "audio"
| "error"
| "audio_interrupted"
| "agent_start"
| "agent_end"
| "agent_handoff"
| "agent_tool_start"
| "agent_tool_end"
| "transport_event"
| "audio_start"
| "audio_stopped"
| "guardrail_tripped"
| "history_updated"
| "history_added"
| "tool_approval_requested")[]

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) ]

( | "audio" | "error" | "audio_interrupted" | "agent_start" | "agent_end" | "agent_handoff" | "agent_tool_start" | "agent_tool_end" | "transport_event" | "audio_start" | "audio_stopped" | "guardrail_tripped" | "history_updated" | "history_added" | "tool_approval_requested")[]

v6.0.0

RuntimeEventEmitter.eventNames

getMaxListeners(): number

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

number

v1.0.0

RuntimeEventEmitter.getMaxListeners

interrupt(): void

Interrupt the session artificially for example if you want to build a “stop talking” button.

void


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 Parameter

K

Parameter Type Description

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

The name of the event being listened for

listener?

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

The event handler function

number

v3.2.0

RuntimeEventEmitter.listenerCount

listeners<K>(eventName): K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (...args) => void : never : never[]

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 Parameter

K

Parameter Type

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never[]

v0.1.26

RuntimeEventEmitter.listeners

mute(muted): void

Mute the session.

Parameter Type Description

muted

boolean

Whether to mute the session.

void


off<K>(eventName, listener): this

Alias for emitter.removeListener().

Type Parameter

K

Parameter Type

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

listener

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

this

v10.0.0

RuntimeEventEmitter.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 Parameter

K

Parameter Type Description

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

The name of the event.

listener

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

The callback function

this

v0.1.101

RuntimeEventEmitter.on

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 Parameter

K

Parameter Type Description

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

The name of the event.

listener

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

The callback function

this

v0.3.0

RuntimeEventEmitter.once

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 Parameter

K

Parameter Type Description

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

The name of the event.

listener

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

The callback function

this

v6.0.0

RuntimeEventEmitter.prependListener

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 Parameter

K

Parameter Type Description

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

The name of the event.

listener

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

The callback function

this

v6.0.0

RuntimeEventEmitter.prependOnceListener

rawListeners<K>(eventName): K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (...args) => void : never : never[]

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');
Type Parameter

K

Parameter Type

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never[]

v9.4.0

RuntimeEventEmitter.rawListeners

reject(approvalItem, options): Promise<void>

Reject a tool call. This will also trigger the tool call to the agent.

Parameter Type Description

approvalItem

RunToolApprovalItem

The approval item to reject.

options

{ alwaysReject: boolean; }

Additional options.

options.alwaysReject?

boolean

Whether to always reject the tool call.

Promise<void>


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.

Parameter Type

eventName?

unknown

this

v0.1.26

RuntimeEventEmitter.removeAllListeners

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 Parameter

K

Parameter Type

eventName

keyof RealtimeSessionEventTypes<TBaseContext> | K

listener

K extends keyof RealtimeSessionEventTypes<TBaseContext> ? RealtimeSessionEventTypes<TBaseContext>[K<K>] extends unknown[] ? (…args) => void : never : never

this

v0.1.26

RuntimeEventEmitter.removeListener

sendAudio(audio, options): void

Send audio to the session.

Parameter Type Description

audio

ArrayBuffer

The audio to send.

options

{ commit: boolean; }

Additional options.

options.commit?

boolean

Whether to finish the turn with this audio.

void


sendMessage(message, otherEventData): void

Send a message to the session.

Parameter Type Description

message

RealtimeUserInput

The message to send.

otherEventData

Record<string, any>

Additional event data to send.

void


setMaxListeners(n): 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.

Parameter Type

n

number

this

v0.3.5

RuntimeEventEmitter.setMaxListeners

updateAgent(newAgent): Promise<RealtimeAgent<TBaseContext>>
Parameter Type

newAgent

RealtimeAgent<TBaseContext>

Promise<RealtimeAgent<TBaseContext>>


updateHistory(newHistory): void

Update the history of the session.

Parameter Type Description

newHistory

| RealtimeItem[] | (history) => RealtimeItem[]

The new history to set.

void


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 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]();
}
}
Parameter Type

signal

AbortSignal

resource

(event) => void

Disposable

Disposable that removes the abort listener.

v20.5.0

RuntimeEventEmitter.addAbortListener

static getEventListeners(emitter, name): 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] ]
}
Parameter Type

emitter

EventTarget | EventEmitter<DefaultEventMap>

name

string | symbol

Function[]

v15.2.0, v14.17.0

RuntimeEventEmitter.getEventListeners

static getMaxListeners(emitter): 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
}
Parameter Type

emitter

EventTarget | EventEmitter<DefaultEventMap>

number

v19.9.0

RuntimeEventEmitter.getMaxListeners

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
Parameter Type Description

emitter

EventEmitter

The emitter to query

eventName

string | symbol

The event name

number

v0.9.12

RuntimeEventEmitter.listenerCount

static on(
emitter,
eventName,
options?): 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'
Parameter Type

emitter

EventEmitter

eventName

string | symbol

options?

StaticEventEmitterIteratorOptions

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

v13.6.0, v12.16.0

RuntimeEventEmitter.on
static on(
emitter,
eventName,
options?): 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'
Parameter Type

emitter

EventTarget

eventName

string

options?

StaticEventEmitterIteratorOptions

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

v13.6.0, v12.16.0

RuntimeEventEmitter.on

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 event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
Parameter Type

emitter

EventEmitter

eventName

string | symbol

options?

StaticEventEmitterOptions

Promise<any[]>

v11.13.0, v10.16.0

RuntimeEventEmitter.once
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 event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
Parameter Type

emitter

EventTarget

eventName

string

options?

StaticEventEmitterOptions

Promise<any[]>

v11.13.0, v10.16.0

RuntimeEventEmitter.once

static setMaxListeners(n?, ...eventTargets?): void
import { setMaxListeners, EventEmitter } from 'node:events';
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
Parameter Type Description

n?

number

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

eventTargets?

(EventTarget | EventEmitter<DefaultEventMap>)[]

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.

void

v15.4.0

RuntimeEventEmitter.setMaxListeners