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.

Example

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',
});

Extends

EventEmitter<RealtimeSessionEventTypes<TBaseContext>>

Type Parameters

Type Parameter	Default type
`TBaseContext`	`unknown`

Constructors

Constructor

new RealtimeSession<TBaseContext>(initialAgent, options): RealtimeSession<TBaseContext>;

Parameters

Parameter	Type
`initialAgent`	\| `RealtimeAgent`<`TBaseContext`> \| `RealtimeAgent`<`RealtimeContextData`<`TBaseContext`>>
`options`	`Partial`<`RealtimeSessionOptions`<`TBaseContext`>>

Returns

RealtimeSession<TBaseContext>

Overrides

RuntimeEventEmitter<RealtimeSessionEventTypes<TBaseContext>>.constructor

Properties

initialAgent

readonly initialAgent:
  | RealtimeAgent<TBaseContext>
| RealtimeAgent<RealtimeContextData<TBaseContext>>;

options

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

captureRejections

static captureRejections: boolean;

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

Since

v13.4.0, v12.16.0

Inherited from

RuntimeEventEmitter.captureRejections

captureRejectionSymbol

readonly static captureRejectionSymbol: typeof captureRejectionSymbol;

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

See how to write a custom rejection handler.

Since

v13.4.0, v12.16.0

Inherited from

RuntimeEventEmitter.captureRejectionSymbol

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'.

Since

v0.11.2

Inherited from

RuntimeEventEmitter.defaultMaxListeners

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.

Since

v13.6.0, v12.17.0

Inherited from

RuntimeEventEmitter.errorMonitor

Accessors

context

Get Signature

get context(): RunContext<RealtimeContextData<TBaseContext>>;

The current context of the session.

Returns

RunContext<RealtimeContextData<TBaseContext>>

currentAgent

Get Signature

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

The current agent in the session.

Returns

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

history

Get Signature

get history(): RealtimeItem[];

The history of the session.

Returns

RealtimeItem[]

muted

Get Signature

get muted(): null | boolean;

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

Returns

null | boolean

transport

Get Signature

get transport(): RealtimeTransportLayer;

The transport layer used by the session.

Returns

RealtimeTransportLayer

usage

Get Signature

get usage(): Usage;

The current usage of the session.

Returns

Usage

Methods

[captureRejectionSymbol]()?

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

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`error`	`Error`
`event`	keyof RealtimeSessionEventTypes<TBaseContext> \| `K`
…`args`	`K` extends keyof `RealtimeSessionEventTypes`<`TBaseContext`> ? `RealtimeSessionEventTypes`<`TBaseContext`>[`K`<`K`>] : `never`

Returns

void

Inherited from

RuntimeEventEmitter.[captureRejectionSymbol]

addListener()

addListener<K>(eventName, listener): this;

Alias for emitter.on(eventName, listener).

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	keyof RealtimeSessionEventTypes<TBaseContext> \| `K`
`listener`	`K` extends keyof `RealtimeSessionEventTypes`<`TBaseContext`> ? `RealtimeSessionEventTypes`<`TBaseContext`>[`K`<`K`>] extends `unknown`[] ? (…`args`) => `void` : `never` : `never`

Returns

this

Since

v0.1.26

Inherited from

RuntimeEventEmitter.addListener

approve()

approve(approvalItem, options): Promise<void>;

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

Parameters

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.

Returns

Promise<void>

close()

close(): void;

Disconnect from the session.

Returns

void

connect()

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.

Parameters

Parameter	Type	Description
`options`	`RealtimeSessionConnectOptions`	The options for the connection.

Returns

Promise<void>

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 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 Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	keyof RealtimeSessionEventTypes<TBaseContext> \| `K`
…`args`	`K` extends keyof `RealtimeSessionEventTypes`<`TBaseContext`> ? `RealtimeSessionEventTypes`<`TBaseContext`>[`K`<`K`>] : `never`

Returns

boolean

Since

v0.1.26

Inherited from

RuntimeEventEmitter.emit

eventNames()

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) ]

Returns

Since

v6.0.0

Inherited from

RuntimeEventEmitter.eventNames

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

number

Since

v1.0.0

Inherited from

RuntimeEventEmitter.getMaxListeners

interrupt()

interrupt(): void;

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

Returns

void

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

Type Parameter
`K`

Parameters

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

Returns

number

Since

v3.2.0

Inherited from

RuntimeEventEmitter.listenerCount

listeners()

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 Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	keyof RealtimeSessionEventTypes<TBaseContext> \| `K`

Returns

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

Since

v0.1.26

Inherited from

RuntimeEventEmitter.listeners

mute()

mute(muted): void;

Mute the session.

Parameters

Parameter	Type	Description
`muted`	`boolean`	Whether to mute the session.

Returns

void

off()

off<K>(eventName, listener): this;

Alias for emitter.removeListener().

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	keyof RealtimeSessionEventTypes<TBaseContext> \| `K`
`listener`	`K` extends keyof `RealtimeSessionEventTypes`<`TBaseContext`> ? `RealtimeSessionEventTypes`<`TBaseContext`>[`K`<`K`>] extends `unknown`[] ? (…`args`) => `void` : `never` : `never`

Returns

this

Since

v10.0.0

Inherited from

RuntimeEventEmitter.off

on()

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

Type Parameter
`K`

Parameters

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

Returns

this

Since

v0.1.101

Inherited from

RuntimeEventEmitter.on

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

Type Parameter
`K`

Parameters

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

Returns

this

Since

v0.3.0

Inherited from

RuntimeEventEmitter.once

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

Type Parameter
`K`

Parameters

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

Returns

this

Since

v6.0.0

Inherited from

RuntimeEventEmitter.prependListener

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

Type Parameter
`K`

Parameters

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

Returns

this

Since

v6.0.0

Inherited from

RuntimeEventEmitter.prependOnceListener

rawListeners()

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 Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	keyof RealtimeSessionEventTypes<TBaseContext> \| `K`

Returns

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

Since

v9.4.0

Inherited from

RuntimeEventEmitter.rawListeners

reject()

reject(approvalItem, options): Promise<void>;

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

Parameters

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.

Returns

Promise<void>

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

Parameter	Type
`eventName?`	`unknown`

Returns

this

Since

v0.1.26

Inherited from

RuntimeEventEmitter.removeAllListeners

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

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	keyof RealtimeSessionEventTypes<TBaseContext> \| `K`
`listener`	`K` extends keyof `RealtimeSessionEventTypes`<`TBaseContext`> ? `RealtimeSessionEventTypes`<`TBaseContext`>[`K`<`K`>] extends `unknown`[] ? (…`args`) => `void` : `never` : `never`

Returns

this

Since

v0.1.26

Inherited from

RuntimeEventEmitter.removeListener

sendAudio()

sendAudio(audio, options): void;

Send audio to the session.

Parameters

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.

Returns

void

sendMessage()

sendMessage(message, otherEventData): void;

Send a message to the session.

Parameters

Parameter	Type	Description
`message`	`RealtimeUserInput`	The message to send.
`otherEventData`	`Record`<`string`, `any`>	Additional event data to send.

Returns

void

setMaxListeners()

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.

Parameters

Parameter	Type
`n`	`number`

Returns

this

Since

v0.3.5

Inherited from

RuntimeEventEmitter.setMaxListeners

updateAgent()

updateAgent(newAgent): Promise<RealtimeAgent<TBaseContext>>;

Parameters

Parameter	Type
`newAgent`	`RealtimeAgent`<`TBaseContext`>

Returns

Promise<RealtimeAgent<TBaseContext>>

updateHistory()

updateHistory(newHistory): void;

Update the history of the session.

Parameters

Parameter	Type	Description
`newHistory`	\| `RealtimeItem`[] \| (`history`) => `RealtimeItem`[]	The new history to set.

Returns

void

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 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]();
  }
}

Parameters

Parameter	Type
`signal`	`AbortSignal`
`resource`	(`event`) => `void`

Returns

Disposable

Disposable that removes the abort listener.

Since

v20.5.0

Inherited from

RuntimeEventEmitter.addAbortListener

getEventListeners()

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] ]
}

Parameters

Parameter	Type
`emitter`	`EventTarget` \| `EventEmitter`<`DefaultEventMap`>
`name`	`string` \| `symbol`

Returns

Function[]

Since

v15.2.0, v14.17.0

Inherited from

RuntimeEventEmitter.getEventListeners

getMaxListeners()

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
}

Parameters

Parameter	Type
`emitter`	`EventTarget` \| `EventEmitter`<`DefaultEventMap`>

Returns

number

Since

v19.9.0

Inherited from

RuntimeEventEmitter.getMaxListeners

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

Parameter	Type	Description
`emitter`	`EventEmitter`	The emitter to query
`eventName`	`string` \| `symbol`	The event name

Returns

number

Since

v0.9.12

Inherited from

RuntimeEventEmitter.listenerCount

on()

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 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'

Parameters

Parameter	Type
`emitter`	`EventEmitter`
`eventName`	`string` \| `symbol`
`options?`	`StaticEventEmitterIteratorOptions`

Returns

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

RuntimeEventEmitter.on

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 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

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'

Parameters

Parameter	Type
`emitter`	`EventTarget`
`eventName`	`string`
`options?`	`StaticEventEmitterIteratorOptions`

Returns

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

RuntimeEventEmitter.on

once()

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

Parameters

Parameter	Type
`emitter`	`EventEmitter`
`eventName`	`string` \| `symbol`
`options?`	`StaticEventEmitterOptions`

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

RuntimeEventEmitter.once

Call Signature

static once(
   emitter,
   eventName,
options?): Promise<any[]>;

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);
}

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!

Parameters

Parameter	Type
`emitter`	`EventTarget`
`eventName`	`string`
`options?`	`StaticEventEmitterOptions`

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

RuntimeEventEmitter.once

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

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.

Returns

void

Since

v15.4.0

Inherited from

RuntimeEventEmitter.setMaxListeners