Class: RTUSocket<ListenEvents, EmitEvents, ServerSideEvents, SocketData>

This is the main object for interacting with a client.

A Socket belongs to a given Namespace and uses an underlying Client to communicate.

Within each Namespace, you can also define arbitrary channels (called "rooms") that the Socket can join and leave. That provides a convenient way to broadcast to a group of socket instances.

Example

io.on("connection", (socket) => {
  console.log(`socket ${socket.id} connected`);

  // send an event to the client
  socket.emit("foo", "bar");

  socket.on("foobar", () => {
    // an event was received from the client
  });

  // join the room named "room1"
  socket.join("room1");

  // broadcast to everyone in the room named "room1"
  io.to("room1").emit("hello");

  // upon disconnection
  socket.on("disconnect", (reason) => {
    console.log(`socket ${socket.id} disconnected due to ${reason}`);
  });
});

Extends

StrictEventEmitter<ListenEvents, EmitEvents, SocketReservedEventsMap>

Type Parameters

Type Parameter	Default type
`ListenEvents` extends `EventsMap`	`DefaultEventsMap`
`EmitEvents` extends `EventsMap`	`ListenEvents`
`ServerSideEvents` extends `EventsMap`	`DefaultEventsMap`
`SocketData`	`any`

Constructors

Constructor

new RTUSocket<ListenEvents, EmitEvents, ServerSideEvents, SocketData>(
   nsp: Namespace<ListenEvents, EmitEvents, ServerSideEvents>,
   client: Client<ListenEvents, EmitEvents, ServerSideEvents>,
   auth: Record<string, unknown>,
previousSession?: Session): Socket<ListenEvents, EmitEvents, ServerSideEvents, SocketData>;

Interface to a Client for a given Namespace.

Parameters

Parameter	Type	Description
`nsp`	`Namespace`<`ListenEvents`, `EmitEvents`, `ServerSideEvents`>
`client`	`Client`<`ListenEvents`, `EmitEvents`, `ServerSideEvents`>
`auth`	`Record`<`string`, `unknown`>
`previousSession`?	`Session`	-

Returns

Socket<ListenEvents, EmitEvents, ServerSideEvents, SocketData>

Package

Overrides

StrictEventEmitter<ListenEvents, EmitEvents, SocketReservedEventsMap>.constructor

Properties

Property	Modifier	Type	Description	Inherited from
`client`	`readonly`	`Client`<`ListenEvents`, `EmitEvents`, `ServerSideEvents`>	-	-
`connected`	`public`	`boolean`	Whether the socket is currently connected or not. Example `io.use((socket, next) => { console.log(socket.connected); // false next(); }); io.on("connection", (socket) => { console.log(socket.connected); // true });`	-
`data`	`public`	`SocketData`	Additional information that can be attached to the Socket instance and which will be used in the Server.fetchSockets() method.	-
`handshake`	`readonly`	`Handshake`	The handshake details.	-
`id`	`readonly`	`string`	An unique identifier for the session.	-
`nsp`	`readonly`	`Namespace`<`ListenEvents`, `EmitEvents`, `ServerSideEvents`>	-	-
`recovered`	`readonly`	`boolean`	Whether the connection state was recovered after a temporary disconnection. In that case, any missed packets will be transmitted to the client, the data attribute and the rooms will be restored.	-
`captureRejections`	`static`	`boolean`	Value: boolean Change the default `captureRejections` option on all new `EventEmitter` objects. Since v13.4.0, v12.16.0	`StrictEventEmitter.captureRejections`
`captureRejectionSymbol`	`readonly`	typeof `captureRejectionSymbol`	Value: `Symbol.for('nodejs.rejection')` See how to write a custom `rejection handler`. Since v13.4.0, v12.16.0	`StrictEventEmitter.captureRejectionSymbol`
`defaultMaxListeners`	`static`	`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 all`EventEmitter` 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	`StrictEventEmitter.defaultMaxListeners`
`errorMonitor`	`readonly`	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	`StrictEventEmitter.errorMonitor`

Accessors

broadcast

Get Signature

get broadcast(): BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>;

Sets a modifier for a subsequent event emission that the event data will only be broadcast to every sockets but the sender.

Example

io.on("connection", (socket) => {
  // the “foo” event will be broadcast to all connected clients, except this socket
  socket.broadcast.emit("foo", "bar");
});

Returns

BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>

a new BroadcastOperator instance for chaining

conn

Get Signature

get conn(): Socket;

A reference to the underlying Client transport connection (Engine.IO Socket object).

Example

io.on("connection", (socket) => {
  console.log(socket.conn.transport.name); // prints "polling" or "websocket"

  socket.conn.once("upgrade", () => {
    console.log(socket.conn.transport.name); // prints "websocket"
  });
});

Returns

Socket

disconnected

Get Signature

get disconnected(): boolean;

Whether the socket is currently disconnected

Returns

boolean

local

Get Signature

get local(): BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>;

Sets a modifier for a subsequent event emission that the event data will only be broadcast to the current node.

Example

io.on("connection", (socket) => {
  // the “foo” event will be broadcast to all connected clients on this node, except this socket
  socket.local.emit("foo", "bar");
});

Returns

BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>

a new BroadcastOperator instance for chaining

request

Get Signature

get request(): IncomingMessage;

A reference to the request that originated the underlying Engine.IO Socket.

Returns

IncomingMessage

rooms

Get Signature

get rooms(): Set<string>;

Returns the rooms the socket is currently in.

Example

io.on("connection", (socket) => {
  console.log(socket.rooms); // Set { <socket.id> }

  socket.join("room1");

  console.log(socket.rooms); // Set { <socket.id>, "room1" }
});

Returns

Set<string>

volatile

Get Signature

get volatile(): this;

Sets a modifier for a subsequent event emission that the event data may be lost if the client is not ready to receive messages (because of network slowness or other issues, or because they’re connected through long polling and is in the middle of a request-response cycle).

Example

io.on("connection", (socket) => {
  socket.volatile.emit("hello"); // the client may or may not receive it
});

Returns

this

self

Methods

[captureRejectionSymbol]()?

optional [captureRejectionSymbol]<K>(
   error: Error,
   event: string | symbol, ...
   args: AnyRest): void;

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`error`	`Error`
`event`	`string` \| `symbol`
...`args`	`AnyRest`

Returns

void

Inherited from

StrictEventEmitter.[captureRejectionSymbol]

addListener()

addListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this;

Alias for emitter.on(eventName, listener).

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	`string` \| `symbol`
`listener`	(...`args`: `any`[]) => `void`

Returns

this

Since

v0.1.26

Inherited from

StrictEventEmitter.addListener;

compress()

compress(compress: boolean): this;

Sets the compress flag.

Parameters

Parameter	Type	Description
`compress`	`boolean`	if `true`, compresses the sending data

Returns

this

self

Example

io.on("connection", (socket) => {
  socket.compress(false).emit("hello");
});

disconnect()

disconnect(close?: boolean): this;

Disconnects this client.

Parameters

Parameter	Type	Description
`close`?	`boolean`	if `true`, closes the underlying connection

Returns

this

self

Example

io.on("connection", (socket) => {
  // disconnect this socket (the connection might be kept alive for other namespaces)
  socket.disconnect();

  // disconnect this socket and close the underlying connection
  socket.disconnect(true);
});

emit()

emit<Ev>(ev: Ev, ...args: EventParams<EmitEvents, Ev>): boolean;

Emits to this client.

Type Parameters

Type Parameter
`Ev` extends `string` \| `symbol`

Parameters

Parameter	Type
`ev`	`Ev`
...`args`	`EventParams`<`EmitEvents`, `Ev`>

Returns

boolean

Always returns true.

Example

io.on("connection", (socket) => {
  socket.emit("hello", "world");

  // all serializable datastructures are supported (no need to call JSON.stringify)
  socket.emit("hello", 1, "2", { 3: ["4"], 5: Buffer.from([6]) });

  // with an acknowledgement from the client
  socket.emit("hello", "world", (val) => {
    // ...
  });
});

Overrides

StrictEventEmitter.emit;

emitWithAck()

emitWithAck<Ev>(ev: Ev, ...args: AllButLast<EventParams<EmitEvents, Ev>>): Promise<FirstNonErrorArg<Last<EventParams<EmitEvents, Ev>>>>;

Emits an event and waits for an acknowledgement

Type Parameters

Type Parameter
`Ev` extends `string` \| `symbol`

Parameters

Parameter	Type
`ev`	`Ev`
...`args`	`AllButLast`<`EventParams`<`EmitEvents`, `Ev`>>

Returns

Promise<FirstNonErrorArg<Last<EventParams<EmitEvents, Ev>>>>

a Promise that will be fulfilled when the client acknowledges the event

Example

io.on("connection", async (socket) => {
  // without timeout
  const response = await socket.emitWithAck("hello", "world");

  // with a specific timeout
  try {
    const response = await socket.timeout(1000).emitWithAck("hello", "world");
  } catch (err) {
    // the client did not acknowledge the event in the given delay
  }
});

eventNames()

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

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

import { EventEmitter } from "node:events";

const myEE = new EventEmitter();
myEE.on("foo", () => {});
myEE.on("bar", () => {});

const sym = Symbol("symbol");
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

Returns

(string | symbol)[]

Since

v6.0.0

Inherited from

StrictEventEmitter.eventNames;

except()

except(room: string | string[]): BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>;

Excludes a room when broadcasting.

Parameters

Parameter	Type	Description
`room`	`string` \| `string`[]	a room, or an array of rooms

Returns

BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>

a new BroadcastOperator instance for chaining

Example

io.on("connection", (socket) => {
  // the "foo" event will be broadcast to all connected clients, except the ones that are in the "room-101" room
  // and this socket
  socket.except("room-101").emit("foo", "bar");

  // with an array of rooms
  socket.except(["room-101", "room-102"]).emit("foo", "bar");

  // with multiple chained calls
  socket.except("room-101").except("room-102").emit("foo", "bar");
});

getMaxListeners()

getMaxListeners(): number;

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

Returns

number

Since

v1.0.0

Inherited from

StrictEventEmitter.getMaxListeners;

in()

in(room: string | string[]): BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>;

Targets a room when broadcasting. Similar to to(), but might feel clearer in some cases:

Parameters

Parameter	Type	Description
`room`	`string` \| `string`[]	a room, or an array of rooms

Returns

BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>

a new BroadcastOperator instance for chaining

Example

io.on("connection", (socket) => {
  // disconnect all clients in the "room-101" room, except this socket
  socket.in("room-101").disconnectSockets();
});

join()

join(rooms: string | string[]):
  | void
| Promise<void>;

Joins a room.

Parameters

Parameter	Type	Description
`rooms`	`string` \| `string`[]	room or array of rooms

Returns

| void | Promise<void>

a Promise or nothing, depending on the adapter

Example

io.on("connection", (socket) => {
  // join a single room
  socket.join("room1");

  // join multiple rooms
  socket.join(["room1", "room2"]);
});

leave()

leave(room: string):
  | void
| Promise<void>;

Leaves a room.

Parameters

Parameter	Type	Description
`room`	`string`

Returns

| void | Promise<void>

a Promise or nothing, depending on the adapter

Example

io.on("connection", (socket) => {
  // leave a single room
  socket.leave("room1");

  // leave multiple rooms
  socket.leave("room1").leave("room2");
});

listenerCount()

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

Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type	Description
`eventName`	`string` \| `symbol`	The name of the event being listened for
`listener`?	`Function`	The event handler function

Returns

number

Since

v3.2.0

Inherited from

StrictEventEmitter.listenerCount;

listeners()

listeners<Ev>(event: Ev): FallbackToUntypedListener<Ev extends "error" | "disconnect" | "disconnecting" ? SocketReservedEventsMap[Ev<Ev>] : Ev extends EventNames<ListenEvents> ? ListenEvents[Ev<Ev>] : never>[];

Returns the listeners listening to an event.

Type Parameters

Type Parameter
`Ev` extends `string` \| `symbol`

Parameters

Parameter	Type	Description
`event`	`Ev`	Event name

Returns

FallbackToUntypedListener<Ev extends "error" | "disconnect" | "disconnecting" ? SocketReservedEventsMap[Ev<Ev>] : Ev extends EventNames<ListenEvents> ? ListenEvents[Ev<Ev>] : never>[]

Array of listeners subscribed to event

Inherited from

StrictEventEmitter.listeners;

listenersAny()

listenersAny(): (...args: any[]) => void[];

Returns an array of listeners that are listening for any event that is specified. This array can be manipulated, e.g. to remove listeners.

Returns

(...args: any[]) => void[]

listenersAnyOutgoing()

listenersAnyOutgoing(): (...args: any[]) => void[];

Returns an array of listeners that are listening for any event that is specified. This array can be manipulated, e.g. to remove listeners.

Returns

(...args: any[]) => void[]

off()

off<K>(eventName: string | symbol, listener: (...args: any[]) => void): this;

Alias for emitter.removeListener().

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	`string` \| `symbol`
`listener`	(...`args`: `any`[]) => `void`

Returns

this

Since

v10.0.0

Inherited from

StrictEventEmitter.off;

offAny()

offAny(listener?: (...args: any[]) => void): this;

Removes the listener that will be fired when any event is received.

Parameters

Parameter	Type	Description
`listener`?	(...`args`: `any`[]) => `void`

Returns

this

Example

io.on("connection", (socket) => {
  const catchAllListener = (event, ...args) => {
    console.log(`got event ${event}`);
  };

  socket.onAny(catchAllListener);

  // remove a specific listener
  socket.offAny(catchAllListener);

  // or remove all listeners
  socket.offAny();
});

offAnyOutgoing()

offAnyOutgoing(listener?: (...args: any[]) => void): this;

Removes the listener that will be fired when any event is sent.

Parameters

Parameter	Type	Description
`listener`?	(...`args`: `any`[]) => `void`	the catch-all listener

Returns

this

Example

io.on("connection", (socket) => {
  const catchAllListener = (event, ...args) => {
    console.log(`sent event ${event}`);
  };

  socket.onAnyOutgoing(catchAllListener);

  // remove a specific listener
  socket.offAnyOutgoing(catchAllListener);

  // or remove all listeners
  socket.offAnyOutgoing();
});

on()

on<Ev>(ev: Ev, listener: FallbackToUntypedListener<Ev extends "error" | "disconnect" | "disconnecting" ? SocketReservedEventsMap[Ev<Ev>] : Ev extends EventNames<ListenEvents> ? ListenEvents[Ev<Ev>] : never>): this;

Adds the listener function as an event listener for ev.

Type Parameters

Type Parameter
`Ev` extends `string` \| `symbol`

Parameters

Parameter	Type	Description
`ev`	`Ev`	Name of the event
`listener`	`FallbackToUntypedListener`<`Ev` extends `"error"` \| `"disconnect"` \| `"disconnecting"` ? `SocketReservedEventsMap`[`Ev`<`Ev`>] : `Ev` extends `EventNames`<`ListenEvents`> ? `ListenEvents`[`Ev`<`Ev`>] : `never`>	Callback function

Returns

this

Inherited from

StrictEventEmitter.on;

onAny()

onAny(listener: (...args: any[]) => void): this;

Adds a listener that will be fired when any event is received. The event name is passed as the first argument to the callback.

Parameters

Parameter	Type	Description
`listener`	(...`args`: `any`[]) => `void`

Returns

this

Example

io.on("connection", (socket) => {
  socket.onAny((event, ...args) => {
    console.log(`got event ${event}`);
  });
});

onAnyOutgoing()

onAnyOutgoing(listener: (...args: any[]) => void): this;

Adds a listener that will be fired when any event is sent. The event name is passed as the first argument to the callback.

Note: acknowledgements sent to the client are not included.

Parameters

Parameter	Type	Description
`listener`	(...`args`: `any`[]) => `void`

Returns

this

Example

io.on("connection", (socket) => {
  socket.onAnyOutgoing((event, ...args) => {
    console.log(`sent event ${event}`);
  });
});

once()

once<Ev>(ev: Ev, listener: FallbackToUntypedListener<Ev extends "error" | "disconnect" | "disconnecting" ? SocketReservedEventsMap[Ev<Ev>] : Ev extends EventNames<ListenEvents> ? ListenEvents[Ev<Ev>] : never>): this;

Adds a one-time listener function as an event listener for ev.

Type Parameters

Type Parameter
`Ev` extends `string` \| `symbol`

Parameters

Parameter	Type	Description
`ev`	`Ev`	Name of the event
`listener`	`FallbackToUntypedListener`<`Ev` extends `"error"` \| `"disconnect"` \| `"disconnecting"` ? `SocketReservedEventsMap`[`Ev`<`Ev`>] : `Ev` extends `EventNames`<`ListenEvents`> ? `ListenEvents`[`Ev`<`Ev`>] : `never`>	Callback function

Returns

this

Inherited from

StrictEventEmitter.once;

prependAny()

prependAny(listener: (...args: any[]) => void): this;

Adds a listener that will be fired when any event is received. The event name is passed as the first argument to the callback. The listener is added to the beginning of the listeners array.

Parameters

Parameter	Type	Description
`listener`	(...`args`: `any`[]) => `void`

Returns

this

prependAnyOutgoing()

prependAnyOutgoing(listener: (...args: any[]) => void): this;

Adds a listener that will be fired when any event is emitted. The event name is passed as the first argument to the callback. The listener is added to the beginning of the listeners array.

Parameters

Parameter	Type	Description
`listener`	(...`args`: `any`[]) => `void`

Returns

this

Example

io.on("connection", (socket) => {
  socket.prependAnyOutgoing((event, ...args) => {
    console.log(`sent event ${event}`);
  });
});

prependListener()

prependListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this;

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.prependListener("connection", (stream) => {
  console.log("someone connected!");
});

Returns a reference to the EventEmitter, so that calls can be chained.

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type	Description
`eventName`	`string` \| `symbol`	The name of the event.
`listener`	(...`args`: `any`[]) => `void`	The callback function

Returns

this

Since

v6.0.0

Inherited from

StrictEventEmitter.prependListener;

prependOnceListener()

prependOnceListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this;

Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener("connection", (stream) => {
  console.log("Ah, we have our first user!");
});

Returns a reference to the EventEmitter, so that calls can be chained.

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type	Description
`eventName`	`string` \| `symbol`	The name of the event.
`listener`	(...`args`: `any`[]) => `void`	The callback function

Returns

this

Since

v6.0.0

Inherited from

StrictEventEmitter.prependOnceListener;

rawListeners()

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

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

import { EventEmitter } from "node:events";
const emitter = new EventEmitter();
emitter.once("log", () => console.log("log once"));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners("log");
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on("log", () => console.log("log persistently"));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners("log");

// Logs "log persistently" twice
newListeners[0]();
emitter.emit("log");

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	`string` \| `symbol`

Returns

Function[]

Since

v9.4.0

Inherited from

StrictEventEmitter.rawListeners;

removeAllListeners()

removeAllListeners(eventName?: string | symbol): this;

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

Parameter	Type
`eventName`?	`string` \| `symbol`

Returns

this

Since

v0.1.26

Inherited from

StrictEventEmitter.removeAllListeners;

removeListener()

removeListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this;

Removes the specified listener from the listener array for the event named eventName.

const callback = (stream) => {
  console.log("someone connected!");
};
server.on("connection", callback);
// ...
server.removeListener("connection", callback);

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

import { EventEmitter } from "node:events";
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log("A");
  myEmitter.removeListener("event", callbackB);
};

const callbackB = () => {
  console.log("B");
};

myEmitter.on("event", callbackA);

myEmitter.on("event", callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit("event");
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit("event");
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

import { EventEmitter } from "node:events";
const ee = new EventEmitter();

function pong() {
  console.log("pong");
}

ee.on("ping", pong);
ee.once("ping", pong);
ee.removeListener("ping", pong);

ee.emit("ping");
ee.emit("ping");

Returns a reference to the EventEmitter, so that calls can be chained.

Type Parameters

Type Parameter
`K`

Parameters

Parameter	Type
`eventName`	`string` \| `symbol`
`listener`	(...`args`: `any`[]) => `void`

Returns

this

Since

v0.1.26

Inherited from

StrictEventEmitter.removeListener;

send()

send(...args: Parameters<EmitEvents["message"]>): this;

Sends a message event.

This method mimics the WebSocket.send() method.

Parameters

Parameter	Type
...`args`	`Parameters`<`EmitEvents`[`"message"`]>

Returns

this

self

See

https://developer.mozilla.org/en-US/docs/Web/API/WebSocket/send

Example

io.on("connection", (socket) => {
  socket.send("hello");

  // this is equivalent to
  socket.emit("message", "hello");
});

setMaxListeners()

setMaxListeners(n: number): this;

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

Parameter	Type
`n`	`number`

Returns

this

Since

v0.3.5

Inherited from

StrictEventEmitter.setMaxListeners;

timeout()

timeout(timeout: number): Socket<ListenEvents, DecorateAcknowledgements<EmitEvents>, ServerSideEvents, SocketData>;

Sets a modifier for a subsequent event emission that the callback will be called with an error when the given number of milliseconds have elapsed without an acknowledgement from the client:

Parameters

Parameter	Type
`timeout`	`number`

Returns

Socket<ListenEvents, DecorateAcknowledgements<EmitEvents>, ServerSideEvents, SocketData>

self

Example

io.on("connection", (socket) => {
  socket.timeout(5000).emit("my-event", (err) => {
    if (err) {
      // the client did not acknowledge the event in the given delay
    }
  });
});

to()

to(room: string | string[]): BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>;

Targets a room when broadcasting.

Parameters

Parameter	Type	Description
`room`	`string` \| `string`[]	a room, or an array of rooms

Returns

BroadcastOperator<DecorateAcknowledgementsWithMultipleResponses<EmitEvents>, SocketData>

a new BroadcastOperator instance for chaining

Example

io.on("connection", (socket) => {
  // the “foo” event will be broadcast to all connected clients in the “room-101” room, except this socket
  socket.to("room-101").emit("foo", "bar");

  // the code above is equivalent to:
  io.to("room-101").except(socket.id).emit("foo", "bar");

  // with an array of rooms (a client will be notified at most once)
  socket.to(["room-101", "room-102"]).emit("foo", "bar");

  // with multiple chained calls
  socket.to("room-101").to("room-102").emit("foo", "bar");
});

use()

use(fn: (event: Event, next: (err?: Error) => void) => void): this;

Sets up socket middleware.

Parameters

Parameter	Type	Description
`fn`	(`event`: `Event`, `next`: (`err`?: `Error`) => `void`) => `void`	middleware function (event, next)

Returns

this

self

Example

io.on("connection", (socket) => {
  socket.use(([event, ...args], next) => {
    if (isUnauthorized(event)) {
      return next(new Error("unauthorized event"));
    }
    // do not forget to call next
    next();
  });

  socket.on("error", (err) => {
    if (err && err.message === "unauthorized event") {
      socket.disconnect();
    }
  });
});

write()

write(...args: Parameters<EmitEvents["message"]>): this;

Sends a message event. Alias of send.

Parameters

Parameter	Type
...`args`	`Parameters`<`EmitEvents`[`"message"`]>

Returns

this

self

addAbortListener()

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

Experimental

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`: `Event`) => `void`

Returns

Disposable

Disposable that removes the abort listener.

Since

v20.5.0

Inherited from

StrictEventEmitter.addAbortListener;

getEventListeners()

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

Returns a copy of the array of listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

import { getEventListeners, EventEmitter } from "node:events";

{
  const ee = new EventEmitter();
  const listener = () => console.log("Events are fun");
  ee.on("foo", listener);
  console.log(getEventListeners(ee, "foo")); // [ [Function: listener] ]
}
{
  const et = new EventTarget();
  const listener = () => console.log("Events are fun");
  et.addEventListener("foo", listener);
  console.log(getEventListeners(et, "foo")); // [ [Function: listener] ]
}

Parameters

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

Returns

Function[]

Since

v15.2.0, v14.17.0

Inherited from

StrictEventEmitter.getEventListeners;

getMaxListeners()

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

Returns the currently set max amount of listeners.

For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.

For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.

import { getMaxListeners, setMaxListeners, EventEmitter } from "node:events";

{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}

Parameters

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

Returns

number

Since

v19.9.0

Inherited from

StrictEventEmitter.getMaxListeners;

listenerCount()

static listenerCount(emitter: EventEmitter, eventName: string | symbol): 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

Deprecated

Since v3.2.0 - Use listenerCount instead.

Inherited from

StrictEventEmitter.listenerCount;

on()

Call Signature

static on(
   emitter: EventEmitter,
   eventName: string | symbol,
options?: StaticEventEmitterIteratorOptions): AsyncIterableIterator<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

AsyncIterableIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

StrictEventEmitter.on;

Call Signature

static on(
   emitter: EventTarget,
   eventName: string,
options?: StaticEventEmitterIteratorOptions): AsyncIterableIterator<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

AsyncIterableIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

StrictEventEmitter.on;

once()

Call Signature

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

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

import { once, EventEmitter } from "node:events";
import process from "node:process";

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit("myevent", 42);
});

const [value] = await once(ee, "myevent");
console.log(value);

const err = new Error("kaboom");
process.nextTick(() => {
  ee.emit("error", err);
});

try {
  await once(ee, "myevent");
} catch (err) {
  console.error("error happened", err);
}

The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

import { EventEmitter, once } from "node:events";

const ee = new EventEmitter();

once(ee, "error")
  .then(([err]) => console.log("ok", err.message))
  .catch((err) => console.error("error", err.message));

ee.emit("error", new Error("boom"));

// Prints: ok boom

An AbortSignal can be used to cancel waiting for the event:

import { EventEmitter, once } from "node:events";

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log("event emitted!");
  } catch (error) {
    if (error.name === "AbortError") {
      console.error("Waiting for the event was canceled!");
    } else {
      console.error("There was an error", error.message);
    }
  }
}

foo(ee, "foo", ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit("foo"); // Prints: Waiting for the event was canceled!

Parameters

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

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

StrictEventEmitter.once;

Call Signature

static once(
   emitter: EventTarget,
   eventName: string,
options?: StaticEventEmitterOptions): 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

StrictEventEmitter.once;

setMaxListeners()

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

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

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

setMaxListeners(5, target, emitter);

Parameters

Parameter	Type	Description
`n`?	`number`	A non-negative number. The maximum number of listeners per `EventTarget` event.
...`eventTargets`?	(`EventEmitter`<`DefaultEventMap`> \| `EventTarget`)[]	-

Returns

void

Since

v15.4.0

Inherited from

StrictEventEmitter.setMaxListeners;

Class: RTUSocket<ListenEvents, EmitEvents, ServerSideEvents, SocketData> ​

Example ​

Extends ​

Type Parameters ​

Constructors ​

Constructor ​

Parameters ​

Returns ​

Package ​

Overrides ​

Properties ​

Accessors ​

broadcast ​

Get Signature ​

Example ​

Returns ​

conn ​

Get Signature ​

Example ​

Returns ​

disconnected ​

Get Signature ​

Returns ​

local ​

Get Signature ​

Example ​

Returns ​

request ​

Get Signature ​

Returns ​

rooms ​

Get Signature ​

Example ​

Returns ​

volatile ​

Get Signature ​

Example ​

Returns ​

Methods ​

[captureRejectionSymbol]()? ​

Type Parameters ​

Parameters ​

Returns ​

Inherited from ​

addListener() ​

Type Parameters ​

Parameters ​

Returns ​

Since ​

Inherited from ​

compress() ​

Parameters ​

Returns ​

Example ​

disconnect() ​

Parameters ​

Returns ​

Example ​

emit() ​

Type Parameters ​

Parameters ​

Returns ​

Example ​

Overrides ​

emitWithAck() ​

Type Parameters ​

Parameters ​

Returns ​

Example ​

eventNames() ​

Returns ​

Since ​

Inherited from ​

except() ​

Parameters ​

Returns ​

Example ​

getMaxListeners() ​

Returns ​

Since ​

Class: RTUSocket<ListenEvents, EmitEvents, ServerSideEvents, SocketData>

Example

Extends

Type Parameters

Constructors

Constructor

Parameters

Returns

Package

Overrides

Properties

Accessors

broadcast

Get Signature

Example

Returns

conn

Get Signature

Example

Returns

disconnected

Get Signature

Returns

local

Get Signature

Example

Returns

request

Get Signature

Returns

rooms

Get Signature

Example

Returns

volatile

Get Signature

Example

Returns

Methods

[captureRejectionSymbol]()?

Type Parameters

Parameters

Returns

Inherited from

addListener()

Type Parameters

Parameters

Returns

Since

Inherited from

compress()

Parameters

Returns

Example

disconnect()

Parameters

Returns

Example

emit()

Type Parameters

Parameters

Returns

Example

Overrides

emitWithAck()

Type Parameters

Parameters

Returns

Example

eventNames()

Returns

Since

Inherited from

except()

Parameters

Returns

Example

getMaxListeners()

Returns

Since