Version: 4.x

Client API

IO

The io method is bound to the global scope in the standalone build:

<script src="/socket.io/socket.io.js"></script>
<script>
  const socket = io();
</script>

An ESM bundle is also available since version 4.3.0:

<script type="module">
  import { io } from "https://cdn.socket.io/4.4.1/socket.io.esm.min.js";

  const socket = io();
</script>

With an import map:

<script type="importmap">
  {
    "imports": {
      "socket.io-client": "https://cdn.socket.io/4.4.1/socket.io.esm.min.js"
    }
  }
</script>

<script type="module">
  import { io } from "socket.io-client";

  const socket = io();
</script>

Else, in all other cases (with some build tools, in Node.js or React Native), it can be imported from the socket.io-client package:

// ES modules
import { io } from "socket.io-client";

// CommonJS
const { io } = require("socket.io-client");

io.protocol

<number>

The protocol revision number (currently: 5).

The protocol defines the format of the packets exchanged between the client and the server. Both the client and the server must use the same revision in order to understand each other.

You can find more information here.

io([url][, options])

url <string> (defaults to window.location)
options <Object>
- forceNew <boolean> whether to create a new connection
Returns <Socket>

Creates a new Manager for the given URL, and attempts to reuse an existing Manager for subsequent calls, unless the multiplex option is passed with false. Passing this option is the equivalent of passing "force new connection": true or forceNew: true.

A new Socket instance is returned for the namespace specified by the pathname in the URL, defaulting to /. For example, if the url is http://localhost/users, a transport connection will be established to http://localhost and a Socket.IO connection will be established to /users.

Query parameters can also be provided, either with the query option or directly in the url (example: http://localhost/users?token=abc).

To understand what happens under the hood, the following example:

import { io } from "socket.io-client";

const socket = io("ws://example.com/my-namespace", {
  reconnectionDelayMax: 10000,
  auth: {
    token: "123"
  },
  query: {
    "my-key": "my-value"
  }
});

is the short version of:

import { Manager } from "socket.io-client";

const manager = new Manager("ws://example.com", {
  reconnectionDelayMax: 10000,
  query: {
    "my-key": "my-value"
  }
});

const socket = manager.socket("/my-namespace", {
  auth: {
    token: "123"
  }
});

The complete list of available options can be found here.

Manager

The Manager manages the Engine.IO client instance, which is the low-level engine that establishes the connection to the server (by using transports like WebSocket or HTTP long-polling).

The Manager handles the reconnection logic.

A single Manager can be used by several Sockets. You can find more information about this multiplexing feature here.

Please note that, in most cases, you won't use the Manager directly but use the Socket instance instead.

new Manager(url[, options])

url <string>
options <Object>
Returns <Manager>

The complete list of available options can be found here.

import { Manager } from "socket.io-client";

const manager = new Manager("https://example.com");

const socket = manager.socket("/"); // main namespace
const adminSocket = manager.socket("/admin"); // admin namespace

manager.reconnection([value])

value <boolean>
Returns <Manager> | <boolean>

Sets the reconnection option, or returns it if no parameters are passed.

manager.reconnectionAttempts([value])

value <number>
Returns <Manager> | <number>

Sets the reconnectionAttempts option, or returns it if no parameters are passed.

manager.reconnectionDelay([value])

value <number>
Returns <Manager> | <number>

Sets the reconnectionDelay option, or returns it if no parameters are passed.

manager.reconnectionDelayMax([value])

value <number>
Returns <Manager> | <number>

Sets the reconnectionDelayMax option, or returns it if no parameters are passed.

manager.timeout([value])

value <number>
Returns <Manager> | <number>

Sets the timeout option, or returns it if no parameters are passed.

manager.open([callback])

callback <Function>
Returns <Manager>

If the manager was initiated with autoConnect to false, launch a new connection attempt.

The callback argument is optional and will be called once the attempt fails/succeeds.

import { Manager } from "socket.io-client";

const manager = new Manager("https://example.com", {
  autoConnect: false
});

const socket = manager.socket("/");

manager.open((err) => {
  if (err) {
    // an error has occurred
  } else {
    // the connection was successfully established
  }
});

manager.connect([callback])

Synonym of manager.open([callback]).

manager.socket(nsp, options)

nsp <string>
options <Object>
Returns <Socket>

Creates a new Socket for the given namespace. Only auth ({ auth: {key: "value"} }) is read from the options object. Other keys will be ignored and should be passed when instancing a new Manager(nsp, options).

Event: 'error'

error <Error> error object

Fired upon a connection error.

socket.io.on("error", (error) => {
  // ...
});

Event: 'reconnect'

attempt <number> reconnection attempt number

Fired upon a successful reconnection.

socket.io.on("reconnect", (attempt) => {
  // ...
});

Event: 'reconnect_attempt'

attempt <number> reconnection attempt number

Fired upon an attempt to reconnect.

socket.io.on("reconnect_attempt", (attempt) => {
  // ...
});

Event: 'reconnect_error'

error <Error> error object

Fired upon a reconnection attempt error.

socket.io.on("reconnect_error", (error) => {
  // ...
});

Event: 'reconnect_failed'

Fired when couldn't reconnect within reconnectionAttempts.

socket.io.on("reconnect_failed", () => {
  // ...
});

Event: 'ping'

Fired when a ping packet is received from the server.

socket.io.on("ping", () => {
  // ...
});

Socket

A Socket is the fundamental class for interacting with the server. A Socket belongs to a certain Namespace (by default /) and uses an underlying Manager to communicate.

A Socket is basically an EventEmitter which sends events to — and receive events from — the server over the network.

socket.emit("hello", { a: "b", c: [] });

socket.on("hey", (...args) => {
  // ...
});

More information can be found here.

socket.id

<string>

An unique identifier for the socket session. Set after the connect event is triggered, and updated after the reconnect event.

const socket = io("http://localhost");

console.log(socket.id); // undefined

socket.on("connect", () => {
  console.log(socket.id); // "G5p5..."
});

socket.connected

<boolean>

Whether or not the socket is connected to the server.

const socket = io("http://localhost");

socket.on("connect", () => {
  console.log(socket.connected); // true
});

socket.disconnected

<boolean>

Whether or not the socket is disconnected from the server.

const socket = io("http://localhost");

socket.on("connect", () => {
  console.log(socket.disconnected); // false
});

socket.io

<Manager>

A reference to the underlying Manager.

socket.on("connect", () => {
  const engine = socket.io.engine;
  console.log(engine.transport.name); // in most cases, prints "polling"

  engine.once("upgrade", () => {
    // called when the transport is upgraded (i.e. from HTTP long-polling to WebSocket)
    console.log(engine.transport.name); // in most cases, prints "websocket"
  });

  engine.on("packet", ({ type, data }) => {
    // called for each packet received
  });

  engine.on("packetCreate", ({ type, data }) => {
    // called for each packet sent
  });

  engine.on("drain", () => {
    // called when the write buffer is drained
  });

  engine.on("close", (reason) => {
    // called when the underlying connection is closed
  });
});

socket.connect()

Added in v1.0.0

Returns Socket

Manually connects the socket.

const socket = io({
  autoConnect: false
});

// ...
socket.connect();

It can also be used to manually reconnect:

socket.on("disconnect", () => {
  socket.connect();
});

socket.open()

Added in v1.0.0

Synonym of socket.connect().

socket.send([...args][, ack])

args <any[]>
ack <Function>
Returns <Socket>

Sends a message event. See socket.emit(eventName[, ...args][, ack]).

socket.emit(eventName[, ...args][, ack])

eventName <string> | <symbol>
args <any[]>
ack <Function>
Returns true

Emits an event to the socket identified by the string name. Any other parameters can be included. All serializable datastructures are supported, including Buffer.

socket.emit("hello", "world");
socket.emit("with-binary", 1, "2", { 3: "4", 5: Buffer.from([6, 7, 8]) });

The ack argument is optional and will be called with the server answer.

Client

socket.emit("hello", "world", (response) => {
  console.log(response); // "got it"
});

Server

io.on("connection", (socket) => {
  socket.on("hello", (arg, callback) => {
    console.log(arg); // "world"
    callback("got it");
  });
});

socket.on(eventName, callback)

Inherited from the EventEmitter class.

eventName <string> | <symbol>
listener <Function>
Returns <Socket>

socket.on("news", (data) => {
  console.log(data);
});

// with multiple arguments
socket.on("news", (arg1, arg2, arg3, arg4) => {
  // ...
});
// with callback
socket.on("news", (cb) => {
  cb(0);
});

socket.once(eventName, callback)

Inherited from the EventEmitter class.

eventName <string> | <symbol>
listener <Function>
Returns <Socket>

Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

socket.once("my-event", () => {
  // ...
});

socket.off([eventName][, listener])

Inherited from the EventEmitter class.

eventName <string> | <symbol>
listener <Function>
Returns <Socket>

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

const myListener = () => {
  // ...
}

socket.on("my-event", myListener);

// then later
socket.off("my-event", myListener);

The listener argument can also be omitted:

// remove all listeners for that event
socket.off("my-event");

// remove all listeners for all events
socket.off();

socket.listeners(eventName)

Inherited from the EventEmitter class.

eventName <string> | <symbol>
Returns <Function[]>

Returns the array of listeners for the event named eventName.

socket.on("my-event", () => {
  // ...
});

console.log(socket.listeners("my-event")); // prints [ [Function] ]

socket.onAny(callback)

callback <Function>

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

socket.prependAny(callback)

callback <Function>

socket.prependAny((event, ...args) => {
  console.log(`got ${event}`);
});

socket.offAny([listener])

listener <Function>

Removes the previously registered listener. If no listener is provided, all catch-all listeners are removed.

const myListener = () => { /* ... */ };

socket.onAny(myListener);

// then, later
socket.offAny(myListener);

socket.offAny();

socket.listenersAny()

Returns <Function[]>

Returns the list of registered catch-all listeners.

const listeners = socket.listenersAny();

socket.compress(value)

value <boolean>
Returns <Socket>

Sets a modifier for a subsequent event emission that the event data will only be compressed if the value is true. Defaults to true when you don't call the method.

socket.compress(false).emit("an event", { some: "data" });

socket.timeout(value)

Added in v4.4.0

value <number>
Returns <Socket>

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 server:

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

socket.disconnect()

Added in v1.0.0

Returns <Socket>

Manually disconnects the socket. In that case, the socket will not try to reconnect.

Associated disconnection reason:

client-side: "io client disconnect"
server-side: "client namespace disconnect"

If this is the last active Socket instance of the Manager, the low-level connection will be closed.

socket.close()

Added in v1.0.0

Synonym of socket.disconnect().

Flag: 'volatile'

Added in v3.0.0

Sets a modifier for the subsequent event emission indicating that the packet may be dropped if:

the socket is not connected
the low-level transport is not writable (for example, when a POST request is already running in HTTP long-polling mode)

socket.volatile.emit(/* ... */); // the server may or may not receive it

Event: 'connect'

Fired upon connection to the Namespace (including a successful reconnection).

socket.on("connect", () => {
  // ...
});

caution

Please note that you shouldn't register event handlers in the connect handler itself, as a new handler will be registered every time the Socket reconnects:

// BAD
socket.on("connect", () => {
  socket.on("data", () => { /* ... */ });
});

// GOOD
socket.on("connect", () => { /* ... */ });
socket.on("data", () => { /* ... */ });

Event: 'disconnect'

reason <string>

Fired upon disconnection. The list of possible disconnection reasons:

Reason	Description
`io server disconnect`	The server has forcefully disconnected the socket with socket.disconnect()
`io client disconnect`	The socket was manually disconnected using socket.disconnect()
`ping timeout`	The server did not send a PING within the `pingInterval + pingTimeout` range
`transport close`	The connection was closed (example: the user has lost connection, or the network was changed from WiFi to 4G)
`transport error`	The connection has encountered an error (example: the server was killed during a HTTP long-polling cycle)

In the first two cases (explicit disconnection), the client will not try to reconnect and you need to manually call socket.connect().

In all other cases, the client will wait for a small random delay and then try to reconnect:

socket.on("disconnect", (reason) => {
  if (reason === "io server disconnect") {
    // the disconnection was initiated by the server, you need to reconnect manually
    socket.connect();
  }
  // else the socket will automatically try to reconnect
});

Event: 'connect_error'

connect_error <Error> error object

Fired when a namespace middleware error occurs.

socket.on("connect_error", (error) => {
  // ...
});

IO​

io.protocol​

io([url][, options])​

Manager​

new Manager(url[, options])​

manager.reconnection([value])​

manager.reconnectionAttempts([value])​

manager.reconnectionDelay([value])​

manager.reconnectionDelayMax([value])​

manager.timeout([value])​

manager.open([callback])​

manager.connect([callback])​

manager.socket(nsp, options)​

Event: 'error'​

Event: 'reconnect'​

Event: 'reconnect_attempt'​

Event: 'reconnect_error'​

Event: 'reconnect_failed'​

Event: 'ping'​

Socket​

socket.id​

socket.connected​

socket.disconnected​

socket.io​

socket.connect()​

socket.open()​

socket.send([...args][, ack])​

socket.emit(eventName[, ...args][, ack])​

socket.on(eventName, callback)​

socket.once(eventName, callback)​

socket.off([eventName][, listener])​

socket.listeners(eventName)​

socket.onAny(callback)​

socket.prependAny(callback)​

socket.offAny([listener])​

socket.listenersAny()​

socket.compress(value)​

socket.timeout(value)​

socket.disconnect()​

socket.close()​

Flag: 'volatile'​

Event: 'connect'​

caution

Event: 'disconnect'​

Event: 'connect_error'​

IO

io.protocol

io([url][, options])

Manager

new Manager(url[, options])

manager.reconnection([value])

manager.reconnectionAttempts([value])

manager.reconnectionDelay([value])

manager.reconnectionDelayMax([value])

manager.timeout([value])

manager.open([callback])

manager.connect([callback])

manager.socket(nsp, options)

Event: 'error'

Event: 'reconnect'

Event: 'reconnect_attempt'

Event: 'reconnect_error'

Event: 'reconnect_failed'

Event: 'ping'

Socket

socket.id

socket.connected

socket.disconnected

socket.io

socket.connect()

socket.open()

socket.send([...args][, ack])

socket.emit(eventName[, ...args][, ack])

socket.on(eventName, callback)

socket.once(eventName, callback)

socket.off([eventName][, listener])

socket.listeners(eventName)

socket.onAny(callback)

socket.prependAny(callback)

socket.offAny([listener])

socket.listenersAny()

socket.compress(value)

socket.timeout(value)

socket.disconnect()

socket.close()

Flag: 'volatile'

Event: 'connect'

Event: 'disconnect'

Event: 'connect_error'