Version: 2.x

Server API

Server

Exposed by require('socket.io').

new Server(httpServer[, options])

httpServer (http.Server) the server to bind to.
options (Object)

Works with and without new:

const io = require('socket.io')();
// or
const Server = require('socket.io');
const io = new Server();

Available options:

Option	Default value	Description
`path`	`/socket.io`	name of the path to capture
`serveClient`	`true`	whether to serve the client files
`adapter`	-	the adapter to use. Defaults to an instance of the `Adapter` that ships with socket.io which is memory based. See socket.io-adapter
`origins`	`*`	the allowed origins
`parser`	-	the parser to use. Defaults to an instance of the `Parser` that ships with socket.io. See socket.io-parser.

Available options for the underlying Engine.IO server:

Option	Default value	Description
`pingTimeout`	`5000`	how many ms without a pong packet to consider the connection closed
`pingInterval`	`25000`	how many ms before sending a new ping packet
`upgradeTimeout`	`10000`	how many ms before an uncompleted transport upgrade is cancelled
`maxHttpBufferSize`	`10e7`	how many bytes or characters a message can be, before closing the session (to avoid DoS).
`allowRequest`		A function that receives a given handshake or upgrade request as its first parameter, and can decide whether to continue or not. The second argument is a function that needs to be called with the decided information: `fn(err, success)`, where `success` is a boolean value where false means that the request is rejected, and err is an error code.
`transports`	`['polling', 'websocket']`	transports to allow connections to
`allowUpgrades`	`true`	whether to allow transport upgrades
`perMessageDeflate`	`false`	Whether to enable the permessage-deflate extension for the WebSocket transport. This extension is known to add a significant overhead in terms of performance and memory consumption, so we suggest to only enable it if it is really needed. Defaults to `false` since version `2.4.0`.
`httpCompression`	`true`	parameters of the http compression for the polling transports (see zlib api docs). Set to `false` to disable.
`cookie`	`io`	name of the HTTP cookie that contains the client sid to send as part of handshake response headers. Set to `false` to not send one.
`cookiePath`	`/`	path of the above `cookie` option. If false, no path will be sent, which means browsers will only send the cookie on the engine.io attached path (`/engine.io`). Set false to not save io cookie on all requests.
`cookieHttpOnly`	`true`	if `true` HttpOnly io cookie cannot be accessed by client-side APIs, such as JavaScript. This option has no effect if `cookie` or `cookiePath` is set to `false`.
`wsEngine`	`ws`	what WebSocket server implementation to use. Specified module must conform to the `ws` interface (see ws module api docs). Default value is `ws`. An alternative c++ addon is also available by installing the eiows module.

Among those options:

The pingTimeout and pingInterval parameters will impact the delay before a client knows the server is not available anymore. For example, if the underlying TCP connection is not closed properly due to a network issue, a client may have to wait up to pingTimeout + pingInterval ms before getting a disconnect event.
The order of the transports array is important. By default, a long-polling connection is established first, and then upgraded to WebSocket if possible. Using ['websocket'] means there will be no fallback if a WebSocket connection cannot be opened.

const server = require('http').createServer();

const io = require('socket.io')(server, {
  path: '/test',
  serveClient: false,
  // below are engine.IO options
  pingInterval: 10000,
  pingTimeout: 5000,
  cookie: false
});

server.listen(3000);

new Server(port[, options])

port (Number) a port to listen to (a new http.Server will be created)
options (Object)

See above for the list of available options.

const io = require('socket.io')(3000, {
  path: '/test',
  serveClient: false,
  // below are engine.IO options
  pingInterval: 10000,
  pingTimeout: 5000,
  cookie: false
});

new Server(options)

options (Object)

See above for the list of available options.

const io = require('socket.io')({
  path: '/test',
  serveClient: false,
});

// either
const server = require('http').createServer();

io.attach(server, {
  pingInterval: 10000,
  pingTimeout: 5000,
  cookie: false
});

server.listen(3000);

// or
io.attach(3000, {
  pingInterval: 10000,
  pingTimeout: 5000,
  cookie: false
});

server.sockets

(Namespace)

An alias for the default (/) namespace.

io.sockets.emit('hi', 'everyone');
// is equivalent to
io.of('/').emit('hi', 'everyone');

server.serveClient([value])

value (Boolean)
Returns Server|Boolean

If value is true the attached server (see Server#attach) will serve the client files. Defaults to true. This method has no effect after attach is called. If no arguments are supplied this method returns the current value.

// pass a server and the `serveClient` option
const io = require('socket.io')(http, { serveClient: false });

// or pass no server and then you can call the method
const io = require('socket.io')();
io.serveClient(false);
io.attach(http);

server.path([value])

value (String)
Returns Server|String

Sets the path value under which engine.io and the static files will be served. Defaults to /socket.io. If no arguments are supplied this method returns the current value.

const io = require('socket.io')();
io.path('/myownpath');

// client-side
const socket = io({
  path: '/myownpath'
});

server.adapter([value])

value (Adapter)
Returns Server|Adapter

Sets the adapter value. Defaults to an instance of the Adapter that ships with socket.io which is memory based. See socket.io-adapter. If no arguments are supplied this method returns the current value.

const io = require('socket.io')(3000);
const redis = require('socket.io-redis');
io.adapter(redis({ host: 'localhost', port: 6379 }));

server.origins([value])

value (String|String[])
Returns Server|String

Sets the allowed origins value. Defaults to any origins being allowed. If no arguments are supplied this method returns the current value.

io.origins(['https://foo.example.com:443']);

server.origins(fn)

fn (Function)
Returns Server

Provides a function taking two arguments origin:String and callback(error, success), where success is a boolean value indicating whether origin is allowed or not. If success is set to false, error must be provided as a string value that will be appended to the server response, e.g. "Origin not allowed".

Potential drawbacks:

in some situations, when it is not possible to determine origin it may have value of *
As this function will be executed for every request, it is advised to make this function work as fast as possible
If socket.io is used together with Express, the CORS headers will be affected only for socket.io requests. For Express you can use cors.

io.origins((origin, callback) => {
  if (origin !== 'https://foo.example.com') {
    return callback('origin not allowed', false);
  }
  callback(null, true);
});

server.attach(httpServer[, options])

httpServer (http.Server) the server to attach to
options (Object)

Attaches the Server to an engine.io instance on httpServer with the supplied options (optionally).

server.attach(port[, options])

port (Number) the port to listen on
options (Object)

Attaches the Server to an engine.io instance on a new http.Server with the supplied options (optionally).

server.listen(httpServer[, options])

Synonym of server.attach(httpServer[, options]).

server.listen(port[, options])

Synonym of server.attach(port[, options]).

server.bind(engine)

engine (engine.Server)
Returns Server

Advanced use only. Binds the server to a specific engine.io Server (or compatible API) instance.

server.onconnection(socket)

socket (engine.Socket)
Returns Server

Advanced use only. Creates a new socket.io client from the incoming engine.io (or compatible API) Socket.

server.of(nsp)

nsp (String|RegExp|Function)
Returns Namespace

Initializes and retrieves the given Namespace by its pathname identifier nsp. If the namespace was already initialized it returns it immediately.

const adminNamespace = io.of('/admin');

A regex or a function can also be provided, in order to create namespace in a dynamic way:

const dynamicNsp = io.of(/^\/dynamic-\d+$/).on('connection', (socket) => {
  const newNamespace = socket.nsp; // newNamespace.name === '/dynamic-101'

  // broadcast to all clients in the given sub-namespace
  newNamespace.emit('hello');
});

// client-side
const socket = io('/dynamic-101');

// broadcast to all clients in each sub-namespace
dynamicNsp.emit('hello');

// use a middleware for each sub-namespace
dynamicNsp.use((socket, next) => { /* ... */ });

With a function:

io.of((name, query, next) => {
  // the checkToken method must return a boolean, indicating whether the client is able to connect or not.
  next(null, checkToken(query.token));
}).on('connection', (socket) => { /* ... */ });

server.close([callback])

callback (Function)

Closes the socket.io server. The callback argument is optional and will be called when all connections are closed.

const Server = require('socket.io');
const PORT   = 3030;
const server = require('http').Server();

const io = Server(PORT);

io.close(); // Close current server

server.listen(PORT); // PORT is free to use

io = Server(server);

server.engine.generateId

Overwrites the default method to generate your custom socket id.

The function is called with a node request object (http.IncomingMessage) as first parameter.

io.engine.generateId = (req) => {
  return "custom:id:" + custom_id++; // custom id must be unique
}

Namespace

Represents a pool of sockets connected under a given scope identified by a pathname (eg: /chat).

A client always connects to / (the main namespace), then potentially connect to other namespaces (while using the same underlying connection).

For the how and why, please take a look at: Rooms and Namespaces.

namespace.name

(String)

The namespace identifier property.

namespace.connected

_(Object<Socket>)_

The hash of Socket objects that are connected to this namespace, indexed by id.

namespace.adapter

(Adapter)

The Adapter used for the namespace. Useful when using the Adapter based on Redis, as it exposes methods to manage sockets and rooms across your cluster.

Note: the adapter of the main namespace can be accessed with io.of('/').adapter.

namespace.to(room)

room (String)
Returns Namespace for chaining

Sets a modifier for a subsequent event emission that the event will only be broadcasted to clients that have joined the given room.

To emit to multiple rooms, you can call to several times.

const io = require('socket.io')();
const adminNamespace = io.of('/admin');

adminNamespace.to('level1').emit('an event', { some: 'data' });

namespace.in(room)

Synonym of namespace.to(room).

namespace.emit(eventName[, ...args])

eventName (String)
args

Emits an event to all connected clients. The following two are equivalent:

const io = require('socket.io')();
io.emit('an event sent to all connected clients'); // main namespace

const chat = io.of('/chat');
chat.emit('an event sent to all connected clients in chat namespace');

Note: acknowledgements are not supported when emitting from namespace.

namespace.clients(callback)

callback (Function)

Gets a list of client IDs connected to this namespace (across all nodes if applicable).

const io = require('socket.io')();
io.of('/chat').clients((error, clients) => {
  if (error) throw error;
  console.log(clients); // => [PZDoMHjiu8PYfRiKAAAF, Anw2LatarvGVVXEIAAAD]
});

An example to get all clients in namespace's room:

io.of('/chat').in('general').clients((error, clients) => {
  if (error) throw error;
  console.log(clients); // => [Anw2LatarvGVVXEIAAAD]
});

As with broadcasting, the default is all clients from the default namespace ('/'):

io.clients((error, clients) => {
  if (error) throw error;
  console.log(clients); // => [6em3d4TJP8Et9EMNAAAA, G5p55dHhGgUnLUctAAAB]
});

namespace.use(fn)

fn (Function)

Registers a middleware, which is a function that gets executed for every incoming Socket, and receives as parameters the socket and a function to optionally defer execution to the next registered middleware.

Errors passed to middleware callbacks are sent as special error packets to clients.

io.use((socket, next) => {
  if (socket.request.headers.cookie) return next();
  next(new Error('Authentication error'));
});

Event: 'connect'

socket (Socket) socket connection with client

Fired upon a connection from client.

io.on('connection', (socket) => {
  // ...
});

io.of('/admin').on('connection', (socket) => {
  // ...
});

Event: 'connection'

Synonym of Event: 'connect'.

Flag: 'volatile'

Sets a modifier for a subsequent event emission that the event data may be lost if the clients are 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).

io.volatile.emit('an event', { some: 'data' }); // the clients may or may not receive it

Flag: 'binary'

Specifies whether there is binary data in the emitted data. Increases performance when specified. Can be true or false.

io.binary(false).emit('an event', { some: 'data' });

Flag: 'local'

Sets a modifier for a subsequent event emission that the event data will only be broadcast to the current node (when the Redis adapter is used).

io.local.emit('an event', { some: 'data' });

Socket

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

It should be noted the Socket doesn't relate directly to the actual underlying TCP/IP socket and it is only the name of the class.

Within each Namespace, you can also define arbitrary channels (called room) that the Socket can join and leave. That provides a convenient way to broadcast to a group of Sockets (see Socket#to below).

The Socket class inherits from EventEmitter. The Socket class overrides the emit method, and does not modify any other EventEmitter method. All methods documented here which also appear as EventEmitter methods (apart from emit) are implemented by EventEmitter, and documentation for EventEmitter applies.

socket.id

(String)

A unique identifier for the session, that comes from the underlying Client.

socket.rooms

(Object)

A hash of strings identifying the rooms this client is in, indexed by room name.

io.on('connection', (socket) => {
  socket.join('room 237', () => {
    let rooms = Object.keys(socket.rooms);
    console.log(rooms); // [ <socket.id>, 'room 237' ]
  });
});

socket.client

(Client)

A reference to the underlying Client object.

socket.conn

(engine.Socket)

A reference to the underlying Client transport connection (engine.io Socket object). This allows access to the IO transport layer, which still (mostly) abstracts the actual TCP/IP socket.

socket.request

(Request)

A getter proxy that returns the reference to the request that originated the underlying engine.io Client. Useful for accessing request headers such as Cookie or User-Agent.

const cookie = require('cookie');

io.on('connection', (socket) => {
  const cookies = cookie.parse(socket.request.headers.cookie || '');
});

socket.handshake

(Object)

The handshake details:

{
  headers: /* the headers sent as part of the handshake */,
  time: /* the date of creation (as string) */,
  address: /* the ip of the client */,
  xdomain: /* whether the connection is cross-domain */,
  secure: /* whether the connection is secure */,
  issued: /* the date of creation (as unix timestamp) */,
  url: /* the request URL string */,
  query: /* the query object */
}

Usage:

io.use((socket, next) => {
  let handshake = socket.handshake;
  // ...
});

io.on('connection', (socket) => {
  let handshake = socket.handshake;
  // ...
});

socket.use(fn)

fn (Function)

Registers a middleware, which is a function that gets executed for every incoming Packet and receives as parameter the packet and a function to optionally defer execution to the next registered middleware.

Errors passed to middleware callbacks are sent as special error packets to clients.

io.on('connection', (socket) => {
  socket.use((packet, next) => {
    if (packet.doge === true) return next();
    next(new Error('Not a doge error'));
  });
});

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

args
ack (Function)
Returns Socket

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

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

(overrides EventEmitter.emit)

eventName (String)
args
ack (Function)
Returns Socket

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

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

The ack argument is optional and will be called with the client's answer.

io.on('connection', (socket) => {
  socket.emit('an event', { some: 'data' });

  socket.emit('ferret', 'tobi', (data) => {
    console.log(data); // data will be 'woot'
  });

  // the client code
  // client.on('ferret', (name, fn) => {
  //   fn('woot');
  // });

});

socket.on(eventName, callback)

(inherited from EventEmitter)

eventName (String)
callback (Function)
Returns Socket

socket.on('news', (data) => {
  console.log(data);
});
// with several arguments
socket.on('news', (arg1, arg2, arg3) => {
  // ...
});
// or with acknowledgement
socket.on('news', (data, callback) => {
  callback(0);
});

socket.once(eventName, listener)

socket.removeListener(eventName, listener)

socket.removeAllListeners([eventName])

socket.eventNames()

Inherited from EventEmitter (along with other methods not mentioned here). See the Node.js documentation for the events module.

socket.join(room[, callback])

room (String)
callback (Function)
Returns Socket for chaining

Adds the client to the room, and fires optionally a callback with err signature (if any).

io.on('connection', (socket) => {
  socket.join('room 237', () => {
    let rooms = Object.keys(socket.rooms);
    console.log(rooms); // [ <socket.id>, 'room 237' ]
    io.to('room 237').emit('a new user has joined the room'); // broadcast to everyone in the room
  });
});

The mechanics of joining rooms are handled by the Adapter that has been configured (see Server#adapter above), defaulting to socket.io-adapter.

For your convenience, each socket automatically joins a room identified by its id (see Socket#id). This makes it easy to broadcast messages to other sockets:

io.on('connection', (socket) => {
  socket.on('say to someone', (id, msg) => {
    // send a private message to the socket with the given id
    socket.to(id).emit('my message', msg);
  });
});

socket.join(rooms[, callback])

rooms (Array)
callback (Function)
Returns Socket for chaining

Adds the client to the list of room, and fires optionally a callback with err signature (if any).

io.on('connection', (socket) => {
  socket.join(['room 237', 'room 238'], () => {
    const rooms = Object.keys(socket.rooms);
    console.log(rooms); // [ <socket.id>, 'room 237', 'room 238' ]
    io.to('room 237').to('room 238').emit('a new user has joined the room'); // broadcast to everyone in both rooms
  });
});

socket.leave(room[, callback])

room (String)
callback (Function)
Returns Socket for chaining

Removes the client from room, and fires optionally a callback with err signature (if any).

io.on('connection', (socket) => {
  socket.leave('room 237', () => {
    io.to('room 237').emit(`user ${socket.id} has left the room`);
  });
});

Rooms are left automatically upon disconnection.

socket.to(room)

room (String)
Returns Socket for chaining

Sets a modifier for a subsequent event emission that the event will only be broadcasted to clients that have joined the given room (the socket itself being excluded).

To emit to multiple rooms, you can call to several times.

io.on('connection', (socket) => {

  // to one room
  socket.to('others').emit('an event', { some: 'data' });

  // to multiple rooms
  socket.to('room1').to('room2').emit('hello');

  // a private message to another socket
  socket.to(/* another socket id */).emit('hey');

  // WARNING: `socket.to(socket.id).emit()` will NOT work, as it will send to everyone in the room
  // named `socket.id` but the sender. Please use the classic `socket.emit()` instead.
});

Note: acknowledgements are not supported when broadcasting.

socket.in(room)

Synonym of socket.to(room).

socket.compress(value)

value (Boolean) whether to following packet will be compressed
Returns Socket for chaining

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.

io.on('connection', (socket) => {
  socket.compress(false).emit('uncompressed', "that's rough");
});

socket.disconnect(close)

close (Boolean) whether to close the underlying connection
Returns Socket

Disconnects this client. If value of close is true, closes the underlying connection. Otherwise, it just disconnects the namespace.

io.on('connection', (socket) => {
  setTimeout(() => socket.disconnect(true), 5000);
});

Flag: 'broadcast'

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

io.on('connection', (socket) => {
  socket.broadcast.emit('an event', { some: 'data' }); // everyone gets it but the sender
});

Flag: 'volatile'

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).

io.on('connection', (socket) => {
  socket.volatile.emit('an event', { some: 'data' }); // the client may or may not receive it
});

Flag: 'binary'

Specifies whether there is binary data in the emitted data. Increases performance when specified. Can be true or false.

const io = require('socket.io')();
io.on('connection', (socket) => {
  socket.binary(false).emit('an event', { some: 'data' }); // The data to send has no binary data
});

Event: 'disconnect'

reason (String) the reason of the disconnection (either client or server-side)

Fired upon disconnection.

io.on('connection', (socket) => {
  socket.on('disconnect', (reason) => {
    // ...
  });
});

Possible reasons:

Reason	Side	Description
`transport error`	Server Side	Transport error
`server namespace disconnect`	Server Side	Server performs a `socket.disconnect()`
`client namespace disconnect`	Client Side	Got disconnect packet from client
`ping timeout`	Client Side	Client stopped responding to pings in the allowed amount of time (per the `pingTimeout` config setting)
`transport close`	Client Side	Client stopped sending data

Event: 'error'

error (Object) error object

Fired when an error occurs.

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

Event: 'disconnecting'

reason (String) the reason of the disconnection (either client or server-side)

Fired when the client is going to be disconnected (but hasn't left its rooms yet).

io.on('connection', (socket) => {
  socket.on('disconnecting', (reason) => {
    let rooms = Object.keys(socket.rooms);
    // ...
  });
});

These are reserved events (along with connect, newListener and removeListener) which cannot be used as event names.

Client

The Client class represents an incoming transport (engine.io) connection. A Client can be associated with many multiplexed Sockets that belong to different Namespaces.

client.conn

(engine.Socket)

A reference to the underlying engine.io Socket connection.

client.request

(Request)

A getter proxy that returns the reference to the request that originated the engine.io connection. Useful for accessing request headers such as Cookie or User-Agent.

Server​

new Server(httpServer[, options])​

new Server(port[, options])​

new Server(options)​

server.sockets​

server.serveClient([value])​

server.path([value])​

server.adapter([value])​

server.origins([value])​

server.origins(fn)​

server.attach(httpServer[, options])​

server.attach(port[, options])​

server.listen(httpServer[, options])​

server.listen(port[, options])​

server.bind(engine)​

server.onconnection(socket)​

server.of(nsp)​

server.close([callback])​

server.engine.generateId​

Namespace​

namespace.name​

namespace.connected​

namespace.adapter​

namespace.to(room)​

namespace.in(room)​

namespace.emit(eventName[, ...args])​

namespace.clients(callback)​

namespace.use(fn)​

Event: 'connect'​

Event: 'connection'​

Flag: 'volatile'​

Flag: 'binary'​

Flag: 'local'​

Socket​

socket.id​

socket.rooms​

socket.client​

socket.conn​

socket.request​

socket.handshake​

socket.use(fn)​

socket.send([...args][, ack])​

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

socket.on(eventName, callback)​

socket.once(eventName, listener)​

socket.removeListener(eventName, listener)​

socket.removeAllListeners([eventName])​

socket.eventNames()​

socket.join(room[, callback])​

socket.join(rooms[, callback])​

socket.leave(room[, callback])​

socket.to(room)​

socket.in(room)​

socket.compress(value)​

socket.disconnect(close)​

Flag: 'broadcast'​

Flag: 'volatile'​

Flag: 'binary'​

Event: 'disconnect'​

Event: 'error'​

Event: 'disconnecting'​

Client​

client.conn​

client.request​

Server

new Server(httpServer[, options])

new Server(port[, options])

new Server(options)

server.sockets

server.serveClient([value])

server.path([value])

server.adapter([value])

server.origins([value])

server.origins(fn)

server.attach(httpServer[, options])

server.attach(port[, options])

server.listen(httpServer[, options])

server.listen(port[, options])

server.bind(engine)

server.onconnection(socket)

server.of(nsp)

server.close([callback])

server.engine.generateId

Namespace

namespace.name

namespace.connected

namespace.adapter

namespace.to(room)

namespace.in(room)

namespace.emit(eventName[, ...args])

namespace.clients(callback)

namespace.use(fn)

Event: 'connect'

Event: 'connection'

Flag: 'volatile'

Flag: 'binary'

Flag: 'local'

Socket

socket.id

socket.rooms

socket.client

socket.conn

socket.request

socket.handshake

socket.use(fn)

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

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

socket.on(eventName, callback)

socket.once(eventName, listener)

socket.removeListener(eventName, listener)

socket.removeAllListeners([eventName])

socket.eventNames()

socket.join(room[, callback])

socket.join(rooms[, callback])

socket.leave(room[, callback])

socket.to(room)

socket.in(room)

socket.compress(value)

socket.disconnect(close)

Flag: 'broadcast'

Flag: 'volatile'

Flag: 'binary'

Event: 'disconnect'

Event: 'error'

Event: 'disconnecting'

Client

client.conn

client.request