socket.io

What is socket.io?

The socket.io npm package enables real-time, bidirectional and event-based communication between web clients and servers. It is primarily used to build real-time web applications and has features like broadcasting to multiple sockets, storing data associated with each client, and asynchronous I/O.

What are socket.io's main functionalities?

Real-time bidirectional event-based communication

This feature allows the server to establish a WebSocket connection with the client for real-time communication. The server listens for events like 'connection', 'chat message', and 'disconnect' to react accordingly.

const io = require('socket.io')(3000);

io.on('connection', (socket) => {
  console.log('a user connected');
  socket.on('chat message', (msg) => {
    io.emit('chat message', msg);
  });
  socket.on('disconnect', () => {
    console.log('user disconnected');
  });
});

Broadcasting

Broadcasting allows a server to send a message to all connected clients except for the one that triggered the message. This is useful for notifying all users about the actions of one.

io.on('connection', (socket) => {
  socket.broadcast.emit('user connected', 'A new user has joined the chat');
});

Namespaces and Rooms

Socket.IO allows for the creation of Namespaces and Rooms which can be used to divide the clients into different groups for targeted broadcasting and communication.

const chat = io.of('/chat').on('connection', (socket) => {
  socket.join('some room');
  chat.to('some room').emit('some event');
});

Other packages similar to socket.io

ws

The 'ws' package is a simple WebSocket library for Node.js. Unlike socket.io, it does not provide high-level features like broadcasting to multiple sockets or automatic reconnection.

engine.io

Engine.io is the low-level engine that powers socket.io. It provides the bare WebSocket-like API and is responsible for handling the transport logistics. It is less feature-rich compared to socket.io.

sockjs

SockJS is a JavaScript library that provides a WebSocket-like object. It is similar to socket.io in that it offers a fallback mechanism for environments where WebSockets are not supported.

faye-websocket

Faye is a set of tools for simple publish-subscribe messaging between web clients. It's more focused on the pub/sub paradigm and lacks some of the real-time communication features that socket.io offers.

README

socket.io

How to use

The following example attaches socket.io to a plain Node.JS HTTP server listening on port 3000.

var server = require('http').createServer();
var io = require('socket.io')(server);
io.on('connection', function(socket){
  socket.on('event', function(data){});
  socket.on('disconnect', function(){});
});
server.listen(3000);

Standalone

var io = require('socket.io')();
io.on('connection', function(socket){});
io.listen(3000);

In conjunction with Express

Starting with 3.0, express applications have become request handler functions that you pass to http or http Server instances. You need to pass the Server to socket.io, and not the express application function.

var app = require('express')();
var server = require('http').createServer(app);
var io = require('socket.io')(server);
io.on('connection', function(){ /* … */ });
server.listen(3000);

In conjunction with Koa

Like Express.JS, Koa works by exposing an application as a request handler function, but only by calling the callback method.

var app = require('koa')();
var server = require('http').createServer(app.callback());
var io = require('socket.io')(server);
io.on('connection', function(){ /* … */ });
server.listen(3000);

API

Server

Exposed by require('socket.io').

Server()

Creates a new Server. Works with and without new:

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

Server(opts:Object)

Optionally, the first or second argument (see below) of the Server constructor can be an options object.

The following options are supported:

• serveClient sets the value for Server#serveClient()
• path sets the value for Server#path()

The same options passed to socket.io are always passed to the engine.io Server that gets created. See engine.io options as reference.

Server(srv:http#Server, opts:Object)

Creates a new Server and attaches it to the given srv. Optionally opts can be passed.

Server(port:Number, opts:Object)

Binds socket.io to a new http.Server that listens on port.

Server#serveClient(v:Boolean):Server

If v 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.

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

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

If no arguments are supplied this method returns the current value.

Server#path(v:String):Server

Sets the path v 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.

Server#adapter(v:Adapter):Server

Sets the adapter v. 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.

Server#origins(v:String):Server

Sets the allowed origins v. Defaults to any origins being allowed.

If no arguments are supplied this method returns the current value.

Server#origins(v:Function):Server

Sets the allowed origins as dynamic function. Function takes two arguments origin:String and callback(error, success), where success is a boolean value indicating whether origin is allowed or not.

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 can use cors

Server#sockets:Namespace

The default (/) namespace.

Server#attach(srv:http#Server, opts:Object):Server

Attaches the Server to an engine.io instance on srv with the supplied opts (optionally).

Server#attach(port:Number, opts:Object):Server

Attaches the Server to an engine.io instance that is bound to port with the given opts (optionally).

Server#listen

Synonym of Server#attach.

Server#bind(srv:engine#Server):Server

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

Server#onconnection(socket:engine#Socket):Server

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

Server#of(nsp:String):Namespace

Initializes and retrieves the given Namespace by its pathname identifier nsp.

If the namespace was already initialized it returns it right away.

Server#emit

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

var io = require('socket.io')();
io.sockets.emit('an event sent to all connected clients');
io.emit('an event sent to all connected clients');

For other available methods, see Namespace below.

Server#close

Closes socket server

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

var io = Server(PORT);

io.close(); // Close current server

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

io = Server(server);

Server#use

See Namespace#use below.

Namespace

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

By default the client always connects to /.

Events

• connection / connect. Fired upon a connection.

  Parameters:

  • Socket the incoming socket.

Namespace#name:String

The namespace identifier property.

Namespace#connected:Object

Hash of Socket objects that are connected to this namespace indexed by id.

Namespace#use(fn:Function):Namespace

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

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

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

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.

Socket#rooms:Array

A list of strings identifying the rooms this socket is in.

Socket#client:Client

A reference to the underlying Client object.

Socket#conn:Socket

A reference to the underlying Client transport connection (engine.io Socket object).

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.

Socket#id:String

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

Socket#emit(name:String[, …]):Socket

Emits an event to the socket identified by the string name. Any other parameters can be included.

All datastructures are supported, including Buffer. JavaScript functions can't be serialized/deserialized.

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

Socket#join(name:String[, fn:Function]):Socket

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

The socket is automatically a member of a room identified with its session id (see Socket#id).

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

Socket#leave(name:String[, fn:Function]):Socket

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

Rooms are left automatically upon disconnection.

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

Socket#to(room:String):Socket

Socket#in(room:String):Socket

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

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

var io = require('socket.io')();
io.on('connection', function(socket){
  socket.to('others').emit('an event', { some: 'data' });
});

Client

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

Client#conn

A reference to the underlying engine.io Socket connection.

Client#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.

Debug / logging

Socket.IO is powered by debug. In order to see all the debug output, run your app with the environment variable DEBUG including the desired scope.

To see the output from all of Socket.IO's debugging scopes you can use:

DEBUG=socket.io* node myapp

License

MIT