How to use

Installing

$ npm install socket.io

Using with Node http server

var app = require('http').createServer(handler)
var io = require('socket.io')(app);
var fs = require('fs');

app.listen(80);

function handler (req, res) {
  fs.readFile(__dirname + '/index.html',
  function (err, data) {
    if (err) {
      res.writeHead(500);
      return res.end('Error loading index.html');
    }

    res.writeHead(200);
    res.end(data);
  });
}

io.on('connection', function (socket) {
  socket.emit('news', { hello: 'world' });
  socket.on('my other event', function (data) {
    console.log(data);
  });
});

<script src="/socket.io/socket.io.js"></script>
<script>
  var socket = io('http://localhost');
  socket.on('news', function (data) {
    console.log(data);
    socket.emit('my other event', { my: 'data' });
  });
</script>

Using with Express 3/4

var app = require('express')();
var server = require('http').Server(app);
var io = require('socket.io')(server);

server.listen(80);

app.get('/', function (req, res) {
  res.sendfile(__dirname + '/index.html');
});

io.on('connection', function (socket) {
  socket.emit('news', { hello: 'world' });
  socket.on('my other event', function (data) {
    console.log(data);
  });
});

<script src="/socket.io/socket.io.js"></script>
<script>
  var socket = io.connect('http://localhost');
  socket.on('news', function (data) {
    console.log(data);
    socket.emit('my other event', { my: 'data' });
  });
</script>

Using with the Express framework

var app = require('express').createServer();
var io = require('socket.io')(app);

app.listen(80);

app.get('/', function (req, res) {
  res.sendfile(__dirname + '/index.html');
});

io.on('connection', function (socket) {
  socket.emit('news', { hello: 'world' });
  socket.on('my other event', function (data) {
    console.log(data);
  });
});

<script src="/socket.io/socket.io.js"></script>
<script>
  var socket = io.connect('http://localhost');
  socket.on('news', function (data) {
    console.log(data);
    socket.emit('my other event', { my: 'data' });
  });
</script>

Sending and receiving events

Socket.IO allows you to emit and receive custom events. Besides connect, message and disconnect, you can emit custom events:

Server

// note, io(<port>) will create a http server for you
var io = require('socket.io')(80);

io.on('connection', function (socket) {
  io.emit('this', { will: 'be received by everyone'});

  socket.on('private message', function (from, msg) {
    console.log('I received a private message by ', from, ' saying ', msg);
  });

  socket.on('disconnect', function () {
    io.emit('user disconnected');
  });
});

Restricting yourself to a namespace

If you have control over all the messages and events emitted for a particular application, using the default / namespace works. If you want to leverage 3rd-party code, or produce code to share with others, socket.io provides a way of namespacing a socket.

This has the benefit of multiplexing a single connection. Instead of socket.io using two WebSocket connections, it’ll use one.

var io = require('socket.io')(80);
var chat = io
  .of('/chat')
  .on('connection', function (socket) {
    socket.emit('a message', {
        that: 'only'
      , '/chat': 'will get'
    });
    chat.emit('a message', {
        everyone: 'in'
      , '/chat': 'will get'
    });
  });

var news = io
  .of('/news')
  .on('connection', function (socket) {
    socket.emit('item', { news: 'item' });
  });

<script>
  var chat = io.connect('http://localhost/chat')
    , news = io.connect('http://localhost/news');
  
  chat.on('connect', function () {
    chat.emit('hi!');
  });
  
  news.on('news', function () {
    news.emit('woot');
  });
</script>

Sending volatile messages

Sometimes certain messages can be dropped. Let’s say you have an app that shows realtime tweets for the keyword bieber.

If a certain 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), if they doesn’t receive ALL the tweets related to bieber your application won’t suffer.

In that case, you might want to send those messages as volatile messages.

Server

var io = require('socket.io')(80);

io.on('connection', function (socket) {
  var tweets = setInterval(function () {
    getBieberTweet(function (tweet) {
      socket.volatile.emit('bieber tweet', tweet);
    });
  }, 100);

  socket.on('disconnect', function () {
    clearInterval(tweets);
  });
});

Sending and getting data (acknowledgements)

Sometimes, you might want to get a callback when the client confirmed the message reception.

To do this, simply pass a function as the last parameter of .send or .emit. What’s more, when you use .emit, the acknowledgement is done by you, which means you can also pass data along:

var io = require('socket.io')(80);

io.on('connection', function (socket) {
  socket.on('ferret', function (name, fn) {
    fn('woot');
  });
});

<script>
  var socket = io(); // TIP: io() with no args does auto-discovery
  socket.on('connect', function () { // TIP: you can avoid listening on `connect` and listen on events directly too!
    socket.emit('ferret', 'tobi', function (data) {
      console.log(data); // data will be 'woot'
    });
  });
</script>

Broadcasting messages

To broadcast, simply add a broadcast flag to emit and send method calls. Broadcasting means sending a message to everyone else except for the socket that starts it.

Server

var io = require('socket.io')(80);

io.on('connection', function (socket) {
  socket.broadcast.emit('user connected');
});

Using it just as a cross-browser WebSocket

If you just want the WebSocket semantics, you can do that too. Simply leverage send and listen on the message event:

var io = require('socket.io')(80);

io.on('connection', function (socket) {
  socket.on('message', function () { });
  socket.on('disconnect', function () { });
});

<script>
  var socket = io('http://localhost/');
  socket.on('connect', function () {
    socket.send('hi');

    socket.on('message', function (msg) {
      // my msg
    });
  });
</script>

If you don’t care about reconnection logic and such, take a look at Engine.IO, which is the WebSocket semantics transport layer Socket.IO uses.