How to send a message to a particular client with socket.io

You can use socket.io rooms. From the client side emit an event ("join" in this case, can be anything) with any unique identifier (email, id).

Client Side:

var socket = io.connect('http://localhost');
socket.emit('join', {email: user1@example.com});

Now, from the server side use that information to create an unique room for that user

Server Side:

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

io.sockets.on('connection', function (socket) {
  socket.on('join', function (data) {
    socket.join(data.email); // We are using room of socket io
  });
});

So, now every user has joined a room named after user's email. So if you want to send a specific user a message you just have to

Server Side:

io.sockets.in('user1@example.com').emit('new_msg', {msg: 'hello'});

The last thing left to do on the client side is listen to the "new_msg" event.

Client Side:

socket.on("new_msg", function(data) {
    alert(data.msg);
}

I hope you get the idea.

When a user connects, it should send a message to the server with a username which has to be unique, like an email.

A pair of username and socket should be stored in an object like this:

var users = {
    'userA@example.com': [socket object],
    'userB@example.com': [socket object],
    'userC@example.com': [socket object]
}

On the client, emit an object to the server with the following data:

{
    to:[the other receiver's username as a string],
    from:[the person who sent the message as string],
    message:[the message to be sent as string]
}

On the server, listen for messages. When a message is received, emit the data to the receiver.

users[data.to].emit('receivedMessage', data)

On the client, listen for emits from the server called 'receivedMessage', and by reading the data you can handle who it came from and the message that was sent.

You can refer to socket.io rooms. When you handshaked socket - you can join him to named room, for instance "user.#{userid}".

After that, you can send private message to any client by convenient name, for instance:

io.sockets.in('user.125').emit('new_message', {text: "Hello world"})

In operation above we send "new_message" to user "125".

thanks.

You could keep a record of each client connection by implementing a custom "handshake", something like:

server.js:

var connections = {};

io.sockets.on('connection', function (socket) {
    socket.emit('give_me_your_name', {});    
    socket.on('give_me_your_name', function (data) {
        connections[data.my_name] = socket;
    });
});

client.js:

socket.on('give_me_your_name', function (data) {
    socket.emit('give_me_your_name', {my_name : 'Bob'});
}

Now, every time you want to send a message to Bob you look in connections and use the proper socket. It's also useful to know that each socket object has an id attribute.

Disclaimer: This is the way you would solve your issue in SockJS; the solution provided by Vladimir Kurijov might be more suitable for Socket.IO.