How to private chat using node.js and socket.io

Create a room with conversation_id and make users to subscribe to that room, 

so that you can emit a private message to that room it by,

client
------
                 
var socket = io.connect('http://ip:port');

socket.emit('subscribe', conversation_id);

socket.emit('send message', {
    room: conversation_id,
    message: "Some message"
});

socket.on('conversation private post', function(data) {
    //display data.message
});

Server
-------
socket.on('subscribe', function(room) {
    console.log('joining room', room);
    socket.join(room);
});

socket.on('send message', function(data) {
    console.log('sending room post', data.room);
    socket.broadcast.to(data.room).emit('conversation private post', {
        message: data.message
    });
});

Here is my Stackoverflow answer : http://stackoverflow.com/a/23623724/453486