socket.io private message

The easiest way I can think of is to have an hash of all the users on using their id or name as the key and have their socket as part of the value then when you want to send a message to just them you pull that socket and emit on it... something like this:

users[toUser].emit('msg',"Hello, "+toUser+"!");

No tutorial needed. The Socket.IO FAQ is pretty straightforward on this one:

socket.emit('news', { hello: 'world' });

EDIT: Folks are linking to this question when asking about how to get that socket object later. There is no need to. When a new client connects, save a reference to that socket on whatever object you're keeping your user information on. My comment from below:

In the top of your script somewhere, setup an object to hold your users' information.

var connectedUsers = {};

In your .on('connection') function, add that socket to your new object. connectedUsers[USER_NAME_HERE] = socket; Then you can easily retrieve it later. connectedUsers[USER_NAME_HERE].emit('something', 'something');

Although we have nice answers here. However, I couldn't grasp the whole client server unique user identification pretty fast, so I'm posting this simple steps in order to help whoever is struggling as i did.....

At the client side, Get the user's ID, in my case I'm getting the username...

Client side user registration

//Connect socket.io     
var systemUrl = 'http://localhost:4000';
var socket = io.connect(systemUrl);

//Collect User identity from the client side
var username = prompt('Enter your Username');
socket.emit('register',username);

The Listen to register on the server side to register user's socket to connected socket

Serve side code User registration

/*Craete an empty object to collect connected users*/
var connectedUsers = {};

io.on('connection',function(socket){

/*Register connected user*/
    socket.on('register',function(username){
        socket.username = username;
        connectedUsers[username] = socket;
    });
});

Send Message from the client side

$(document).on('click','.username',function(){
    var username = $(this).text(),
        message = prompt("type your message");

    socket.emit('private_chat',{
        to : username,
        message : message
    });
});

Receive message on server and emit it to private user

/*Private chat*/
socket.on('private_chat',function(data){
    const to = data.to,
            message = data.message;

    if(connectedUsers.hasOwnProperty(to)){
        connectedUsers[to].emit('private_chat',{
            //The sender's username
            username : socket.username,

            //Message sent to receiver
            message : message
        });
    }

}); 

Receive message on client and display it

/*Received private messages*/
socket.on('private_chat',function(data){
    var username = data.username;
    var message = data.message;

    alert(username+': '+message);
});

This is not the best, however you can start from here....

Here's a code snippet that should help:

Client-side (sending message)

socket.emit("private", { msg: chatMsg.val(), to: selected.text() });

where to refers to the id to send a private message to and msg is the content.

Client-side (receiving message)

socket.on("private", function(data) {   
   chatLog.append('<li class="private"><em><strong>'+ data.from +' -> '+ data.to +'</strong>: '+ data.msg +'</em></li>');
});

where chatLog is a div displaying the chat messages.

Server-side

client.on("private", function(data) {       
    io.sockets.sockets[data.to].emit("private", { from: client.id, to: data.to, msg: data.msg });
    client.emit("private", { from: client.id, to: data.to, msg: data.msg });
});