In socket.io 1.0 you can do that with following code:

if (io.sockets.connected[socketid]) {
io.sockets.connected[socketid].emit('message', 'for your eyes only');
}

Update:

@MustafaDokumacı's answer contains a better solution.

In socket.io 1.0 they provide a better way for this. Each socket automatically joins a default room by self id. Check documents: http://socket.io/docs/rooms-and-namespaces/#default-room

So you can emit to a socket by id with following code:

io.to(socketid).emit('message', 'for your eyes only');

@Mustafa Dokumacı and @Curious already provided enough information, I am adding how you can get socket id.

To get socket id use socket.id:

var chat = io.of("/socket").on('connection',onSocketConnected);


function onSocketConnected(socket){
console.log("connected :"+socket.id);
}

If you have used a namespace I found that the following works :

//Defining the namespace <br>
var nsp = io.of('/my-namespace');


//targeting the message to socket id <br>
nsp.to(socket id of the intended recipient).emit('private message', 'hello');

More about namespaces: http://socket.io/docs/rooms-and-namespaces/

I believe both @Curious and @MustafaDokumacı provided solutions that work well. The difference though is that with @MustafaDokumacı's solution the message is broadcasted to a room and not only to a particular client.

The difference is prominent when an acknowledgement is requested.

io.sockets.connected[socketid].emit('message', 'for your eyes only', function(data) {...});

works as expected, while

io.to(socketid).emit('message', 'for your eyes only', function(data) {...});

fails with

Error: Callbacks are not supported when broadcasting

in Node.js --> socket.io --> there is a chat example to download Paste this in line (io on connection) part.. i use this code that works 100%

io.on('connection', function(socket){
socket.on('chat message', function(msg){
console.log(socket.id);
io.to(socket.id).emit('chat message', msg+' you ID is:'+socket.id);
});
});

Update Socket.io v4.0+

var socketById = io.sockets.sockets.get(id);
socketById.emit("message", "hi")

This is the optimal way to get a socket by ID in v4 and emitting to it.