Client Socket.io acknowledgement for server

Question

I know it is possible to use callback functions in socket.io, an example is found in the link http://socket.io/docs

EXAMPLE

Client

socket.emit('ferret', 'tobi', function (data) {
    console.log(data); 
});

Server

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

In this case, the function of callback has the following address: server to client

What I’m trying unsuccessfully is that the function of callback work from client to server

Thank you