client.js

var socket = require('socket.io-client')('http://127.0.0.1:3000');
socket.on('connect', function(io){
    console.log('[CLIENT] connected');

    var data = { name: 'hans' };
    console.log('[CLIENT] sending json:', '(' + typeof data + ')', data);
    socket.emit('json', data);

    var data = 'hello world';
    console.log('[CLIENT] sending string:', '(' + typeof data + ')', data);
    socket.emit('string', data);

    var data = 'client-ack-data';
    console.log('[CLIENT] sending client-ack:', '(' + typeof data + ')', data);
    socket.emit('client-ack', data, function (data) {
        console.log('[CLIENT] got client-ack:', '(' + typeof data + ')', data);

        var data = { from: 'client', value: 'ack-data' };
        console.log('[CLIENT] sending client-ack-json:', '(' + typeof data + ')', data);
        socket.emit('client-ack-json', data, function (data) {
            console.log('[CLIENT] got client-ack-json:', '(' + typeof data + ')', data);
        });
    });

    socket.on('server-ack', function(data, ack) {
        console.log('[CLIENT] got server-ack:', '(' + typeof data + ')', data);

        var respo = 'C-' + (Math.random() * 1000);
        console.log('[CLIENT] sending server-ack response:', '(' + typeof respo + ')', respo);

        ack(respo);
    });
});
socket.on('event', function(data){
    console.log('[CLIENT] event', data);
});
socket.on('disconnect', function(){
    console.log('[CLIENT] disconnected');
});

package.json

{
  "dependencies": {
    "socket.io": "^1.3.7",
    "socket.io-client": "^1.3.7"
  }
}

server.js

var io = require('socket.io')();
io.on('connection', function(socket) {
    console.log('\n\n');
    console.log('[SERVER] client connected');

    socket.on('json', function(data) {
        console.log('[SERVER] got json:', '(' + typeof data + ')', data);
    });

    socket.on('string', function(data) {
        console.log('[SERVER] got string:', '(' + typeof data + ')', data);
    });

    socket.on('client-ack', function(data, ack) {
        console.log('[SERVER] got client-ack:', '(' + typeof data + ')', data);

        var respo = 'S-' + (Math.random() * 1000);
        console.log('[SERVER] sending client-ack response:', '(' + typeof respo + ')', respo);

        ack(respo);
    });

    socket.on('client-ack-json', function(data, ack) {
        console.log('[SERVER] got client-ack-json:', '(' + typeof data + ')', data);

        var respo = { from: 'server', value: Math.random() * 1000 };
        console.log('[SERVER] sending client-ack-json response:', '(' + typeof respo + ')', respo);

        ack(respo);

        setTimeout(function() {
            console.log('[SERVER] sending server-ack');
            socket.emit('server-ack', 'server-ack-data', function(data) {
                console.log('[SERVER] got server-ack:', '(' + typeof data + ')', data);
            });
        }, 50);
    });
});
io.listen(3000);
console.log('[SERVER] listening on 127.0.0.1:3000');