Simplified version of socket chat

server.js

var express = require('express');
var app = express();
var server = require('http').Server(app);
var io = require('socket.io')(server);
var port = process.env.PORT || 8120;

server.listen(port, function () {
    console.log('Chat is listening on port %d', port);
});

app.use(express.static(__dirname + '/public'));

var usernames = {};
var numUsers = 0;

io.on('connection', function (socket) {
    var addedUser = false;

    socket.on('new message', function (data) {
        socket.broadcast.emit('new message', {
            username: socket.username,
            message: data.message,
            timestamp: Date.now()
        });
        console.log(socket.username + ": " + data.message);
    });

    socket.on('add user', function (username) {
        socket.username = username;
        usernames[username] = username;
        ++numUsers;
        addedUser = true;
        socket.emit('login', {
            numUsers: numUsers
        });
        socket.broadcast.emit('user joined', {
            username: socket.username,
            numUsers: numUsers
        });
    });

    socket.on('disconnect', function () {
        if (addedUser) {
            delete usernames[socket.username];
            --numUsers;

            socket.broadcast.emit('user left', {
                username: socket.username,
                numUsers: numUsers
            });
        }
    });
});