Example node + socket.io chat app using express.

app.js

#!/usr/bin/env node

var express     = require('express');
var app         = express();
var server_port = process.env.PORT || 5000;
var server      = require('http').createServer(app);
var io          = require('socket.io').listen(server);
var users       = {};

server.listen(server_port);

console.log('###Express server listening on port %d', server_port);

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

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

    socket.on('set nickname', function(nickname) {

        user = getUserByNick(nickname);

        if (user) {
            socket.emit('ready', { id: user.id } );
        } else {
            user = registerUser(nickname, socket.id);
        }

        socket.set('nickname', user.nickname, function() {
            socket.set('userID', user.id, function() {
                socket.emit('ready', { id: user.id } );
            });
        });
    });

    socket.on('update visible users', function(users) {
        for (var i in users) {
            socket.join(users[i]);
        }
    });

    socket.on('msg', function(msg) {
        socket.get('nickname', function(err, nickname) {
            var user = users[nickname];

            if (user) {
                socket.broadcast.to(user.id).emit('chat update', { nickname: user.nickname, text: msg });
            }
        });
    });
});

function getUserByNick(nickname) {
    return users[nickname];
}

function registerUser(nickname, socketID) {
    if (users[nickname] == undefined) {
        var userId = Object.keys(users).length + 100;
        users[nickname] = {
            id: userId,
            nickname: nickname,
            socketId: socketID
        };
    }

    return users[nickname];
}