server.js

// Setup basic express server
var express = require('express'),
    app = express(),
    session = require('express-session'),
    cookieParser = require('cookie-parser'),
    sessionStore = new session.MemoryStore(),
    bodyParser = require('body-parser');
var server = require('http').createServer(app);
var io = require('socket.io')(server);
var port = process.env.PORT || 3000;
var mongo = require('mongodb').MongoClient;

var MONGO_PATH = "mongodb://localhost:27017/mydb";
var CHAT_COLLECTION = "chat";

var sessionMiddleware = session({
    name: 'sid',
    store: sessionStore, // MemoryStore; for example Redis should be used for production
    secret: 's3cr37',
    saveUninitialized: true,
    resave: false,
});


//a way to use the same session in both express and socket.io
io.use(function (socket, next) {
    sessionMiddleware(socket.request, socket.request.res, next);
});

app.use(sessionMiddleware);
// parse application/json
app.use(bodyParser.json())

server.listen(port, function () {
    console.log('Server listening at port %d', port);
});

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

var numUsers = 0;

var getUsernameFromSession = function (socketRequest) {
    var sessionId = socketRequest.sessionID;
    try {
        var cookie = JSON.parse(socketRequest.sessionStore.sessions[sessionId]);
    } catch (err) {
        return null;
    }
    return cookie.username;
};

app.post('/login', function (req, res) {
    req.session.username = req.body.user;
    res.sendStatus(200);
});

app.get('/logout', function (req, res) {
    req.session.username = null
    res.sendStatus(200);
});

io.on('connection', function (socket) {
    // when the client emits 'new message', this listens and executes
    socket.on('new message', function (data) {
        var usernameSession = getUsernameFromSession(socket.request);
        var date = new Date();

        mongo.connect(MONGO_PATH, function (err, db) {
            if (!err) {
                // console.log("We are connected");
                var collection = db.collection(CHAT_COLLECTION);
                collection.insert({username: usernameSession, message: data}, function (err, o) {
                    if (err) {
                        console.warn(err.message);
                    } else {
                        console.log("chat inserted into db: " + data.text + "@" + data.datetime);
                    }
                });
            }
        });

        // we tell the client to execute 'new message'
        socket.broadcast.emit('new message', {
            username: usernameSession,
            message: data
        });
    });

    // when the client emits 'add user', this listens and executes
    socket.on('add user', function (username) {
        var usernameSession = getUsernameFromSession(socket.request);
        ++numUsers;
        socket.emit('login', {
            numUsers: numUsers
        });
        // echo globally (all clients) that a person has connected
        socket.broadcast.emit('user joined', {
            username: usernameSession,
            numUsers: numUsers
        });

        mongo.connect(MONGO_PATH, function (err, db) {
            var collection = db.collection(CHAT_COLLECTION);
            collection.find().sort({"message.datetime": -1}).limit(10).toArray(function (err, messages) {
                if (err) {
                    return;
                }
                for (var i = messages.length - 1; i >= 0; --i) {
                    socket.emit('new message', messages[i]);
                }
            });
            console.log('<<<<<<<<<<<<<>>>>>>>>>>>>>>>');
        });
    });

    //when client gets back online, give him messages he didn't receive while being offline
    socket.on('back online', function (lastDatetime) {

        if (lastDatetime != null) {
            console.log(usernameSession + ' is back online. Give him the messages he did not receive');
            var usernameSession = getUsernameFromSession(socket.request);
            if (usernameSession != undefined) {
                mongo.connect(MONGO_PATH, function (err, db) {
                    var collection = db.collection(CHAT_COLLECTION);
                    collection.find({
                        "message.datetime": {$gt: lastDatetime},
                        "username": {$ne: usernameSession}
                    }).sort({"message.datetime": -1}).toArray(function (err, messages) {
                        if (err) {
                            return;
                        }
                        for (var i = messages.length - 1; i >= 0; --i) {
                            socket.emit('new message', messages[i]);
                        }
                    });
                });
            }
        }
    });

    // when the client emits 'typing', we broadcast it to others
    socket.on('typing', function () {
        var usernameSession = getUsernameFromSession(socket.request);
        socket.broadcast.emit('typing', {
            username: usernameSession
        });
    });

    // when the client emits 'stop typing', we broadcast it to others
    socket.on('stop typing', function () {
        var usernameSession = getUsernameFromSession(socket.request);
        socket.broadcast.emit('stop typing', {
            username: usernameSession
        });
    });

    // when the user disconnects..
    socket.on('disconnect', function () {
        console.log(socket.username + ' has disconnected');
        var usernameSession = getUsernameFromSession(socket.request);
        if (usernameSession != null) {
            --numUsers;
            //inform everyone that this client has left
            socket.broadcast.emit('user left', {
                username: usernameSession,
                numUsers: numUsers
            });
        }
    });

});