// Set up the Session middleware using a MongoDB session store
expressSession = require("express-session");
var sessionMiddleware = expressSession({
    name: "COOKIE_NAME_HERE",
    secret: "COOKIE_SECRET_HERE",
    store: new (require("connect-mongo")(expressSession))({
        url: "mongodb://localhost/DATABASE_NAME_HERE"
    })
});

// Set up the Express server
var app = require("express")()
    .use(sessionMiddleware)
    .use(passport.initialize())
    .use(passport.session())
    // ... more middleware ...
    .listen(8000);

// Set up the Socket.IO server
var io = require("socket.io")(app)
    .use(function(socket, next){
        // Wrap the express middleware
        sessionMiddleware(socket.request, {}, next);
    })
    .on("connection", function(socket){
        var userId = socket.request.session.passport.user;
        console.log("Your User ID is", userId);
    });