What does passport.session() middleware do?

Question

I am building an authentication system using Passport.js using Easy Node Authentication: Setup and Local tutorial.

I am confused about what passport.session() does.

After playing around with the different middleware I came to understand that express.session() is what sends a session ID over cookies to the client, but I'm confused about what passport.session() does and why it is required in addition to express.session().

Here is how I set up my application:

// Server.js configures the application and sets up the webserver

//importing our modules
var express = require('express');
var app = express();
var port = process.env.PORT || 8080;
var mongoose = require('mongoose');
var passport = require('passport');
var flash = require('connect-flash');

var configDB = require('./config/database.js');

//Configuration of Databse and App

mongoose.connect(configDB.url); //connect to our database

require('./config/passport')(passport); //pass passport for configuration

app.configure(function() {

    //set up our express application

    app.use(express.logger('dev')); //log every request to the console
    app.use(express.cookieParser()); //read cookies (needed for auth)
    app.use(express.bodyParser()); //get info from html forms

    app.set('view engine', 'ejs'); //set up ejs for templating

    //configuration for passport
    app.use(express.session({ secret: 'olhosvermelhoseasenhaclassica', maxAge:null })); //session secret
    app.use(passport.initialize());
    app.use(passport.session()); //persistent login session
    app.use(flash()); //use connect-flash for flash messages stored in session

});

//Set up routes
require('./app/routes.js')(app, passport);

//launch
app.listen(port);
console.log("Server listening on port" + port);

Answer 1

passport.session() acts as a middleware to alter the req object and change the 'user' value that is currently the session id (from the client cookie) into the true deserialized user object.

Whilst the other answers make some good points I thought that some more specific detail could be provided.

app.use(passport.session());

is equivalent to

app.use(passport.authenticate('session'));

Where 'session' refers to the following strategy that is bundled with passportJS.

https://github.com/jaredhanson/passport/blob/master/lib/strategies/session.js

Specifically lines 59-60:

var property = req._passport.instance._userProperty || 'user';
req[property] = user;

Where it essentially acts as a middleware and alters the value of the 'user' property in the req object to contain the deserialized identity of the user. To allow this to work correctly you must include serializeUser and deserializeUser functions in your custom code.

passport.serializeUser(function (user, done) {
    done(null, user.id);
});

passport.deserializeUser(function (user, done) {
    //If using Mongoose with MongoDB; if other you will need JS specific to that schema.
    User.findById(user.id, function (err, user) {
        done(err, user);
    });
});

This will find the correct user from the database and pass it as a closure variable into the callback done(err,user); so the above code in the passport.session() can replace the 'user' value in the req object and pass on to the next middleware in the pile.