JAVASCRIPT

passport serializeUser

passport.serializeUser(function(user, done) {
    done(null, user.id);
});              │
                 │ 
                 │
                 └─────────────────┬──→ saved to session
                                   │    req.session.passport.user = {id: '..'}
                                   │
                                   ↓           
passport.deserializeUser(function(id, done) {
                   ┌───────────────┘
                   │
                   ↓ 
    User.findById(id, function(err, user) {
        done(err, user);
    });            └──────────────→ user object attaches to the request as req.user   
});

passportjs serializeuser

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

passport.deserializeUser(function(id, done) {
  User.findById(id, function(err, user) {
    done(err, user);
  });
});