// Check if the user is authenticated: I have a middleware function named CheckAuth which I use on every route that needs the user to be authenticated:
function checkAuth(req, res, next) {
  if (!req.session.user_id) {
    res.send('You are not authorized to view this page');
  } else {
    next();
  }
}

// I use this function in my routes like this:
app.get('/my_secret_page', checkAuth, function (req, res) {
  res.send('if you are viewing this page it means you are logged in');
});

// 2) The login route:
app.post('/login', function (req, res) {
  var post = req.body;
  if (post.user === 'john' && post.password === 'johnspassword') {
    req.session.user_id = johns_user_id_here;
    res.redirect('/my_secret_page');
  } else {
    res.send('Bad user/pass');
  }
});

// 3) The logout route:
app.get('/logout', function (req, res) {
  delete req.session.user_id;
  res.redirect('/login');
});