gistfile1.js

app.post('/login', function(req, res) {
    console.log(res);
    passport.authenticate('local', function(err, user) {
      if (req.xhr) {
        //thanks @jkevinburton
        if (err)   { return res.json({ error: err.message }); }
        if (!user) { return res.json({error : "Invalid Login"}); }
        req.login(user, {}, function(err) {
          if (err) { return res.json({error:err}); }
          return res.json(
            { user: {
                      id: req.user.id,
                      email: req.user.email,
                      joined: req.user.joined
              },
              success: true
            });
        });
      } else {
        if (err)   { return res.redirect('/login'); }
        if (!user) { return res.redirect('/login'); }
        req.login(user, {}, function(err) {
          if (err) { return res.redirect('/login'); }
          return res.redirect('/');
        });
      }
    })(req, res);
  });