Express.js middleware the hard way

update-user.js

app.post('/user/:id', function (req, res) {
  User.findOne({_id: ObjectId(req.params.id)}, function (err, user) {
    validator.run(rules, req.body, function (errorCount, errors) {
      user.merge(req.body);
      user.save(function(err, savedUser) {
        EmailClient.sendEmail(user.email, 'Your account was updated', function (err) {
          res.status(200).send({
            message: 'User updated'
          });
        });
      });
    });
  });
});