Express Post user with validation middleware with promises.

// Service
module.exports = {
  createUser: createUser
};

function createUser(req, res, next) {
  return User.findOne({neki id})
    .then(user => {
      if (user.exists) return res.status(500).send('zajebo si');
    })
    .then(() => {
      return User.create(req.body);
    })
    .then((user) => {
      return res.status(200).send(user);
    })
    .catch(err => next(new Error(err)));
}


// validate user middleware
function validateUser(req, res, next) {
  if (typeof req.body.email === 'string' &&
      typeof req.body.fullName === 'string' &&
      typeof req.body.username === 'string') 
    return next();

  return res.status(500).send('Please enter a valid User.');
}
module.exports = validateUser;


// Controller
var express = require('express');
var UserController = express.Router();
var UserService = require('path/to/UserService');
var validateUser = require('path/to/validateUser');

UserController.post('/api/users', validateUser, UserService.createUser);