Medium Sails UserController after error handling to UserService:

UserController.js

module.exports = {
  register: (req, res) => {
    const error = UserService.handleError(req)
    if (error) {
      return res.status(400).send(error)
    }
    ... continue processing registration    
  },
  
  login: (req, res) => {    
    const error = UserService.handleError(req)
    if (error) {
      return res.status(400).send(error)
    }
    ... continue processing login
  }
  ...
}