promises1.js

signup: function (req, res) {

    User.findOne({
      where: {
        username: req.body.username
      }
    }).then(function (user){
      if (user) {
        sails.log.silly(user);
        return res.json(409, { 'error': 'Sorry, that username is already taken' });
      }
    }).then(() => UserService.create(req.body))
    }).catch(function (error) {
      if (process.env.NODE_ENV == 'development'){
        return res.json(500, { 'error': err });
      } else {
        return res.json(500, { 'error': 'Error in user creation' });
      }
    })
  }
}
// Inside the UserSevice

create: function(data) {
  return new Promise((resolve, reject) => {
    User.create({
      username: data.username,
      password: data.password
    }).then((result) => {
      resolve(result)
    }).catch((err) => {
      reject(err)
    })
  })
}