Beginner: Use of descriptive functions + promise chain

use_of_func.js

const UserController = {
  create(req, res) {
    const userData = req.body;
    const query = {
      where: { email: userData.email }
    };
    const defaultRole = {
      where: { title: 'Writer' }
    };

    Users.findOne(query)
      .then(checkReturnedUser)
      .then(getDefaultRoleInfo)
      .then(createUser)
      .then(generateToken)
      .then(resHandler)
      .catch(errorHandler);


    function checkReturnedUser(user) {
      if (user) {
        throw new Error('checkReturnedUSer');
      }
      return user;
    }

    function getDefaultRoleInfo() {
      return Roles.findOne(defaultRole);
    }

    function createUser(role) {
      return Users.create(Object.assign({}, userData, { roleId: role.id }));
    }

    function generateToken(newUserObj) {
      const payload = { roleId: newUserObj.roleId, email: newUserObj.email };
      return {
        newUser: newUserObj,
        token: jwt.sign(payload, config.jwtSecret, { expiresIn: '24h' })
      };
    }

    function resHandler(resObj) {
      return res.status(201).send(Object.assign({}, resObj, {
        message: 'User signup completed successfully'
      }));
    }

    function errorHandler(error) {
      const message = error.message;
      if (message === 'checkReturnedUSer') {
        return res.status(400).send({
          error: 'An account with that email address already exists'
        });
      }
      return res.status(500).send({ error });
    }
  }
};