i don't understand promises

express-response-flow.js

export const register = (req, res) => {
  const newUser = new User(res.locals.db)({
    name: req.body.name,
    email: req.body.email,
    password: req.body.password
  });

  User(res.locals.db)
    .findOne({ email: newUser.email })
    .then(checkDuplicateUser(res))
    .catch(makeError(409, res))
    .then(validateUser(newUser))
    .catch(makeError(400, res))
    .then(generatePassword(req.body.password))
    .catch(makeError(500, res))
    .then(saveWithPassword(newUser))
    .then(respond(res))
    .catch(() => {});
};

const makeError = (status, res) => {
  return error => Promise.reject(res.status(status).json({error}));
}

const checkDuplicateUser = () => {
  return user => {
    return new Promise((resolve, reject) => {
      if (!user) resolve();
      reject('email already exists');
    });
  }
}

export const validateUser = user => {
  return () => {
    return new Promise((resolve, reject) => {
      if (!validateEmail(user.email))
        return reject('email invalid format');
  
      if (user.password.length < 8)
        return reject('password must be >= 8 characters long');
  
      return resolve()
    });
  };
};

export const generatePassword = password => {
  return () => {
    return new Promise((resolve, reject) => {
      generateSalt(parseInt(process.env.BCRYPT_COST, 10))
        .then(salt => generateHash(salt, password))
        .then(resolve)
        .catch(() => reject('password generation failed'));
    });
  };
};

const saveWithPassword = (user) => {
  return hash => {
    user.password = hash;
    return user.save();
  }
}

const respond = (res) => {
  return user => res.json({ name: user.name, email: user.email });
}