一个真实的Async/Await示例

译者按： 通过真实的代码示例感受Async/Await的力量。

原文: Async/await - A thorough example
译者: Fundebug

为了保证可读性，本文采用意译而非直译。另外，本文版权归原作者所有，翻译仅用于学习。

既然Node.js 8已经LTS了，我想大家是时候试一试Async/Await特性了，真的很好用！它可以帮助我们用同步的方式写异步代码，极大地提高了代码的可读性。在过去的2年时间里，Promise给我们带来了不少便利，同时也让我们有一些失望。

这边博客，我将介绍一个真实的代码示例，它是一个REST API的controller。通过展示我们如何从Promise切换到async/await，你讲能够体会到Async/Await的神奇之处！

Promise示例

下面是我的工作项目中真实的Controller代码：

const BPromise = require('bluebird');

const { WrongCredentialsError, DBConnectionError, EmailError } = require('./../errors');

/**
 * Emulate an Express.js route call as an example
 */
loginController({}, { json: response => console.log(response) }, null)

function loginController (req, res, err) {
  const { email, password } = req;

  let user;

  BPromise.try(() => validateUserInput(req))
    .then(() => fetchUserByEmail(email))
    .then(fetchedUser => user = fetchedUser)
    .then(() => comparePasswords(req.password, user.password))
    .then(() => markLoggedInTimestamp(user.userId))
    .then(() => sendEmail(user.userId))
    .then(() => generateJWT(user))
    .then(token => res.json({ success: true, token }))
    .catch(WrongCredentialsError, () => res.json({ success: false, error: 'Invalid email and/or password' }))
    .catch(EmailError, DBConnectionError, () => res.json({ success: false, error: 'Unexpected error, please try again' }))
    .catch(() => res.json({ success: false }))
}

/**
 * Validate input from Request
 *
 * @param {Object} input
 * @throws {WrongCredentialsError}
 * @returns {Void}
 */
function validateUserInput(input) {
  if (!input.email || !input.password) {
    throw new WrongCredentialsError();
  }
}

/**
 * Fetch a User from the DB by Email
 *
 * @throws WrongCredentialsError
 * @throws DBConnectionError
 * @returns {BPromise}
 */
function fetchUserByEmail(email) {
  const user = {
    userId: 'DUMMY_ID',
    email: 'konmpar@gmail.com',
    password: 'DUMMY_PASSWORD_HASH'
  }
  return new BPromise(resolve => resolve(user));
}

/**
 * Compare two password
 *
 * @param {String} inputPwd
 * @param {String} storedPwd
 * @throws {WrongCredentialsError}
 * @returns {Void}
 */
function comparePasswords(inputPwd, storedPwd) {
  if (hashPassword(inputPwd) !== storedPwd) {
    throw new WrongCredentialsError();
  }
}

/**
 * Hash password
 *
 * @param {String} password
 * @returns {String}
 */
function hashPassword(password) {
  return password;
}

/**
 * Mark a user's logged in timestamp
 *
 * @param {String} userId
 * @throws DBConnectionError
 * @returns {BPromise}
 */
function markLoggedInTimestamp(userId) {
  return new BPromise(resolve => resolve());
}

/**
 * Send a follow up email
 *
 * @param {String} userId
 * @throws EmailError
 * @returns {BPromise}
 */
function sendEmail(userId) {
  return new BPromise(resolve => resolve());
}