用Node.js中的promises替换回调

Question

我有一个简单的节点模块,它连接到一个数据库,并有几个接收数据的函数,例如这个函数:

---

dbConnection.js:

import mysql from 'mysql';

const connection = mysql.createConnection({
  host: 'localhost',
  user: 'user',
  password: 'password',
  database: 'db'
});

export default {
  getUsers(callback) {
    connection.connect(() => {
      connection.query('SELECT * FROM Users', (err, result) => {
        if (!err){
          callback(result);
        }
      });
    });
  }
};

该模块将从不同的节点模块以这种方式调用:

---

app.js:

import dbCon from './dbConnection.js';

dbCon.getUsers(console.log);

我想使用promises而不是回调来返回数据.到目前为止,我已经阅读了以下线程中的嵌套promise:使用嵌套Promise编写清洁代码,但我找不到任何对这个用例来说足够简单的解决方案.result使用承诺返回的正确方法是什么？

Answer 1

使用Promise课程

我建议看看MDN的Promise文档,它为使用Promise提供了一个很好的起点.或者,我确信网上有很多教程.:)

注意:现代浏览器已经支持Promises的ECMAScript 6规范(参见上面链接的MDN文档),我假设你想要使用原生实现,没有第三方库.

至于一个实际的例子......

基本原理是这样的:

1. 您的API已被调用
2. 您创建一个新的Promise对象,该对象将一个函数作为构造函数参数
3. 您提供的函数由底层实现调用,函数有两个函数 - resolve和reject
4. 完成逻辑后,您可以调用其中一个来填充Promise或拒绝它并返回错误

这可能看起来很多,所以这是一个实际的例子.

exports.getUsers = function getUsers () {
  // Return the Promise right away, unless you really need to
  // do something before you create a new Promise, but usually
  // this can go into the function below
  return new Promise((resolve, reject) => {
    // reject and resolve are functions provided by the Promise
    // implementation. Call only one of them.

    // Do your logic here - you can do WTF you want.:)
    connection.query('SELECT * FROM Users', (err, result) => {
      // PS. Fail fast! Handle errors first, then move to the
      // important stuff (that's a good practice at least)
      if (err) {
        // Reject the Promise with an error
        return reject(err)
      }

      // Resolve (or fulfill) the promise with data
      return resolve(result)
    })
  })
}

// Usage:
exports.getUsers()  // Returns a Promise!
  .then(users => {
    // Do stuff with users
  })
  .catch(err => {
    // handle errors
  })

使用async/await语言功能(Node.js> = 7.6)

在Node.js 7.6中,使用async/await支持升级了v8 JavaScript编译器.您现在可以将函数声明为async,这意味着它们会自动返回Promise在异步函数完成执行时解析的函数.在此函数中,您可以使用await关键字等待另一个Promise结算.

这是一个例子:

exports.getUsers = async function getUsers() {
  // We are in an async function - this will return Promise
  // no matter what.

  // We can interact with other functions which return a
  // Promise very easily:
  const result = await connection.query('select * from users')

  // Interacting with callback-based APIs is a bit more
  // complicated but still very easy:
  const result2 = await new Promise((resolve, reject) => {
    connection.query('select * from users', (err, res) => {
      return void err ? reject(err) : resolve(res)
    })
  })
  // Returning a value will cause the promise to be resolved
  // with that value
  return result
}

Answer 2

使用bluebird,您可以使用Promise.promisifyAll(和Promise.promisify)向任何对象添加Promise就绪方法.

var Promise = require('bluebird');
// Somewhere around here, the following line is called
Promise.promisifyAll(connection);

exports.getUsersAsync = function () {
    return connection.connectAsync()
        .then(function () {
            return connection.queryAsync('SELECT * FROM Users')
        });
};

并使用这样的:

getUsersAsync().then(console.log);

要么

// Spread because MySQL queries actually return two resulting arguments, 
// which Bluebird resolves as an array.
getUsersAsync().spread(function(rows, fields) {
    // Do whatever you want with either rows or fields.
});

添加处理器

蓝鸟支持很多功能,其中之一是处置器,它可以让你安全地处理一个连接后的帮助下结束了Promise.using和Promise.prototype.disposer.这是我的应用程序中的一个示例:

function getConnection(host, user, password, port) {
    // connection was already promisified at this point

    // The object literal syntax is ES6, it's the equivalent of
    // {host: host, user: user, ... }
    var connection = mysql.createConnection({host, user, password, port});
    return connection.connectAsync()
        // connect callback doesn't have arguments. return connection.
        .return(connection) 
        .disposer(function(connection, promise) { 
            //Disposer is used when Promise.using is finished.
            connection.end();
        });
}

然后像这样使用它:

exports.getUsersAsync = function () {
    return Promise.using(getConnection()).then(function (connection) {
            return connection.queryAsync('SELECT * FROM Users')
        });
};

一旦promise与值一起解析(或拒绝一个Error),这将自动结束连接.

Answer 3

Node.js版本8.0.0+:

您不必再使用bluebird来宣传节点API方法.因为,从版本8+开始,您可以使用本机util.promisify:

const util = require('util');

const connectAsync = util.promisify(connection.connectAsync);
const queryAsync = util.promisify(connection.queryAsync);

exports.getUsersAsync = function () {
    return connectAsync()
        .then(function () {
            return queryAsync('SELECT * FROM Users')
        });
};

现在,不必使用任何第三方库来进行promisify.