eMFtQj hIPNtw jQMBWr 所以在JS的世界里, 很需要一种侵入性不那么强而且易读的异步回调解决方案, ES7的规范里, ECMAScript又提出了async/await, 使用这个方案仍然需要使用Promise来改写异步过程, 但是需要用then来改写执行过程, 而是可以像同步代码一样将结果直接作为执行结果返回, 可阅读性也非常好. 这个方案的 hYcTaL hctuh jMfvbc gFwJbt [object Object],[object Object],[object Object],[object Object] ikkyVr bPjPUf cofShm lmYuZf euExUQ hDyWCg hUVKYO [object Object] uQPrX ekvxDO bpqJVJ iGaadT iGXldS hOxXax emJCYl ddGdPh iBSbYp dwejLW esHzLO"> /* sc-component-id: sc-bdVaJa */ .sc-bdVaJa {} .jTcPxY{height:100%;overflow:auto;} /* sc-component-id: sc-bwzfXH */ .sc-bwzfXH {} .iYgZUj{color:#BBB;margin:10px 0 20px 0;} /* sc-component-id: sc-htpNat */ .sc-htpNat {} .knYzzU{position:relative;padding:10px 0 10px 0;width:calc(100% - 10px);height:50px;-webkit-transition:-webkit-transform 0.6s;-webkit-transition:transform 0.6s;transition:transform 0.6s;} .knYzzU:hover{background-color:#EEE;-webkit-transform:translate(10px,0);-ms-transform:translate(10px,0);transform:translate(10px,0);}.null{position:relative;padding:10px 0 10px 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Life of xhu

当Promise遇上闭包

Dec 20, 2015  |  JavaScript  |  Promise  |  Closure

这个博客的Projects页面,数据并不是写死的,而是从GitHub抓取过来的,因为官方的Nodejs GitHub API库并不支持Promise的方式,所以我一开始是这样写的:

var fetchReposPromise = new Promise((resolve, reject) => {
  githubAPI.request('...', (data) => {
    resolve(data);
  });
});

然后在需要请求数据的时候,直接fetchReposPromise.then就行了,对这段代码我期望的行为是,每次进入resolve函数的时候,都能重新发一次请求,然而事实并不是这样,我发现每次最后进入resolve函数的都是同一份数据,于是我写了下面一段代码来验证:

// server
var express = require('express');
var app = express();

app.get('/', (req, res) => {
  res.send({num: Math.random()});
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

// client
var request = require('request');

var promise =  new Promise((resolve, reject) => {
  request.get('http://localhost:3000', function (err, res, body) {
    resolve(body);
  });
});

promise.then(value => console.log(value));
promise.then(value => console.log(value));
// {"num":0.6926063213031739}
// {"num":0.6926063213031739}

事实果然是这样,两次then的结果并没有区别,也就是说这个get请求只执行了一次,所以进入resolve函数的data还是一样的。

于是我又写了下面这段代码来验证我的想法:

var a = 1;
var p = new Promise(resolve => resolve(a));

a = 2;
p.then(value => console.log(value));   // => 1

这个结果出来后,终于算是真相大白了,原来又是闭包!

回到第一段代码,这段代码中的Promise封装了一个异步过程,然后把结果作为参数传入了resolve函数,这段代码看起来每次resolve执行前都会经历请求数据的过程,其实并不是这样,对于一个Promise的resolve和reject函数,之前的执行过程都是完全不可见的,当Promise里的异步过程执行完毕时,最后resolve(data)其实生成了一个闭包,data成了这个闭包的内部变量,之后再调用then函数的时候,都是使用这个data作为参数,所以每次执行的结果都是一样的。

知道了原因,改起来也就轻松了,我们只需要在这个Promise外面套一个function,这样每次执行这个函数的时候都会生成一个新的闭包,以第二段代码中的clien为例:

var request = require('request');

var generatePromise = () => {
  return new Promise((resolve, reject) => {
    request.get('http://localhost:3000', function (err, res, body) {
      resolve(body);
    });
  });
};

generatePromise().then(value => console.log(value));
generatePromise().then(value => console.log(value));
// => {"num":0.4341859801206738}
// => {"num":0.3852167946752161}

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