react 项目中,渲染组件时,显示的数据一直有问题,本来以为是 react 组件的问题,后来才发现罪魁祸首在 fetch 数据的过程,因为我用了 async/await ,而却搭配了 foreach 去循环拉取数据,却导致本以为是同步的操作还是变成了异步。

沿用我之前一篇文章( callback vs async.js vs promise vs async / await )里的例子,来重现这个错误:

let read = function (code) {
   if (code) {
       return true;
   } else {
       return false;
let readFileA = function () {
   return new Promise(function (resolve, reject) {
       if (read(1)) {
           resolve("111");
       } else {
           reject("a fail");
let readFileB = function () {
   return new Promise(function (resolve, reject) {
       if (read(1)) {
           resolve("222");
       } else {
           reject("b fail");
let readFileC = function () {
   return new Promise(function (resolve, reject) {
       if (read(1)) {
           resolve("333");
       } else {
           reject("c fail");
async function test() {
   try {
       let readFileFun = [readFileA(), readFileB(), readFileC()]
       console.log("………………start………………")
       // // 方法一:forEach
       // await readFileFun.forEach(async (func, i) => {
       //     console.log("start:", i+1)
       //     let re = await func;
       //     console.log(re)
       //     console.log("end:", i+1)
       // }) 
       // // 方法二:for loop
       // for (let i = 0; i < readFileFun.length; ++i) {
       //     console.log("start:", i+1)
       //     let re = await readFileFun[i];
       //     console.log(re)
       //     console.log("end:", i+1)
       // // 方法三:for ... of
       // for (const [i, func] of readFileFun.entries()) {
       //     console.log("start:", i+1)
       //     let re = await func;
       //     console.log(re)
       //     console.log("end:", i+1)
       console.log("………………end………………")
   } catch (err) {
       console.log(err); // 如果b失败,return: b fail
test();  

输出结果:

# (错)方法一:
………………start………………
start: 1
start: 2
start: 3
end: 1
end: 2
end: 3
………………end………………
# (对)方法二、三:
………………start………………
start: 1
end: 1
start: 2
end: 2
start: 3
end: 3
………………end………………

为什么 foreach 不行,而 普通 for 循环 和 for…of 却正常呢?

我们得先从 foreach 的源码看起:(https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach>)

// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: http://es5.github.io/#x15.4.4.18
if (!Array.prototype.forEach) {
  Array.prototype.forEach = function(callback/*, thisArg*/) {
    var T, k;
    if (this == null) {
      throw new TypeError('this is null or not defined');
    // 1. Let O be the result of calling toObject() passing the
    // |this| value as the argument.
    var O = Object(this);
    // 2. Let lenValue be the result of calling the Get() internal
    // method of O with the argument "length".
    // 3. Let len be toUint32(lenValue).
    var len = O.length >>> 0;
    // 4. If isCallable(callback) is false, throw a TypeError exception. 
    // See: http://es5.github.com/#x9.11
    if (typeof callback !== 'function') {
      throw new TypeError(callback + ' is not a function');
    // 5. If thisArg was supplied, let T be thisArg; else let
    // T be undefined.
    if (arguments.length > 1) {
      T = arguments[1];
    // 6. Let k be 0.
    k = 0;
    // 7. Repeat while k < len.
    while (k < len) {
      var kValue;
      // a. Let Pk be ToString(k).
      //    This is implicit for LHS operands of the in operator.
      // b. Let kPresent be the result of calling the HasProperty
      //    internal method of O with argument Pk.
      //    This step can be combined with c.
      // c. If kPresent is true, then
      if (k in O) {
        // i. Let kValue be the result of calling the Get internal
        // method of O with argument Pk.
        kValue = O[k];
        // ii. Call the Call internal method of callback with T as
        // the this value and argument list containing kValue, k, and O.
        callback.call(T, kValue, k, O);
      // d. Increase k by 1.
    // 8. return undefined.

摘抄最重要的部分:

O 为传入数组 len 为传入数组长度 callback 为传入回调函数 while (k < len) { var kValue; if (k in O) { kValue = O[k]; callback.call(T, kValue, k, O);

可以看到callback.call(T, kValue, k, O);这一句,callback 其实是我们传入的一个被 async 封装的 promise 对象,而 Array.prototype.forEach 内部并未对这个promise 对象做任何处理,只是忽略它。

如果我们尝试把 Array.prototype.forEach 改造一下,让它不要忽视,就可以达到效果了,如下:

 Array.prototype.forEach = async function(callback/*, thisArg*/) {
   		// ………
			await callback.call(T, kValue, k, O);
   		// ………

你总不能去侵入式的改造Array.prototype.forEach吧!所以最简单的办法就是抛弃 foreach,使用 for…of 或者 for 循环!

https://stackoverflow.com/questions/37576685/using-async-await-with-a-foreach-loop

https://github.com/babel/babel/issues/909