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