爲何 array.foreach 不支持 async/await

時間 2019-11-09

標籤爲何 array.foreach array foreach 不支持 async await 欄目 Java 简体版

原文原文鏈接

1、背景

react 項目中，渲染組件時，顯示的數據一直有問題，原本覺得是 react 組件的問題，後來才發現罪魁禍首在 fetch 數據的過程，由於我用了 async/await ，而卻搭配了 foreach 去循環拉取數據，卻致使本覺得是同步的操做仍是變成了異步。html

2、正文

沿用我以前一篇文章（callback vs async.js vs promise vs async / await）裏的例子，來重現這個錯誤：react

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); // 若是b失敗，return： b fail
   }
}

test();

輸出結果：git

# （錯）方法一：
………………start………………
start: 1
start: 2
start: 3
111
end: 1
222
end: 2
333
end: 3
………………end………………


# （對）方法2、三：
………………start………………
start: 1
111
end: 1
start: 2
222
end: 2
start: 3
333
end: 3
………………end………………

爲何 foreach 不行，而普通 for 循環和 for…of 卻正常呢？github

咱們得先從 foreach 的源碼看起：(https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach>）數組

// 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.
      k++;
    }
    // 8. return undefined.
  };
}

摘抄最重要的部分：promise

/* 
O 爲傳入數組
len 爲傳入數組長度
callback 爲傳入回調函數
*/

while (k < len) {

      var kValue; 
      if (k in O) { 
        kValue = O[k]; 
        callback.call(T, kValue, k, O);
      } 
  
      k++;
}

能夠看到callback.call(T, kValue, k, O);這一句，callback 實際上是咱們傳入的一個被 async 封裝的 promise 對象，而 Array.prototype.forEach 內部並未對這個promise 對象作任何處理，只是忽略它。babel

若是咱們嘗試把 Array.prototype.forEach 改造一下，讓它不要忽視，就能夠達到效果了，以下：異步

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