forEach、map、reduce和promise那點事(上)

forEach、map、reduce和promise那點事(上)

通過此文，您可以學到：

1. forEach、map、reduce 后面能不能帶 async 函數？
2. 怎么實現多個 promise 同步執行，不管有沒有拋出錯誤，都把結果收集起來？

forEach 后面能不能帶async函數？

首先我們來模擬一個異步函數：

const fetch = (forceTrue) => new Promise((resolve, reject) => {
    if (Math.random() > 0.2 || forceTrue) {
        resolve('success');
    } else {
        reject('error');
    }
});

然后我們來試試使用 forEach 來執行多個async函數：

const result = [];

[0,1,2].forEach(async () => {
    const value = await fetch(true);
    result.push(value);
});

console.log(result); // 返回 []

可以看到，我們預期使用 await 獲取 fetch 的結果后存入 result 里面去，但是后面打印出來的卻是空數組。

我們使用 for 試一試：

const result = [];

for (let i = 0; i < 3; ++i) {
    const value = await fetch(true);
    result.push(value);
}

console.log(result); // 返回['success', 'success', 'success']

可以看到，使用 for 的時候按預期返回了。那么為什么用 forEach 就不行呢？我們看一看[mdn上面forEach的polyfill]源碼：

// 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 = thisArg;
    }

    // 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
  };
}

可以看到，里面是通過 while 循環里面多次調用callback.call(T, kValue, k, O);來實現的，由於它前面沒有 await，所以每次循環執行到這里的時候並沒有等待它執行完，最后打印 result 的時候就是空數組了。

map 和 reduce 后面能不能帶async函數？

我們再來看一看 map 和 reduce 的 demo：

// map
const mapResult = [0,1,2].map(async () => await fetch(true));
console.log(mapResult); // 返回 [Promise, Promise, Promise]

// reduce
const reduceResult = [0,1,2].reduce(async (accu) => {
    const value = await fetch(true);
    accu.push(value);
    console.log('accu=====', typeof accu);
    return accu;
}, []); // 報錯 Uncaught (in promise) TypeError: accu.push is not a function
console.log(reduceResult);

可以看到，兩個都沒有正常返回，我們先來看一看[mdn上面map的polyfill]源碼：

// Production steps of ECMA-262, Edition 5, 15.4.4.19
// Reference: http://es5.github.io/#x15.4.4.19
if (!Array.prototype.map) {

  Array.prototype.map = function(callback/*, thisArg*/) {

    var T, A, 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 A be a new array created as if by the expression new Array(len)
    //    where Array is the standard built-in constructor with that name and
    //    len is the value of len.
    A = new Array(len);

    // 7. Let k be 0
    k = 0;

    // 8. Repeat, while k < len
    while (k < len) {

      var kValue, mappedValue;

      // 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. Let mappedValue be the result of calling the Call internal
        //     method of callback with T as the this value and argument
        //     list containing kValue, k, and O.
        mappedValue = callback.call(T, kValue, k, O);

        // iii. Call the DefineOwnProperty internal method of A with arguments
        // Pk, Property Descriptor
        // { Value: mappedValue,
        //   Writable: true,
        //   Enumerable: true,
        //   Configurable: true },
        // and false.

        // In browsers that support Object.defineProperty, use the following:
        // Object.defineProperty(A, k, {
        //   value: mappedValue,
        //   writable: true,
        //   enumerable: true,
        //   configurable: true
        // });

        // For best browser support, use the following:
        A[k] = mappedValue;
      }
      // d. Increase k by 1.
      k++;
    }

    // 9. return A
    return A;
  };
}

可以看到，源碼里面每次 while 循環把 callback 返回的結果賦值給 mappedValue，而 callback 在調用的時候也沒有使用 await，所以結果數組的每一個值都是一個 promise。

我們再看一看[mdn上面reduce的polyfill]源碼：

// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: http://es5.github.io/#x15.4.4.21
// https://tc39.github.io/ecma262/#sec-array.prototype.reduce
if (!Array.prototype.reduce) {
  Object.defineProperty(Array.prototype, 'reduce', {
    value: function(callback /*, initialValue*/) {
      if (this === null) {
        throw new TypeError( 'Array.prototype.reduce ' +
          'called on null or undefined' );
      }
      if (typeof callback !== 'function') {
        throw new TypeError( callback +
          ' is not a function');
      }

      // 1. Let O be ? ToObject(this value).
      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0;

      // Steps 3, 4, 5, 6, 7
      var k = 0;
      var value;

      if (arguments.length >= 2) {
        value = arguments[1];
      } else {
        while (k < len && !(k in o)) {
          k++;
        }

        // 3. If len is 0 and initialValue is not present,
        //    throw a TypeError exception.
        if (k >= len) {
          throw new TypeError( 'Reduce of empty array ' +
            'with no initial value' );
        }
        value = o[k++];
      }

      // 8. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kPresent be ? HasProperty(O, Pk).
        // c. If kPresent is true, then
        //    i.  Let kValue be ? Get(O, Pk).
        //    ii. Let accumulator be ? Call(
        //          callbackfn, undefined,
        //          « accumulator, kValue, k, O »).
        if (k in o) {
          value = callback(value, o[k], k, o);
        }

        // d. Increase k by 1.
        k++;
      }

      // 9. Return accumulator.
      return value;
    }
  });
}

可以看到，源碼里面每次循環的時候也是把 callback 的值賦給累計器 accu，而 callback 是一個 async 函數，所以這里 accu 其實是一個 promise 而不是數組，它沒有數組方法，所以報錯了。