forEach中使用async/await踩坑

在使用forEach的时候,发现在回调里获取异步数据,结果拿不到

    const imgInfoObj = {}
    imgInfo.forEach(async (item) => {
      const imageOpt = {
        width: item.width || 72,
        height: item.height || 72,
        url: item.url,
        offset: item.offset || [0, 0] //图标偏移
      }
      const canvas = await this.getCanvasUrl(imageOpt)
      imgInfoObj[item.name] = {
        canvas,
        imageOpt
      }
    })
    console.log(imgInfoObj) // 输出空数组{}

这是为啥呢?其实是因为js没有对数组的forEach方法做异步处理,在循环调用回调的时候,回调内部的异步操作挂到任务队列里,但不影响forEach方法循环调用回调这个行为本身,也就是说forEach走完了,回调内的异步并不能拿到异步的值,他们还在队列里等着处理呢

最后上一下forEach源码

// 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.
        // 可以看到并没有对callback的调用做异步处理
        callback.call(T, kValue, k, O);
      }
      // d. Increase k by 1.
      k++;
    }
    // 8. return undefined
  };
}