一、promise

二、promise.all

// 输入不仅仅只有Array
function promiseAll (args) {
  return new Promise((resolve, reject) => {
    const promiseResults = [];
    let iteratorIndex = 0;
    // 已完成的数量，用于最终的返回，不能直接用完成数量作为iteratorIndex
    // 输出顺序和完成顺序是两码事
    let fullCount = 0;
    // 用于迭代iterator数据
    for (const item of args) {
      // for of 遍历顺序，用于返回正确顺序的结果
      // 因iterator用forEach遍历后的key和value一样，所以必须存一份for of的 iteratorIndex
      let resultIndex = iteratorIndex;
      iteratorIndex += 1;
      // 包一层，以兼容非promise的情况
      Promise.resolve(item).then(res => {
        promiseResults[resultIndex] = res;
        fullCount += 1;
        // Iterator 接口的数据无法单纯的用length和size判断长度，不能局限于Array和 Map类型中
        if (fullCount === iteratorIndex) {
          resolve(promiseResults)
        }
      }).catch(err => {
        reject(err)
      })
    }
    // 处理空 iterator 的情况
    if(iteratorIndex===0){
      resolve(promiseResults)
    }
  }
  )
}
if (!Promise.all) Promise.all = promiseAll;

三、promise.allSettled

export function allSettled (promises) {
    return new Promise(function(resolve, reject) {
        if (!Array.isArray(promises)) {
            return reject(
                new TypeError("arguments must be an array")
            );
        }
        var resolvedCounter = 0;
        var promiseNum = promises.length;
        var resolvedValues = new Array(promiseNum);
        for (var i = 0; i < promiseNum; i++) {
            (function(i) {
                Promise.resolve(promises[i]).then(
                    function(value) {
                        resolvedCounter++;
                        resolvedValues[i] = value;
                        if (resolvedCounter == promiseNum) {
                            return resolve(resolvedValues);
                        }
                    },
                    function(reason) {
                        resolvedCounter++;
                        resolvedValues[i] = reason;
                        if (resolvedCounter == promiseNum) {
                            return reject(reason);
                        }
                    }
                );
            })(i);
        }
  });
};