重写promise.all

使用

var promise1 = Promise.resolve(3);
var promise2 = 42;
var promise3 = new Promise(function(resolve, reject) {
  setTimeout(resolve, 100, 'foo');
});

Promise.all([promise1, promise2, promise3]).then(function(values) {
  console.log(values);
});

分析

1.参数为iterator类型
2.并发执行
3.等待所有promise返回后才返回结果
4.结果为数组，与传入的promise顺序对应
5.但凡有一个出错，则抛错

实现

Promise.myAll = function (Iterator) {
  return new Promise((resolve, reject) => {
    // 转换成数组，好处是方便获取长度，且方便对应结果数组的index
    const arr = Array.from(Iterator);
    const { length } = arr;
    const result = [];
    // 无length则无法执行for of，所以此处特殊值处理
    if (!length) return result;

    //计数，个数达到length即全部执行完
    let count = 0;

    for (let index = 0; index < length; index++) {
      const promise = arr[index];

      promise
        .then((res) => {
          result[index] = res;
          if (++count === length) resolve(result);
        })
        .catch((err) => {
          reject(err);
        });
    }
  });
};

测试案例

const test = async () => {
  var promise1 = Promise.resolve(3);
  var promise2 = Promise.resolve({ name: "shuaige" });
  var promise3 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 100, "foo");
  });

  Promise.myAll([promise3, promise1, promise2]).then(function (values) {
    console.log(values);
  });
};

test();
//[
///"foo",
//3,
//{ name: "shuaige" }
//]