手写 Promise.all

Promise.all 接受由多个 promise 组成的数组作为参数
当数组中所有的 promise 都 resolve 时，返回这些 promise 结果组成的数组
当任意一个 promise reject 时，则终止执行，抛出错误

Promise.all2 = (promiseList) => {
  return new Promise((resolve, reject) => {
    let result = []
    let length = promiseList.length
    let count = 0
    promiseList.forEach((promise, index) => {
      promise.then(data => {
        result[index] = data
        count += 1
        if(count === length -1) {
          resolve(result)
        }
      }, (reason) => reject(reason))
    })
  })
}
Promise.all2([Promise.resolve(1), Promise.resolve('foo'), Promise.resolve(3)])
.then(data => console.log(data), reason => console.error(reason))
// [1, 'foo', 3]
Promise.all2([Promise.resolve(1), Promise.reject('foo'), Promise.resolve(3)])
.then(data => console.log(data), reason => console.error(reason))
// error foo