promise.all实现限制并发

function asyncPool(limit, array, iteratorFn) {
    let executing = [] // 正在执行的
  let res = [] // 所有结果
  let index = 0 // 当前索引
  let len = array.length // 整体长度
  function enqueue () { //队列函数
      if (index === len) return Promise.resolve()
    let item = array[index++]
    let e = Promise.resolve().then(() => iteratorFn(item))
    res.push(e)
    let p = e.then(() => {
        executing.splice(executing.indexOf(e), 1)
    })
    executing.push(p)
    let r = Promise.resolve()
    if (executing.length >= limit) {
        r = Promise.race(executing)
    }
    return r.then(() => enqueue())
  }
  return enqueue().then(() => Promise.all(res))  // 触发队列执行，并且确保队列执行完之后监控all res
}
const timeout = i => new Promise(resolve => setTimeout(() => {
    return resolve(i)
}, i));
asyncPool(2, [1000, 5000, 3000, 2000], timeout).then(results => {
    console.log(results)
});