Promise.all限制并发数

摘自https://juejin.im/post/5d4a1d30f265da03f564cbf1

class Limit {
  constructor (n) {
    this.limit = n
    this.count = 0
    this.queue = []
  }

  enqueue (fn) {
    // 关键代码: fn, resolve, reject 统一管理
    return new Promise((resolve, reject) => {
      this.queue.push({ fn, resolve, reject })
    })
  }

  dequeue () {
    if (this.count < this.limit && this.queue.length) {
      // 等到 Promise 计数器小于阈值时，则出队执行
      const { fn, resolve, reject } = this.queue.shift()
      this.run(fn).then(resolve).catch(reject)
    }
  }

  // async/await 简化错误处理
  async run (fn) {
    this.count++
    // 维护一个计数器
    const value = await fn()
    this.count--
    // 执行完，看看队列有东西没
    this.dequeue()
    return value
  }

  build (fn) {
    if (this.count < this.limit) {
      // 如果没有到达阈值，直接执行
      return this.run(fn)
    } else {
      // 如果超出阈值，则先扔到队列中，等待有空闲时执行
      return this.enqueue(fn)
    }
  }
}

Promise.map = function (list, fn, { concurrency }) {
  const limit = new Limit(concurrency)
  return Promise.all(list.map((...args) => {
    return limit.build(() => fn(...args))
  }))
}