在限定并发数下用 Promise 并发完成请求

class PromisePool {
    constructor(max, fn) {
      this.max = max; // 最大并发数
      this.fn = fn;   // 自定义的请求函数
      this.pool = []; // 并发池
      this.urls = []; // 剩余的请求地址
    }
  
    start(urls) {
      this.urls = urls;
      // 先循环把并发池塞满
      while (this.pool.length < this.max) {
        let url = this.urls.shift();
        this.setTask(url);
      }
      // 利用Promise.race 方法来获得并发池中某任务完成的信号
      let race = Promise.race(this.pool);
      return this.run(race);
    }
  
    run(race) {
      race
        .then(res => {
          // 每当并发池跑完一个任务，就再塞入一个任务
          let url = this.urls.shift();
          this.setTask(url);
          return this.run(Promise.race(this.pool));
        });
    }
    setTask(url) {
      if (!url) return;
      let task = this.fn(url);
      this.pool.push(task); // 将该任务推入pool并发池中
      console.log(`\x1B[43m ${url} 开始，当前并发数：${this.pool.length}`);
      task.then(res => {
        // 请求结束后将该Promise任务从并发池中移除
        this.pool.splice(this.pool.indexOf(task), 1);
        console.log(`\x1B[43m ${url} 结束，当前并发数：${this.pool.length}`);
      });
    }
  }
  
  // test
  const URLs = [
    'bytedance.com',
    'tencent.com',
    'alibaba.com',
    'microsoft.com',
    'apple.com',
    'hulu.com',
    'amazon.com'
  ];
  let dur = 0;
  // 自定义请求函数
  var requestFn = url => {
    return new Promise(resolve => {
      setTimeout(_ => {
        resolve(`任务 ${url} 完成`);
      }, 1000*dur++)
    }).then(res => {
      console.log('外部逻辑 ', res);
    })
  }
  
  const pool = new PromisePool(5, requestFn); // 并发数为3
  pool.start(URLs);