class BufferControl {
  bufferList = []; //緩沖隊列
  maxLen = 1; //當前允許執(zhí)行的最大并發(fā)數量
  currentQueenCount = 0; //當前正在執(zhí)行的任務數量
  add(fn) {
    this.bufferList.push(fn);
    this.run();
  }

  run() {
    if (!this.bufferList.length) {
      return;
    }
    while (this.bufferList.length && this.currentQueenCount < this.maxLen) {
      let fn = this.bufferList.shift();
      this.currentQueenCount++;
      fn().then(() => {
        this.currentQueenCount--;
        this.run();
      });
    }
  }
}

let queen = new BufferControl();

// 定義幾個異步任務
let fn1 = () => {
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log('fn1', 1000);
      resolve();
    }, 1000);
  });
};
let fn2 = () => {
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log('fn2', 2000);
      resolve();
    }, 1000);
  });
};
let fn3 = () => {
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log('fn3', 3000);
      resolve();
    }, 1000);
  });
};
let fn4 = () => {
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log('fn4', 4000);
      resolve();
    }, 1000);
  });
};
// 追加到緩沖隊列中
queen.add(fn1);
queen.add(fn2);
queen.add(fn3);
queen.add(fn4);