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实现代码

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class Task {
  constructor(count) {
    this.maxCount = count;
    this.runCount = 0;
    this.tasks = [];
  }

  add(task) {
    return new Promise((resolve, reject) => {
      this.tasks.push({
        task,
        resolve,
        reject,
      });
      this.run();
    });
  }

  run() {
    while (this.tasks.length && this.runCount < this.maxCount) {
      const { task, resolve, reject } = this.tasks.shift();
      this.runCount++;
      task()
        .then(resolve, reject)
        .finally(() => {
          this.runCount--;
          this.run();
        });
    }
  }
}

demo

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const task = new Task(2);
function demo(time, index) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(index);
      console.log(index);
    }, time);
  });
}
// 最多同时执行两个任务 所以这里先打印1,2然后打印3，4
task.add(() => demo(1000, 1));
task.add(() => demo(1000, 2));
task.add(() => demo(1000, 3));
task.add(() => demo(1000, 4));