控制并发， 实现 SuperTask

function timeout(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

const superTask = new SuperTask();

function addTask(time, name) {
  superTask
    .add(() => timeout(time))
    .then(() => {
      console.log(`${name} 执行完毕`);
    });
}

addTask(10000, "task1"); // 10000ms后 task1 执行完毕
addTask(5000, "task2"); // 5000ms后 task2 执行完毕
addTask(3000, "task3"); // 8000ms后 task3 执行完毕
addTask(4000, "task4"); // 12000ms后 task4 执行完毕
addTask(5000, "task5"); // 15000ms后 task5 执行完毕

class SuperTask {
  constructor(maxCount = 2) {
    this.maxCount = maxCount;
    this.tasks = [];
    this.penddingCount = 0;
  }

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

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