JSQuestions/solutions/async-scheduler.js

class TaskScheduler {
  constructor(concurrency = 1) {
    this.queue = [];
    this.isProcessing = false;
    this.concurrencyLimit = concurrency;
    this.runningCount = 0;
  }
  
  addTask(taskFn) {
    const { promise, resolve, reject } = Promise.withResolvers();
    this.queue.push({
      taskFn,
      resolve,
      reject,
    });

    this._processQueue();
    return promise;
  }

  _processQueue() {
    if (!this.isProcessing) return;

    while (this.runningCount < this.concurrencyLimit && this.queue.length > 0) {
      const {
        taskFn,
        resolve,
        reject,
      } = this.queue.shift();

      this.runningCount++;

      // Ensure taskFn errors are captured as rejections
      const taskPromise = Promise.resolve().then(() => taskFn());
      taskPromise.then(resolve, reject);
      taskPromise.finally(() => {
        this.runningCount--;
        this._processQueue();
      });
    }
  }

  play(){
    this.isProcessing = true;
    this._processQueue();
  }

  pause(){
    this.isProcessing = false;
  }
}

module.exports = TaskScheduler;