2. class Scheduler { constructor(parallelCount = 2) { this.parallelCount = parallelCount; this.runningCount = 0; this.task = []; } add(task) { return new Promise((resolve, reject) => { this.task.push({ task, resolve, reject, }); this._run(); }); } _run() { if (this.task.length > 0 && this.runningCount < this.parallelCount) { const { task, resolve, reject } = this.task.shift(); this.runningCount++; task() .then(resolve, reject) .finally(() => { this.runningCount--; this._run(); }); } } }