JobQueue using Promises

JobQueue.js

// A job queue that takes an anonymous functions and executes them in parallel
class JobQueue {
  constructor(maxConcurrent = 3) {
    this.maxConcurrent = maxConcurrent;
    this.queue = [];
    this.activeCount = 0;
  }

  // Add task to queue and
  exec(task) {
    return new Promise((resolve, reject) => {
      this.queue.push(() => {
        return task().then(resolve).catch(reject);
      });
      this.next();
    });
  }

  // Run the next task
  next() {
    if (this.activeCount < this.maxConcurrent && this.queue.length > 0) {
      // Grab next task
      const nextTask = this.queue.shift();
      this.activeCount++;

      // Run the task and reduce the active count once it's done
      nextTask().finally(() => {
        this.activeCount--;
        this.next();
      });
    }
  }
}

const sleep = (ms) => new Promise((r) => setTimeout(r, ms));

const Job = async (id, delay) => {
  console.log(`Task ${id} added`);
  try {
    await sleep(delay);
    if (id === 2) {
      throw new Error("err happened");
    }
  } catch (err) {
    // Handle errors but dont kill the queue
    console.error(err);
  }
  console.log(`Task ${id} done`);
};

const queue = new JobQueue(3);
queue.exec(() => Job(1, 1000));
queue.exec(() => Job(2, 750));
queue.exec(() => Job(3, 2000));
queue.exec(() => Job(4, 2500));
queue.exec(() => Job(5, 1000));
queue.exec(() => Job(6, 250));