ConcurrentQueue.ts

type QueueOpts = {
  maxConcurrency: Number;
};

type Task<R> = () => Promise<R>;

class ConcurrentQueue<R> {
  opts: QueueOpts = {
    maxConcurrency: 1
  };

  tasks: Task<R>[] = [];

  results: R[] = [];

  constructor(opts: QueueOpts) {
    this.opts = opts;
  }

  public async run(tasks: Task<R>[]): Promise<R[]> {
    this.tasks = tasks;
    const workers: Promise<R | "done">[] = new Array(this.opts.maxConcurrency)
      .fill(0)
      .map((_) => this.doWork());
    await Promise.all(workers);
    return this.results;
  }

  async doWork(): Promise<R | "done"> {
    let nextTask = this.tasks.shift();
    if (nextTask === undefined) return "done";
    let result = await nextTask();
    this.results.push(result);
    return this.doWork();
  }
}

ExampleUsage.ts

const queue = new ConcurrentQueue({
  maxConcurrency: 2
});

let delay = (ms: number) =>
  new Promise((resolve) => setTimeout(() => resolve(true), ms));

let log = async (ms: number, msg: string) => {
  await delay(ms);
  console.log(msg);
  return msg;
};

let tasks: Task<String>[] = [
  async () => log(2000, "I'm (even later) task #1!"),
  async () => log(1000, "I'm (late) task #2!"),
  async () => log(300, "I'm task #3!"),
  async () => log(400, "I'm task #4!"),
  async () => log(200, "I'm task #5!"),
  async () => log(500, "I'm task #6!"),
  async () => log(50, "I'm task #7!"),
  async () => log(1, "I'm task #8!")
];

let results = await queue.run(tasks);
console.log("Results: ", results);