asyncIterators.ts

class RangeIterator implements Iterator<number> {
  constructor(private n: number, private to: number) {}
  next(): IteratorResult<number> {
    if (this.n <= this.to) {
      return { done: false, value: this.n++ };
    } else {
      return { done: true, value: undefined };
    }
  }
}

class AsyncRangeIterator implements AsyncIterator<number> {
  constructor(private n: number, private to: number) {}
  async next(): Promise<IteratorResult<number>> {
    await new Promise(resolve => setTimeout(resolve, 3000));

    if (this.n <= this.to) {
      return { done: false, value: this.n++ };
    } else {
      return { done: true, value: undefined };
    }
  }
}

class MyRange {
  constructor(private start: number, private end: number) {}

  [Symbol.iterator]() {
    return new RangeIterator(this.start, this.end);
  }

  [Symbol.asyncIterator]() {
    return new AsyncRangeIterator(this.start, this.end);
  }
}

const range = new MyRange(0, 10);

for (let n of range) {
  console.log(n);
}

(async () => {
  for await (let n of range) {
    console.log(n);
  }
})();