Untested prototype for a readahead iterator in typescript

ReadAheadIterator.ts

class ReadAheadIterator<T extends any> implements AsyncIterator<T> {
  currentQ: Promise<T>[] = [];
  constructor(private readNext: Iterator<Promise<T>>, private readAheadCount: number = 5) {
    this.readAhead();
  }

  [Symbol.asyncIterator]() { return this; }

  readAhead() {
    while (this.currentQ.length < this.readAheadCount) {
      let res = this.readNext.next();
      if (!res.done) {
        this.currentQ.push(res.value);
      } else {
        break;
      }
    }
  }

  next(): Promise<IteratorResult<T>> {
    if (!this.currentQ.length) {
      return new Promise(res => res({
        done: true,
        value: void 0
      }));
    }
    let res = this.currentQ.shift();
    this.readAhead();
    return res.then(v => ({
      done: false,
      value: v
    }));
  }
}