How to get multiple random documents from angularfire firestore

I've been looking for solutions but non worked for my application, I'm trying to build a Quiz app using angular and firestore with angularfire, my current approach:

public getRandomQuestion(): Promise<any> {
    let ques: IQuestion;
    return this.firestore.collection('questions', ref => ref.where('randId', '>=', this.getRandomInt(1,999999)).limit(1)).valueChanges().toPromise();
  }
  public getTenRandomQuestion(): IQuestion[]{
    let obs : IQuestion[] = [];
    for (let i: number = 0; i <= 9; i++){
      this.getRandomQuestion().then(res => {
        obs[i] = res;
      });
    }
    console.log(obs);
    return obs;
}

But it always returns an empty array, here's my firestore : Firestore