将Firebase Observable List设置为typescript中的对象/变量

@Injectable()

export class ArticlesService {
  items: FirebaseListObservable<any>;
  private _url = 'https://59176d8f5b63ed0011251710.mockapi.io/api/v1/articles';

  constructor(private _http: Http, db: AngularFireDatabase) {
    this.items = db.list('/');
  }

  getAllArticles(): Observable<any> {
    return this._http.get(this._url)
      .map((response: Response) => response.json())
      .catch((error) => Observable.throw(error.json()));
  }

  getAnArticle(articleId: string): Observable<any> {
    return this._http.get(`${this._url}/${articleId}`)
      .map((response: Response) => response.json())
      .catch((error) => Observable.throw(error.json()));
  }
}
@Injectable()

export class ArticlesService {