async getTopStories(page, pageSize) {
    page = page || 1;
    pageSize = pageSize || this.pageSize;

    const result = await this.request('topstories.json', {
      data: {
        orderBy: '"$key"',
        startAt: `"${pageSize * (page - 1)}"`,
        endAt: `"${pageSize * page - 1}"`,
      },
    });
    return Object.keys(result).map(key => result[key]);
  }
get top story ids
@param {Number} [page] - page number, 1-base
@param {Number} [pageSize] - page count
@return {Promise} id list