image.service.geturl.ts

    return this.http.get(`${API}/${path}`).toPromise().then((snapshot) => {
      this.list = [];

      // prepare a list for intersection observer
      this._indexToObserve = 0;
      const items = snapshot.json();
      this.list = items.reverse().map((item, index) => {
        /* provide 2 additional properties for each image:
              toLoad: boolean, the component to load image,
              index: number, the component to call observe(item) on image loaded
         */
        item.toLoad = index < (this.indexToObserve + INTERSECT_PAGESIZE);
        item.index = index;
        return item;
      });
      return this.list;
    });