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// ...

@Component({
  // ...
})
export class AlbumComponent implements OnInit {

  // ...

  preloadedPhotos: HTMLImageElement[]; // 🅰

  // ...

  ngOnInit() {
    // ...
    // 🅱
    this.preloadedPhotos = new Array<HTMLImageElement>(this.album.photos.length);
    this.preloadPhoto(-1);
    this.preloadPhoto(0);
    this.preloadPhoto(1);
  }

  preloadPhoto(idx) { // 🅲
    if (!this.preloadedPhotos[idx]) {
      this.preloadedPhotos[idx] = new Image();
      this.preloadedPhotos[idx].src = this.photosUrl + this.album.photos[idx];
    }
  }

  slide(offset) {
    // ...
    this.preloadPhoto(this.current + offset); // 🅳
  }

}