Preload images on page

prealoadImages.js

preloadImages() {
    let i = 0;
    const arr = [...Backgrounds, ...Images];
    return new Promise(resolve => {
      arr.map(filename => {
        const img = new Image();
        img.src = filename;
        img.onload = () => (++i == arr.length) ? resolve() : 1;
      });
    });
  }