va/assets/js/cover.js

(function () {
  const className = 'js-cover';
  const covers = Array.from(document.querySelectorAll(`.${className}`));
  const toggleVisibleData = (entries, observer) => {
    entries.forEach(entry => {
      entry.target.setAttribute('data-visible', entry.isIntersecting);
    });
  };

  if ('IntersectionObserver' in window) {
    const observer = new IntersectionObserver(toggleVisibleData);
    
    covers.forEach(t => {
      t.setAttribute('data-observe', '');
      observer.observe(t);
    });
  }
})();