const fetchers = {};

function prefetchCleanup(event) {
  clearTimeout(fetchers[event.target.href]);
  event.target.removeEventListener('mouseleave', prefetchCleanup);
}

document.addEventListener('mouseover', event => {
  let url = event.target.href;
  clearTimeout(fetchers[url]);
  if (!event.target.dataset.prefetch) return;
  if (Turbolinks.controller.cache.has(url)) return;

  event.target.addEventListener('mouseleave', prefetchCleanup);

  fetchers[url] = setTimeout(() => {
    const iframe = document.createElement('iframe');
    iframe.setAttribute('style', 'display:none');
    iframe.setAttribute('src', url);
    iframe.addEventListener('load', () => {
      Turbolinks.controller.cache.put(
        url,
        Turbolinks.Snapshot.fromElement(iframe.contentDocument.documentElement)
      );
      iframe.parentElement.removeChild(iframe);
    });
    document.body.appendChild(iframe);
  }, 600);
});