idlePrefetch.js

    idlePrefetch (originImageUrl) {
      let fetched = false;
      let myNonEssentialWork = deadline => {
          const appendLinkTo = function (url) {
            const linkTag = document.createElement('link')
            linkTag.rel = 'prefetch'
            linkTag.href = url
            document.head.appendChild(linkTag)
          }          
          if (deadline.timeRemaining() > 0) {
            appendLinkTo(originImageUrl);
            fetched = true;

          }
          if (!fetched) {
            requestIdleCallback(myNonEssentialWork);
          }          
        };

      requestIdleCallback(myNonEssentialWork);
    }