sel-deckart-workers-dev

addEventListener("fetch", (event) => {
  event.respondWith(
    handleRequest(event.request).catch(
      (err) => new Response(err.stack, { status: 500 })
    )
  );
});

const addPreloadBefore = (href) => ({
  element: (el) => {
    el.before(`<link rel="preload" href="${href}" as="image" />`, { html: true });
  }
});

async function handleRequest(request) {
  const url = new URL(request.url);
  const host = request.headers.get('x-host');
  const bypassTransform = request.headers.get('x-bypass-transform');
  const acceptHeader = request.headers.get('accept');

  if (!host) {
    return new Response('x-host headers missing', {status: 403});
  }

  url.hostname = host

  if ((acceptHeader
    && acceptHeader.indexOf('text/html') >= 0)
    && (!bypassTransform
    || (bypassTransform
    && bypassTransform.indexOf('true') === -1))) {

    const response = await fetch(url.toString(), request);
    const newResponse = new Response(response.body, response);

    return new HTMLRewriter()
      .on(`link[rel^="dns-prefetch"]`, addPreloadBefore("https://searchengineland.com/images/marquee/keyword_hero_05032021.jpg"))
      .transform(newResponse);
  }

  return fetch(url.toString(), request);
}