Example Cloudflare Worker that removes script preloads

index.js

/* 
 * For more detail see https://andydavies.me/blog/2020/09/22/exploring-site-speed-optimisations-with-webpagetest-and-cloudflare-workers/
 * Started from Pat's example in https://www.slideshare.net/patrickmeenan/getting-the-most-out-of-webpagetest
 *
 * Change site to be site you want to experiment with
 */

const site = 'www.example.com';

async function handleRequest(request) {
   
  // When overrideHost is used in a script, WPT sets x-host to original host i.e. the one we want to proxy 
  const host = request.headers.get('x-host');

  if(host) {
    const url = new URL(request.url)

    url.hostname = host;

    // If it's the original document, rewrite the HTML
    // TODO use a better check?

      let accept = request.headers.get('accept');

      if (host === site && accept && accept.indexOf('text/html') >= 0) {
        const response = await fetch(url.toString(), request)

        return new HTMLRewriter()
          .on('link[rel="preload"][as="script"]', new removeElement())
          .transform(response)
      }

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

  // Otherwise, process request as normal
  return fetch(request)
  
}

class removeElement {
  element(element) {
    element.remove();
  }
}

addEventListener('fetch', event => {

  console.log(event.request.url);

  event.respondWith(handleRequest(event.request))
});