async function wait_for_performance_entries(url) {
  let entries = performance.getEntriesByName(url);
  if (entries.length > 0) {
    return entries;
  }
  return new Promise((resolve) => {
    new PerformanceObserver((list) => {
      const entries = list.getEntriesByName(url);
      if (entries.length > 0) {
        resolve(entries);
      }
    }).observe({ entryTypes: ['resource'] });
  });
}

self.addEventListener('activate', event => {
    event.waitUntil(self.registration.navigationPreload.enable());
  });

self.addEventListener('fetch', event => {
    let headers;
    event.respondWith(
      event.preloadResponse
          .then(response => {
            headers = response.headers;
            return response.text()
          })
          .then(_ => wait_for_performance_entries(event.request.url))
          .then(entries =>
            new Response(
              JSON.stringify({
                decodedBodySize: headers.get('X-Decoded-Body-Size'),
                encodedBodySize: headers.get('X-Encoded-Body-Size'),
                timingEntries: entries
              }),
              {headers: {'Content-Type': 'text/html'}})));
  });