chromium/third_party/blink/web_tests/external/wpt/fenced-frame/resources/background-sync-sw-inner.https.html

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="utils.js"></script>
<title>Fenced frame content to report the result of background sync's register in SW</title>

<body>
<script type="module">
  const [background_sync_register_key] = parseKeylist();
  const searchParams = new URL(location.href).searchParams;
  const method = searchParams.get('method');
  const isPeriodic = searchParams.get('periodic');

  const getController = () => {
    if (navigator.serviceWorker.controller) {
      return navigator.serviceWorker.controller;
    }
    return new Promise(resolve => {
      navigator.serviceWorker.addEventListener('controllerchange', () => {
        resolve(navigator.serviceWorker.controller);
      });
    });
  };

  const sendMessageToServiceWorker = async () => {
    const ctrl = await getController();
    return new Promise(resolve => {
      ctrl.postMessage({method, isPeriodic});
      navigator.serviceWorker.onmessage = e => {
        resolve(e.data);
      }
    });
  };

  await navigator.serviceWorker.register(
    "background-sync-sw.js", { scope: location.href, type: "module" });
  const data = await sendMessageToServiceWorker();

  writeValueToServer(background_sync_register_key, data);
</script>
</body>