background-fetch-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 fetch in SW</title>

<body>
  <script type="module">
      const [background_fetch_register_key, method] = parseKeylist();

      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);
          navigator.serviceWorker.onmessage = e => {
            resolve(e.data);
          }
        });
      };

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

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

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