chromium/third_party/blink/web_tests/http/tests/inspector-protocol/fetch/resources/service-worker-with-fetch.html

<script>
  navigator.serviceWorker.register('service-worker-with-fetch.js');
  navigator.serviceWorker.addEventListener('message', event => {
    console.log('main got message', event.data);
  });
  navigator.serviceWorker.ready.then(registration => {
    registration.active.postMessage('init');
  });
</script>