chromium/content/test/data/workers/create_worker.html

<script>
const params = new URLSearchParams(location.search);
const worker = new Worker(params.get('worker_url'));
const onmessage_promise = new Promise(r => worker.onmessage = r);
async function waitForMessage() {
  const message = await onmessage_promise;
  return message.data;
}
</script>