<!doctype html>
<script>
const baseLocation = window.location;
const parentWorkerScript = `
  const childWorkerScript = 'self.onmessage = async (e) => {' +
    '  const response = await fetch(e.data);' +
    '  const text = await response.text();' +
    '  self.postMessage(text);' +
    '};';
  const blob = new Blob([childWorkerScript], { type: 'text/javascript' });
  const blobUrl = URL.createObjectURL(blob);
  const childWorker = new Worker(blobUrl);

  // When a message comes from the parent frame, sends a resource url to the
  // child worker.
  self.onmessage = (e) => {
    childWorker.postMessage(e.data);
  };
  // When a message comes from the child worker, sends a content of fetch() to
  // the parent frame.
  childWorker.onmessage = (e) => {
    self.postMessage(e.data);
  };
`;
const blob = new Blob([parentWorkerScript], { type: 'text/javascript' });
const blobUrl = URL.createObjectURL(blob);
const worker = new Worker(blobUrl);

function fetch_in_worker(url) {
  const resourceUrl = new URL(url, baseLocation).href;
  return new Promise((resolve) => {
    worker.onmessage = (event) => {
      resolve(event.data);
    };
    worker.postMessage(resourceUrl);
  });
}
</script>