chrome/test/data/workers/fetch_from_worker.js

// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

self.onmessage = async e => {
  const response = await fetch(e.data.url);
  if (!response.ok) {
    self.postMessage('bad response');
    return;
  }
  const text = await response.text();
  self.postMessage(text);
};