worker.js

// DedicatedWorkerGlobalScope
let port;
onmessage = async (e) => {
  "use strict";
  if (!port) {
    [port] = e.ports;
    port.onmessage = (event) => postMessage(event.data);
  }
  const { urls } = e.data;
  // https://github.com/whatwg/streams/blob/master/transferable-streams-explainer.md
  const { readable, writable } = new TransformStream();
  (async (_) => {
    for await (
      const _ of (async function* stream() {
        while (urls.length) {
          yield (await fetch(urls.shift(), { cache: "no-store" })).body.pipeTo(
            writable,
            {
              preventClose: !!urls.length,
            },
          );
        }
      })()
    );
  })();
  port.postMessage(
    {
      readable,
    },
    [readable],
  );
};