1.js

async function mergeResponses(responsePromises) {

  const headers = new Headers();
  headers.append("Content-Type", "text/html");
  const { readable, writable } = new TransformStream();
  const response = new Response(readable, {
    headers: { "Content-Type": "text/html" },
  });

  const done = (async function() {
    for await (const response of responsePromises) {
      await response.body.pipeTo(writable, { preventClose: true });
    }
    writable.getWriter().close();
  })();

  return { done, response };
}