Service worker stream transferring

1-service-worker.js

"use strict";

const worker = new Worker("worker.js");

self.onfetch = e => {
  const transform = new TransformStream(); // creates an identity transform
  e.respondWith(new Response(transform.readable));
  
  // Give the worker the writable end. An identity transform stream will just shuffle
  // bytes written there into transform.readable.
  worker.postMessage(transform.writable, [transform.writable]);
  
  // transform.writable has now been transferred/neutered/detached.
  // This means that transform.writable.getWriter() will never work anymore in this thread.
  // However, the *creator* of transform.writable (i.e., the logic inside
  // new TransformStream()) can still see data that is written to it. That logic
  // will then shuffle it over to transform.readable.
};

2-worker.js

"use strict";

self.onmessage = ({ data: writableStream }) => {
  const writer = writableStream.getWriter();
  writer.write(new Uint8Array([1, 2, 3, 4]));
  writer.close();
  
  // You can, of course, do the writing/closing asynchronously.
};