If you're interested in doing it with React, here's a video explaining how to do a custom backend: https://www.youtube.com/watch?v=eNC0mRYGWgc

You basically need to change BackendIDOperations and ReactEventListener to send/receive messages vs talking to the real APIs. It's already set up in the codebase to do this (because it actually worked at one time).

Webpack, per usual, already thought of this and has an answer for you :) https://github.com/webpack/worker-loader