Building React with webpack 4 and a node module that uses threads.js

I'm trying to build a react website which contains a custom built node module, and I want to make use of worker threads in this node module.

I've added the react plugin to the react webpack config: plugins: [new ThreadsPlugin()] with const ThreadsPlugin = require('threads-plugin'); at the top.

Ive followed the examples and I when it comes to create a worker I get

Refused to execute script from 'http://localhost:3000/static/js/add.js' because its MIME type ('text/html') is not executable.

I'm guessing this is because it's losing the add.js file location within the webpack as when you navigate http://localhost:3000/static/js/add.js it returns a 404 web error, but I don't know how to correct for this.