https://blog.makotoishida.com/2018/11/create-react-appweb-worker.html

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';

import createWorker from './createWorker';
import myWorkerFunc from './myWorker';

ReactDOM.render(<App />, document.getElementById('root'));

// Testing Web Worder
const myWorker = createWorker(myWorkerFunc);
myWorker.addEventListener('message', (m: any) => {
  console.log('msg', m.data.msg);
  if (m.data.count >= 10) {
    console.log('clearing');
    myWorker.postMessage({ action: 'clear' });
  }
});
setInterval(() => {
  myWorker.postMessage({ action: 'count' });
}, 400);