It raised an eyebrow to see a dependency on react-router-dom in the server/handlerRender.js file, since it's a client library being utilized on the server.

import { matchPath } from 'react-router-dom';

From Michael Jackson's guide you referenced: https://reacttraining.com/react-router/web/guides/server-rendering, there's a /server folder which might be more appropriate. As a code reviewer I'd be less concerned about the latter than the former..

import ReactDOMServer from 'react-dom/server'