I'm trying to figure out how to dynamically generate sitemap in reactJS server side (express) web app. I'm using react router.
For simple ReactJS sitemap integration with React Router, you should check out the react-router-sitemap package.
It's fairly configurable as well, as you can see in the docs, so if you need to include change frequency or things like that, you should be able to do it no problem.
Or were you looking for a way to implement one yourself?