As a developer, doesn&#x27;t it make you _itch_ when you repeat yourself? Do you feel that itch when you repeat the same application logic on the server as you do on the client? Getting the client-side validation in-sync with the server validation? This is where JavaScript is fan-fucking-tastic.<p>All of my recent projects have been isomorphic react apps. I build my app like it were a fully client-side single page web app, but I get server rendering when there&#x27;s a new http request. Once the page has rendered, the web app takes over and uses history pushState. There are very few pieces of code that aren&#x27;t used in both places. Once you get the hang of it, you very naturally create mockable interfaces that can be swapped depending on environment.<p>In my opinion, this is pretty much the only category that makes JavaScript &quot;Best-in-class&quot;; It&#x27;s the easiest language to build isomorphic web apps.