As a developer, doesn'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.

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'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'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.

In my opinion, this is pretty much the only category that makes JavaScript "Best-in-class"; It's the easiest language to build isomorphic web apps.