For server side code you can add plugins and/or hook up events where your code writes or changes runnable files (ASPX/ASCX) to paths inside the webapp.

This is very uncommon and depending on how you set it up could be very dangerous and cause frustration when deploying new code.

For javascript you can just add content type string properties that you edit through a plain textarea in Edit mode, this is quite common and typically used for stuff such as campaign tracking and banner ad scripts etc.

It's also common to add a Dynamic content plugin with a textarea that allows any type of ugly client side code (typically embeddable widgets or video etc).

Thanks for the reply, it helped me a lot!

Then it feels like Javascript is absolutely okay to code in a textarea in Edit mode in my situation, I need to do a small validation of a field and have very limited access to the backend.

While server-side code should be left to be done in the backend