javascript is kind of in the middle whether it's design or development.
Development and design kinda blur together, and javascript is the means by which that blurring happens.
Designers need to know a bit to make the interface work nicely, and developers need to use it to for the client-side programming part, especially if you don't want stuff to resubmit every time you click a button.
And that's without considering node.js, a framework for server-side javascript apps