Really? I've never met a designer that wrote js. Engineers should be writing the software. Js web apps are essentially the same as c++ win32 ( or whatever thick client side tech ) apps with some web semantics like urls preserved. I've seen designers write css, and I think the good ones do, but I've never met one that was happy to or was particularly skilled at it.
There is a recent inclination among designers to write JS and take part in writing front-end code. Youtube is full of their talks.
JS/Hypertext was not initially meant for engineering anyting, so it's not analogous to a combination of UI toolkit & proper programming language & a compiler.
