Parent didn't specify, but usually on the front-end. I would disagree that it's all that important, certainly not more important than being able to lay out a page and style it without it turning into a mess. I've cleaned up crappy CSS work before, the only real solution is to throw it out and start fresh.

At my company where I'm the back-end guy, our front-end guy does most of the Javascript, but he has to get my help whenever he has to do anything non-trivial. Only a few projects a year ever has him needing my assistance.

Most companies' Javascript needs should be similar. Every once in awhile there will be a marketing push wih non-trivial Javascript needs, most of the time they can get by with basic jQuery. Hide this element, show it when there's an event.

Unless you're working for a news agency, making Snowfall every month, I'd be surprised if the business actually did need more. Of course, if the front-end guy decides he has to learn React, well, I'd say, go ahead, knock yourself out. But don't kid yourself into believing it's really necessary.