What's your definition of "must learn?" Personally, I believe that to study JavaScript is also essentially to study web anthropology—it's incredibly useful for understanding the tech culture of today, regardless of whether you personally program for the web.
As a language that started client-side and expanded far beyond that, JS has become uniquely accessible to both sides of the experience graph as it's become more complex and multi-paradigm. Largely owing to its user base containing so many semi-technical professionals, the language has evolved specifically to provide simple APIs for all the most common paradigms.
Simultaneously, that is its biggest pitfall, from my perspective as someone who spends a ton of time mentoring newbies: it's so multi-paradigm that it's easy for an unexperienced developer to accidentally code using the worst features of each paradigm simultaneously, instead of the best.
On top of all that, the JS open source community is particularly diverse and interesting, what with the incredible versatility of the language.
So, yeah, just wanted to call attention to the fact that there's a ton to be learned from JS that doesn't even necessarily have to do with using it to code a thing. Whether that's relevant to your needs is your call. ;)