Well if he wanted code navigation for JavaScript he could have used tern. It also integrates nicely with company for auto-completion.
Or tide, which uses the typescript tsserver-backend for all its code-analysis.
Gives you navigation, refactoring, completion, eldoc etc. Basicslly JS on steroids.
cscope may be irrelevant, but ctags/etags has support for JS (and other languages that would be relevant for web developers, like HTML, PHP, Python, Ruby, SQL, ...).