For JS, from GSG: "Filenames should be all lowercase in order to avoid confusion on case-sensitive platforms. Filenames should end in .js, and should contain no punctuation except for - or _ (prefer - to _)." 

I generally see '-' in most people's libraries in Node and client-side JS, so I'd go with that (as it's in-line with the rest of the project as it is already, as well).