@gruz I'm in the process of updating Geany's JavaScript parser to current U-Ctags one. did you check if U-CTgas gives better results?
This said, unfortunately the dynamic nature of JavaScript makes it virtually impossible to get perfect symbols statically, the only way to really get perfect info would be to actually run the code (e.g. in case some code is dynamically extending classes or so). However yes, we can try and improve the results still.