You can't really write UDF in javascript, it's nothing like the VBA integration. (and on a personal note, I still prefer VBA over javascript)

UDFs aren't the only reason VBA is used, though if that's what you're interested in it looks like Microsoft are planning something new in this area:

http://stackoverflow.com/a/40727011

Also, if you don't mind using 3rd party solutions, you've got other options:

http://fcell.io/

http://xlloop.sourceforge.net/

The big difference with VBA and javascript / third party solution is that I can send a xlsm spreadsheet to a colleague or another company and he will be able to run it just as well as me, I don't need to spend the next 5 years doing the support of how to reinstall the addin that is required to make it run.

Yes, that's why I said "if you don't mind using 3rd party solutions". If 3rd party solutions aren't suitable in your workplace (some workplaces would only need the extra features in internal-only documents, meaning the add-in installation could be managed by a single IT department rather than by individual users) then the Microsoft solution that's being planned is more relevant.

Also a somewhat limited version of SQL.