Let's check this "forever", and let me add two cents:
- adding trailing comma in such cases is just a solution that is less error-prone, you never end up with lack thereof when adding/copypasting elements in an array,
- I'd say we (Wikia programmers) tend more often to put this trailing comma, and if a best practice was created based on what we already do I guess it would be a part of such list to put these trailing commas in similar places.