I think you are nearly there. All that seems to have happened is that a urlDecode step has been accidentally missed out.
JS should certainly be supported:
- there are a lot of things that it can do much more straightforwardly and more easily than AppleScript (much easier records, richer libraries, including basics like regular expressions and url encoding/decoding etc).
- The centre of the apple’s gravity is shifting inexorably towards iOS, on which there is no AppleScript at all, but plenty of JS
- JS is the language of web pages – a more generally useful language than AppleScript for users to learn.