Method PathToFiletypeCompleterPluginLoader searches for completers for a given filetype by their folder names. This way you can't have several completers handling one filetype.

I discovered this while I was looking for ways to apply the TypeScript completer for javascript files abandoning Tern altogether (it does not support class parameters).

Since each completer has method SupportedFiletypes, I see it as a better place to test if a completer can be used for a filetype. I would have to extend [ 'typescript' ] to ['typescript', 'javascript']. Of course, you wouldn't want to instantiate each completer to do this check. It makes more sense to make this method static.