"if all you need is a mark, the Python convention to add an underscore in front of your private method or variable works just fine, as does the convention to make constants uppercased. I've seen quite a bit of JS code use these conventions too."

Until you have a user subclass your class and accidentally stomp over one of your private names, causing your class to mysteriously fail. I've been bitten by this before, and it'd be nice for JS to offer a solution.