[dogweather]

I don't design my APIs that way unless the language lets me write each (addClass) version as a separate function 'head'. I.e., Elixir and Haskell.

For the other 99% of programming languages, that kind of interface makes the addClass implementation too complicated and forces unneeded branching into it. Consider: when the app developer is calling addClass, they _know_ which interface variation they're using. So they can easy write e.g. addClasses instead of addClass ... completely removing the branching from the code altogether.

Your input and output types are much simpler and static analysis is much easier as well.