The is operator is for identity, not equality. Your example is just using it weirdly in a way that most people wouldn’t do.

The + operator is for numbers or strings, not arrays. Your example is just using it weirdly in a way that most people wouldn't do.

I'm not defending Javascript's obviously terrible behaviour there. Just pointing out that Python has obviously terrible behaviours too. In both cases the solution is "don't do that, and use static analysis to make sure you don't do it accidentally".

Sometimes I meet junior developers who have only ever used javascript, and it’s like (to borrow another contentious nerd topic) like meeting someone who’s only ever played D&D talking about game design.

Yeah I think you can generalise that to "have only ever used one language". I would say Python and Javascript are pretty close on the "noob level". By which I mean if you meet someone who has only ever written C++, Java, or Rust or whatever they're going to be a class above someone who has only ever written Python or Javascript.