Closures, event bubbling, event loop: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Even...

Basically all the things you otherwise forget about or never need to worry about when writing production JavaScript. Yawn. I sound like a curmudgeon and I am: too many interviews I've been in that involved JavaScript felt like abstracted garbage. Nevertheless, good luck!

have a really good way of talking through how you handle async code and the difference between async and procedural code. Plus, how you can make async code read like procedural (async/await). I always give bonus points if you can describe clearly the difference between using ansync/await and then based promise code (not just saying "syntactic sugar", but actually explaining how the code looks different but yet how they are effectively the same).

How people answer questions around promises in general often gives me a good idea of where they are at in their JavaScript journey.