What to prepare in order to answer "how do you make sure your JS code is right"

All:

I am pretty new to JS testing which is pain in ASS during most of my JOB interview. As beginner JS developer, I have not got chance to take training for testing, however I got asked a lot of time with questions like:

"How do you test your code?"

"What tool do you use when you debug you code?"

"How do you prove your code is right?" ......

I realize they probably mean JS testing, but I do not know how to answer and prepare this area, I am wondering if any experienced could help with certain roadmap of learning JS testing? It does not have to be systematic, just ok for handling beginner common JS tasks and interview, that is it.

Thanks