I have developed javascript applications for a while now and I have never had to use prototype. If I was asked about it in an interview, I would explain that it's a leaky abstraction, that using it in code means you are over-complicating your solution, and that it seems to be a holdover from when people thought your codebase should be modeled according to reality ("we have users, therefore we should have a user model").

You can absolutely be a great javascript developer without ever touching prototype. Prototype is an infection of object-oriented programming into a functional language.