As far as I know google has standard procedure for hiring software engineers regardless of of front end or backend, phone screenings followed by on-site interviews. You are supposed to have good data structure and algorithmic knowledge. I would suggest to go with data structure and algorithms, also System Design is very important and your past projects ( if you are experienced one ). Try to get as much information as possible from your recruiter about your interview that will surely help.
I assumed this was a common enough subject of blog posts and Reddit discussions that a Google search should answer your question. Maybe not though. Too sleepy to check right now, but best of luck!
All I can think of is that there are books written about the logic puzzles and algorithms Google uses in their interviews, but take that with a grain of salt.
All of the above. You should be ready to answer standard ds/alg whiteboard questions, have a strong grasp of JavaScript, and be ready to answer system design questions related to client side development.
https://github.com/jwasham/coding-interview-university