Initial phone screen. Expect to answer a few questions about basic JS, and whatever framework (Angular/React) you may be interviewing for. Understand WebSockets and Observables, how those two can interact.
Next is a code test and architecture questions. You will wire up a fake subscription service in whatever framework you are working in - coding in a browser tool. Do it step by step and share your thoughts, very friendly interview team. Should be able to - accept input to subscribe to somthing, update the dom with subscription updates. The fake service is a little tricky, so check out the payload values and ask explicitly what you should be looking at.
Architecture questions are pretty standard - just start simple and be able to say what's wrong with a simple solution and how to account for that. If you paint yourself in a corner, just share that you've noticed it. Very friendly team.
Check out react-rxjs for bonus points