Frontend
RxJS
Also written as Observables, Reactive Programming, NgRx
A library for handling streams of events over time (clicks, search-box typing, server updates) as 'observables' you can filter and combine. It is built into Angular, and NgRx is Angular's popular state-management tool built on it.
Think of it like
Plumbing for events: you route, filter and merge pipes of data instead of catching each drop by hand.
Junior or senior?
Near-universal on Angular resumes. Real depth shows in talking about operators like switchMap and about unsubscribing to avoid memory leaks — the classic RxJS mistake.
Ask them
“How did you make sure subscriptions were cleaned up so they didn't leak memory?”