Question 1

Why is RxJS central to Angular?

Accepted Answer

HTTP, forms valueChanges, router events, and many APIs expose Observables for async streams. RxJS models push-based async data with operators for transform/cancel/combine. You must manage subscriptions. Signals increasingly coexist for local state.

Question 2

What is the difference between switchMap, mergeMap, concatMap, and exhaustMap?

Accepted Answer

They flatten inner Observables differently: switch cancels, merge concurrent, concat queues, exhaust ignores new until done. Use switchMap for typeahead/HTTP that should cancel. concatMap for ordered writes. mergeMap for independent parallel work. exhaustMap for ignoring double-submits.

Question 3

How do you avoid memory leaks with Observables in components?

Accepted Answer

Unsubscribe via takeUntilDestroyed, async pipe, or firstValueFrom for one-shots. Manual subscribe without teardown leaks. async pipe auto-unsubscribes. Prefer declarative pipelines in services.

Question 4

What does shareReplay do and when is it risky?

Accepted Answer

Multicasts and replays last N values to late subscribers; can retain memory and keep sources alive. Use for caching HTTP among subscribers. Configure refCount carefully. Don't shareReplay(1) forever on huge payloads without strategy.

Question 5

Debug: a valueChanges subscription fires too often. What do you check?

Accepted Answer

Look for distinctUntilChanged, debounceTime, and whether patchValue emits; also OnPush/parent CD loops. Filter empty emissions, compare with distinctUntilChanged, debounce user input. Avoid writing back into the form inside the same stream without care.

Question 6

How would you implement a cancellable search-as-you-type with RxJS?

Accepted Answer

Listen to input events, debounceTime, distinctUntilChanged, switchMap to HTTP. switchMap cancels in-flight requests when the query changes. Handle errors with catchError per inner observable so the outer stream survives.

Question 7

When should a service expose Observable vs signal?

Accepted Answer

Observables for streams/events over time; signals for current synchronous state in the view model. You can bridge with toSignal/toObservable. Don't force everything into one model—use the right abstraction.

RxJS Angular Interview Questions

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