Angular Web Framework RxJS & Observables 4

Question 1

What is the correct way to multicast an Observable to multiple subscribers using a single execution in RxJS?

Accepted Answer

Use the share() operator

Answer

share() wraps the source in a Subject and multicasts it so all subscribers share one underlying execution rather than triggering separate ones.

Question 2

Which operator is best suited for sequential HTTP requests where the order of results must match the order of source emissions?

Accepted Answer

concatMap

Answer

concatMap subscribes to each inner Observable only after the previous one completes, preserving order at the cost of parallelism.

Question 3

What does `BehaviorSubject` require that a regular `Subject` does not?

Accepted Answer

An initial value provided at construction time

Answer

BehaviorSubject must be initialized with a seed value and always emits the most recent value to new subscribers immediately upon subscription.

Question 4

Which RxJS operator emits only the first value that passes a predicate, then completes?

Accepted Answer

first

Answer

first() emits the first value matching an optional predicate and immediately completes the stream.

Question 5

How does `mergeMap` differ from `concatMap` when handling multiple concurrent inner Observables?

Accepted Answer

mergeMap subscribes to all inner Observables simultaneously; concatMap queues them

Answer

mergeMap subscribes to every inner Observable immediately without waiting, allowing concurrent execution unlike concatMap's sequential approach.

Angular Web Framework Practice Test