MEAN RxJS and Reactive Programming

Question 1

What does the RxJS `switchMap` operator do when a new value arrives while a previous inner Observable is still active?

Accepted Answer

It cancels the previous inner Observable and subscribes to the new one

Answer

switchMap cancels (unsubscribes from) any in-flight inner Observable when a new source value arrives, then subscribes to the new inner Observable. This makes it ideal for HTTP search requests where only the latest result matters.

Question 2

Which RxJS creation function converts a JavaScript Promise into an Observable?

Accepted Answer

from()

Answer

from() can convert various iterable structures into Observables, including Promises. When given a Promise, it wraps it so it emits the resolved value and then completes, or errors if the Promise rejects.

Question 3

What is the key difference between a `Subject` and a `BehaviorSubject` in RxJS?

Accepted Answer

A BehaviorSubject requires an initial value and emits it immediately to new subscribers

Answer

A BehaviorSubject holds and replays its current value to any new subscriber immediately upon subscription. It requires an initial value at construction. A plain Subject emits nothing to late subscribers — they only receive future values.

Question 4

Which RxJS operator should you use to handle errors inside an Observable pipeline without terminating the stream?

Accepted Answer

catchError()

Answer

catchError() intercepts an error in the Observable chain, allowing you to return a fallback Observable (e.g., of([]) or EMPTY) so the stream continues or completes gracefully rather than propagating the error to the subscriber.

Question 5

In Angular, what is the correct way to prevent memory leaks when subscribing to an Observable inside a component?

Accepted Answer

Always use the async pipe in templates or unsubscribe in ngOnDestroy

Answer

Long-lived Observables (like those from services) do not automatically unsubscribe when a component is destroyed. You must either use the async pipe (which handles unsubscription automatically) or manually call unsubscribe() in ngOnDestroy, often via a takeUntil pattern.

MEAN - Stack Development Practice Test

MEAN - Stack Development Practice Test

MEAN RxJS and Reactive Programming