Angular 20 Interview Questions and Answers (2025) – Part 2: RxJS, Change Detection & Performance

In Part 1, we covered TypeScript and Angular Core Concepts (Q1–Q50).

Now in Part 2 of Angular 20 Interview Questions and Answers (2025 Edition), we’ll explore:

• RxJS Operators & Advanced Usage (Q51–Q90)

* Angular Change Detection & Performance Optimization (Q91–Q100)

RxJS Questions (Q51–Q90)

Q51. Difference between switchMap, mergeMap, concatMap, and exhaustMap?

• switchMap → cancels previous observable, subscribes to latest.
• mergeMap → runs all observables concurrently.
• concatMap → executes one after another in order.
• exhaustMap → ignores new until current completes.

Q52. What is the difference between forkJoin and combineLatest?

• forkJoin → waits for all observables to complete, emits once.
• combineLatest → emits whenever any observable emits (after all have emitted at least once).

Q53. How do you create an observable in Angular?

• Using new Observable(), creation operators (of, from, interval), or Angular HttpClient.

Q54. What is takeUntil in RxJS?

• Completes an observable when another observable emits (used for unsubscribing).

Q55. How to cancel HTTP requests in Angular using RxJS?

• Use takeUntil with Subject.
• Use AbortSignal (Angular 16+).

Q56. What is shareReplay and why is it useful?

• Replays the last emitted values to new subscribers.
• Used for caching HTTP requests.

Q57. What is a Subject in RxJS?

• Both observer and observable.
• Multicasts to multiple subscribers.

Q58. Difference between BehaviorSubject, ReplaySubject, and AsyncSubject?

• BehaviorSubject → stores last value.
• ReplaySubject → replays all/limited values.
• AsyncSubject → emits last value on completion.

Q59. What is the difference between hot and cold observables?

• Cold → new execution for each subscriber.
• Hot → shared execution across subscribers.

Q60. How do you debug RxJS streams in Angular?

• Use tap(), RxJS DevTools, or console logging.

Q61. What is the difference between map and switchMap?

• map → transforms emitted values.
• switchMap → transforms and flattens inner observable.

Q62. What is debounceTime and where is it useful?

• Ignores values until a set time passes without new emissions.
• Used in search/autocomplete inputs.

Q63. What is distinctUntilChanged?

• Prevents emitting duplicate consecutive values.

Q64. What is filter in RxJS?

• Emits values that pass a condition.

Q65. What is tap operator used for?

• Performs side effects (logging, debugging) without altering values.

Q66. How is startWith used in observables?

• Emits an initial value before other emissions.

Q67. What is catchError in RxJS?

• Catches errors and returns a fallback observable.

Q68. What is retry and retryWhen?

• retry(n) → retries n times on error.
• retryWhen → retries based on custom logic.

Q69. What is the difference between from and of?

• of(1,2,3) → emits values as sequence.
• from([1,2,3]) → emits each array element separately.

Q70. What is the difference between interval and timer?

• interval(ms) → emits numbers repeatedly.
• timer(ms) → emits once or starts interval after delay.

Q71. What is merge in RxJS?

• Combines multiple observables, emits as they come.

Q72. What is concat in RxJS?

• Runs observables one after another.

Q73. What is zip in RxJS?

• Combines values from observables into tuples, emits when all have emitted.

Q74. What is combineLatestWith?

• Variant of combineLatest for cleaner chaining.

Q75. What is withLatestFrom?

• Combines latest value from one observable with another emission.

Q76. How do you handle memory leaks in RxJS?

• Use takeUntil, async pipe, or Subscription.unsubscribe().

Q77. What is the async pipe in Angular?

• Subscribes and unsubscribes automatically.

Q78. What is multicast in RxJS?

• Shares a single subscription among multiple observers.

Q79. What is the difference between publish and share?

• publish → returns a ConnectableObservable.
• share → shorthand for multicast + refCount.

Q80. What is a higher-order observable?

• Observable emitting other observables. Flattened using switchMap, etc.

Q81. What is windowTime vs bufferTime?

• bufferTime → collects values, emits arrays.
• windowTime → collects into inner observables.

Q82. What is throttleTime vs debounceTime?

• throttleTime → emits first value, ignores others for duration.
• debounceTime → waits until silence period ends.

Q83. What is auditTime in RxJS?

• Ignores values until duration ends, then emits last value.

Q84. What is the purpose of finalize operator?

• Executes logic when observable completes/errors/unsubscribes.

Q85. What is pairwise operator?

• Emits previous and current values as pairs.

Q86. What is scan vs reduce in RxJS?

• scan → accumulates values continuously.
• reduce → emits only final accumulated value.

Q87. What is pluck in RxJS?

• Extracts a property from objects in a stream.

Q88. What is switchAll operator?

• Flattens inner observables, similar to switchMap.

Q89. What is the difference between concatAll, mergeAll, and switchAll?

• concatAll → sequential.
• mergeAll → parallel.
• switchAll → latest only.

Q90. What is backpressure in RxJS and how do you handle it?

• Overwhelming emissions handled using throttleTime, debounceTime, or buffering.

---

Change Detection & Performance (Q91–Q100)

Q91. What is Angular Change Detection?

• Updates DOM when component state changes.

Q92. What are Change Detection Strategies?

• Default → checks always.
• OnPush → checks only when inputs change or observables emit.

Q93. What is NgZone in Angular?

• Patches async tasks, triggers change detection automatically.

Q94. What is runOutsideAngular used for?

• Runs code outside Angular zone to avoid unnecessary checks.

Q95. How does ChangeDetectorRef.detectChanges() work?

• Immediately runs change detection for that component subtree.

Q96. What is markForCheck() in Angular?

• Marks component for check in next cycle.

*Q97. What is trackBy in *ngFor and why use it?*

• Optimizes rendering by tracking items with a unique key.

Q98. How do you optimize Angular apps for performance?

• Use OnPush strategy, trackBy, lazy loading, async pipe, and avoid memory leaks.

Q99. What are Angular DevTools used for?

• Profiling change detection and performance bottlenecks.

Q100. What is the role of Singleton Services in performance?

• Avoids duplicate service instances, reduces memory usage.

---

Conclusion

This was Part 2 of Angular 20 Interview Questions and Answers (2025 Edition).

We covered:
RxJS Operators & Advanced Usage (Q51–Q90)
Change Detection & Performance Optimization (Q91–Q100)