Question 1

What is RxJS and what problems does it solve?

Accepted Answer

RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using Observables. It solves complex async challenges like managing multiple event streams, handling race conditions, and composing asynchronous operations with powerful operators.

Question 2

What is the difference between an Observable and a Promise?

Accepted Answer

Observables emit multiple values over time and are lazy (only execute when subscribed), while Promises emit a single value and are eager (execute immediately). Observables are also cancellable through unsubscription, whereas Promises cannot be cancelled.

Question 3

What is the difference between cold and hot Observables?

Accepted Answer

Cold Observables create a new data producer for each subscriber and start from the beginning. Hot Observables share a single data producer among all subscribers, emitting values regardless of subscriptions—like a live broadcast.

Question 4

What is a Subject in RxJS?

Accepted Answer

A Subject is both an Observable and an Observer. It can multicast values to multiple subscribers simultaneously, acting as a bridge between non-reactive code and reactive streams. It's commonly used for event buses and state management.

Question 5

What is the difference between BehaviorSubject, ReplaySubject, and AsyncSubject?

Accepted Answer

BehaviorSubject stores the current value and emits it to new subscribers. ReplaySubject replays a specified number of previous values. AsyncSubject only emits the last value when the stream completes. Regular Subject has no memory of past values.

Question 6

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

Accepted Answer

map transforms values without flattening. mergeMap subscribes to all inner Observables concurrently. switchMap cancels previous inner subscriptions when new values arrive. concatMap waits for each inner Observable to complete before processing the next.

Question 7

When should you use switchMap over mergeMap?

Accepted Answer

Use switchMap for search/typeahead features where only the latest request matters and previous ones should be cancelled. Use mergeMap when all concurrent operations must complete, like saving multiple items simultaneously.

Question 8

What is the difference between merge, concat, and forkJoin?

Accepted Answer

merge combines Observables concurrently, emitting values as they arrive. concat waits for each Observable to complete before subscribing to the next. forkJoin waits for all Observables to complete and emits their final values as an array.

Question 9

How does combineLatest work in RxJS?

Accepted Answer

combineLatest waits for all source Observables to emit at least once, then emits an array of the latest values whenever any source emits. It's ideal for combining multiple state sources into a single derived value.

Question 10

Why is it important to unsubscribe from Observables?

Accepted Answer

Failing to unsubscribe causes memory leaks as subscriptions keep references alive and continue executing callbacks. In Angular components, unsubscribed HTTP or interval Observables can cause errors after component destruction.

Question 11

What is the takeUntil pattern for managing subscriptions?

Accepted Answer

The takeUntil pattern uses a Subject that emits on component destroy. Pipes include takeUntil(destroy$) to automatically complete subscriptions. This centralized approach is cleaner than tracking individual subscriptions manually.

Question 12

What is the catchError operator?

Accepted Answer

catchError intercepts errors in an Observable stream and returns a replacement Observable or rethrows. It allows graceful error recovery—returning cached data, default values, or user-friendly error states instead of breaking the stream.

Question 13

What are higher-order Observables?

Accepted Answer

Higher-order Observables are Observables that emit other Observables. Operators like mergeMap, switchMap, and concatMap flatten them into a single stream. They commonly occur with HTTP requests triggered by user events.

Question 14

How do you debug RxJS streams?

Accepted Answer

Use the tap operator for side-effect logging without affecting the stream. RxJS DevTools extension visualizes marble diagrams. Break complex pipes into named Observables and test operators in isolation for easier debugging.

Question 15

What are common memory leak pitfalls in RxJS?

Accepted Answer

Common causes include: forgetting to unsubscribe from long-lived Observables like intervals, not completing Subjects, subscribing in loops without cleanup, and using shareReplay without refCount. Always clean up subscriptions on component destroy.

RxJS Fundamentals(5 questions)

What is RxJS and what problems does it solve?

What is reactive programming?

What is an Observable and how does it differ from a Promise?

What is the difference between cold and hot Observables?

What is the Observer pattern in RxJS?

Observables and Subscriptions(5 questions)

How do you create an Observable in RxJS?

What is a Subscription and how do you manage it?

Why is it important to unsubscribe from Observables?

What are the different ways to unsubscribe from Observables?

What is the async pipe in Angular and how does it handle subscriptions?

Subjects(4 questions)

What is a Subject in RxJS?

What is the difference between Subject, BehaviorSubject, ReplaySubject, and AsyncSubject?

When would you use a BehaviorSubject over a regular Subject?

What is multicasting and how do Subjects enable it?

Creation Operators(4 questions)

What are creation operators in RxJS?

What is the difference between of, from, and fromEvent?

How do you create an Observable from a Promise?

What is the interval and timer operator?

Transformation Operators(5 questions)

What is the map operator and how does it work?

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

When would you use switchMap over mergeMap?

What is the filter operator?

What is the tap operator and when would you use it?

Combination Operators(4 questions)

What is the difference between merge, concat, and forkJoin?

How does combineLatest work?

What is the withLatestFrom operator?

When would you use zip vs combineLatest?

Error Handling(4 questions)

How do you handle errors in RxJS?

What is the catchError operator?

What is the difference between retry and retryWhen?

How do you implement graceful error recovery in RxJS?

Higher-Order Observables(4 questions)

What are higher-order Observables?

How does exhaustMap differ from switchMap?

What is the mergeAll operator?

When would you use concatMap for sequential processing?

Best Practices and Performance(5 questions)

What are common memory leak pitfalls in RxJS?

How do you debug RxJS streams?

What is the takeUntil pattern for managing subscriptions?

What are the best practices for organizing RxJS code?

How do you test RxJS Observables?