Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is RxJS?
RxJS is a library for reactive programming using Observables, making it easier to compose asynchronous or callback-based code.
Example:
const observable = new Observable(observer => { observer.next('Hello'); });
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Ques 2. Explain what Observables are in RxJS.
Observables are data streams that emit a sequence of values over time. They can be subscribed to, and observers can react to emitted values.
Example:
const observable = new Observable(observer => { observer.next('First'); observer.next('Second'); });
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Ques 3. What is an Observer in RxJS?
An Observer is an interface that defines methods to handle the next, error, and complete events emitted by an Observable.
Example:
const observer = { next: value => console.log(value), error: err => console.error(err), complete: () => console.log('Completed') };
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Ques 4. What is the purpose of the 'pluck' operator in RxJS?
'pluck' is used to extract a specified property from each emitted object in an Observable stream.
Example:
const pluckedObservable = sourceObservable.pipe(pluck('name'));
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Ques 5. What is the purpose of the 'distinctUntilChanged' operator in RxJS?
'distinctUntilChanged' filters out consecutive duplicate values emitted by an Observable, only allowing distinct values to be emitted.
Example:
const distinctObservable = observable.pipe(distinctUntilChanged());
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Ques 6. Explain the use of the 'interval' function in RxJS.
'interval' creates an Observable that emits sequential numbers at a specified interval, creating a timer-like behavior.
Example:
const intervalObservable = interval(1000);
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Ques 7. What is the purpose of the 'take' operator in RxJS?
'take' is used to emit only the first n values emitted by an Observable and then complete.
Example:
const takenObservable = observable.pipe(take(3));
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Intermediate / 1 to 5 years experienced level questions & answers
Ques 8. Explain the concept of operators in RxJS.
Operators are functions that can be applied to Observables to manipulate, transform, and filter the emitted values.
Example:
const modifiedObservable = observable.pipe(map(value => value.toUpperCase()));
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Ques 9. What is the purpose of the 'map' operator in RxJS?
The 'map' operator transforms each value emitted by an Observable by applying a provided function to it.
Example:
const modifiedObservable = observable.pipe(map(value => value * 2));
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Ques 10. Explain the difference between 'map' and 'mergeMap' operators in RxJS.
'map' is used to transform values emitted by an Observable, while 'mergeMap' is used to project each value to an Observable and flatten the resulting Observables into one.
Example:
const modifiedObservable = observable.pipe(mergeMap(value => of(value, value * 2)));
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Ques 11. What is the purpose of the 'filter' operator in RxJS?
The 'filter' operator is used to selectively emit values from an Observable based on a provided predicate function.
Example:
const filteredObservable = observable.pipe(filter(value => value > 5));
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Ques 12. Explain the concept of 'rxjs/operators' and its significance.
'rxjs/operators' is a module in RxJS that provides a collection of operators. These operators are used for transforming, filtering, and combining Observables to perform various operations.
Example:
import { map, filter } from 'rxjs/operators'; const modifiedObservable = observable.pipe(map(value => value * 2), filter(value => value > 5));
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Ques 13. What is the purpose of the 'switchMap' operator in RxJS?
'switchMap' is used to transform each value emitted by an Observable into a new Observable and switch to emitting values from the latest Observable, ignoring previous ones.
Example:
const switchedObservable = observable.pipe(switchMap(value => of(value, value * 2)));
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Ques 14. Explain the concept of 'Subjects' in RxJS.
Subjects are both Observers and Observables. They can multicast values to multiple Observers, making them useful for scenarios where multiple subscribers need to receive the same values.
Example:
const subject = new Subject(); subject.next('First value'); subject.subscribe(value => console.log(value));
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Ques 15. Explain the difference between 'concatMap' and 'mergeMap' operators in RxJS.
'concatMap' concatenates the emissions of mapped Observables in the order they were emitted, while 'mergeMap' merges them as soon as they arrive, possibly interleaving values.
Example:
const concatenatedObservable = observable.pipe(concatMap(value => of(value, value * 2)));
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Ques 16. How does error handling work in RxJS Observables?
Errors in Observables can be handled using the 'catchError' operator, which allows you to catch and handle errors within the Observable stream.
Example:
const errorHandledObservable = observable.pipe(catchError(err => of('Error handled:', err)));
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Ques 17. What is the purpose of the 'zip' operator in RxJS?
'zip' is used to combine multiple Observables by emitting values in a strict sequence, combining corresponding values from each Observable.
Example:
const zippedObservable = zip(observable1, observable2, (value1, value2) => value1 + value2);
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Ques 18. What is the purpose of the 'bufferTime' operator in RxJS?
'bufferTime' is used to collect values emitted by an Observable over a specified time period into arrays and emit these arrays at regular intervals.
Example:
const bufferedObservable = observable.pipe(bufferTime(1000));
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Ques 19. Explain the concept of 'throttleTime' in RxJS.
'throttleTime' emits a value from the source Observable, then ignores subsequent values for a specified duration, ensuring a minimum time between emissions.
Example:
const throttledObservable = inputObservable.pipe(throttleTime(300));
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Ques 20. What is the purpose of the 'combineLatest' operator in RxJS?
'combineLatest' combines the latest values from multiple Observables into an array or using a provided function, emitting a new value whenever any of the combined Observables emit.
Example:
const combinedObservable = combineLatest(observable1, observable2, (value1, value2) => value1 + value2);
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Ques 21. Explain the concept of 'defer' in RxJS.
'defer' is used to create an Observable only when a subscriber subscribes, ensuring that each subscriber gets a fresh Observable instance.
Example:
const deferredObservable = defer(() => of('Deferred value'));
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Experienced / Expert level questions & answers
Ques 22. Explain the concept of multicasting in RxJS.
Multicasting is the process of broadcasting a single source Observable to multiple subscribers, preventing redundant work for shared operations.
Example:
const subject = new Subject(); observable.subscribe(subject);
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Ques 23. What is the difference between 'Cold' and 'Hot' Observables?
'Cold' Observables start producing values only when a subscription is made, while 'Hot' Observables produce values even before any subscriptions.
Example:
Cold: const coldObservable = new Observable(...); Hot: const hotObservable = new Subject();
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Ques 24. Explain the purpose of the 'debounceTime' operator in RxJS.
The 'debounceTime' operator is used to emit a value from an Observable only after a specified amount of time has passed without any new values being emitted.
Example:
const debouncedObservable = inputObservable.pipe(debounceTime(300));
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Ques 25. Explain the concept of 'Schedulers' in RxJS.
Schedulers are used to control the execution context and timing of Observable operations, allowing you to control when and where certain code is executed.
Example:
const scheduledObservable = observable.pipe(delay(1000, asyncScheduler));
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Ques 26. What is the purpose of the 'finalize' operator in RxJS?
'finalize' is used to perform a specified action when an Observable completes, whether it completes normally or due to an error.
Example:
const finalizedObservable = observable.pipe(finalize(() => console.log('Observable completed')));
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Ques 27. Explain the concept of 'ReplaySubjects' in RxJS.
ReplaySubjects are similar to Subjects but can 'replay' a specified number of previously emitted values to new subscribers, ensuring they receive a specific history of values.
Example:
const replaySubject = new ReplaySubject(2); replaySubject.next('First value'); replaySubject.next('Second value'); replaySubject.subscribe(value => console.log(value));
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Ques 28. Explain the difference between 'BehaviorSubjects' and 'ReplaySubjects' in RxJS.
BehaviorSubjects always emit the latest value to new subscribers, while ReplaySubjects can replay a specified number of previous values to new subscribers.
Example:
const behaviorSubject = new BehaviorSubject('Initial value'); behaviorSubject.next('Updated value'); behaviorSubject.subscribe(value => console.log(value));
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Ques 29. What is the purpose of the 'debounce' operator in RxJS?
'debounce' is used to discard values emitted by an Observable that follow each other too quickly, allowing only the last value within a specified time window to be emitted.
Example:
const debouncedObservable = inputObservable.pipe(debounce(() => timer(300)));
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