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Top 100 RxJS Interview Questions and Answers

Top 100 RxJS Interview Questions and Answers

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1. What is RxJS?

Answer:
RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using Observables, widely used in JavaScript and TypeScript applications. It enables the composition of asynchronous and event-based programs using observable sequences.


2. What is an Observable in RxJS?

Answer:
An Observable in RxJS is a representation of a stream of data or events that can be observed over time. It can emit multiple values (synchronously or asynchronously) and notify observers when new values are available.


3. How do you create an Observable in RxJS?

Answer:
There are several ways to create an Observable in RxJS:

  • Using the new Observable constructor.
  • Using creation functions like of, from, or interval.
  • Adapting existing events, promises, or callbacks using functions like fromEvent, fromPromise, or bindCallback.

Example using of:

import { of } from 'rxjs';

const observable = of('Hello', 'World');

4. What is an Observer in RxJS?

Answer:
An Observer in RxJS is an interface that defines a set of callbacks to handle different types of notifications that an Observable can send. It consists of three methods: next (handles next values), error (handles errors), and complete (handles completion of the observable).


5. How do you subscribe to an Observable in RxJS?

Answer:
To subscribe to an Observable in RxJS, you use the subscribe method. This method takes an Observer object or callback functions for handling next, error, and complete notifications.

Example:

import { of } from 'rxjs';

const observable = of('Hello', 'World');

observable.subscribe(
  value => console.log(value), // Handles next values
  error => console.error(error), // Handles errors
  () => console.log('Complete') // Handles completion
);

6. What is the purpose of Operators in RxJS?

Answer:
Operators in RxJS are functions that transform, filter, or combine Observables to create new Observables. They allow you to perform operations on the emitted values, enabling powerful data processing pipelines.

Example using the map operator:

import { of } from 'rxjs';
import { map } from 'rxjs/operators';

const observable = of(1, 2, 3);

observable.pipe(
  map(value => value * 2)
).subscribe(result => console.log(result));

7. How do you handle errors in RxJS?

Answer:
In RxJS, you can use the catchError operator to handle errors emitted by an Observable. This operator allows you to replace an error with another Observable or a default value.


8. What is the purpose of the debounceTime operator?

Answer:
The debounceTime operator is used to filter out a burst of rapidly emitted events, and only emit a value if no new event is emitted for a specified duration.


9. Explain the concept of multicasting in RxJS.

Answer:
Multicasting is the process of sharing a single subscription to an Observable among multiple subscribers. This is achieved using Subjects or operators like share or publish.


10. What is the purpose of the switchMap operator in RxJS?

Answer:
The switchMap operator is used to map each value emitted by an observable to another observable. It cancels the previous subscription and switches to a new one whenever a new value is emitted.


11. How does the mergeMap operator differ from switchMap?

Answer:
Unlike switchMap, the mergeMap operator does not cancel the previous subscription when a new value is emitted. It allows multiple inner observables to be active at the same time.


12. Explain the purpose of the combineLatest operator.

Answer:
The combineLatest operator takes multiple observables as input and whenever any of the input observables emits a value, it combines the latest values from all the observables and emits a new value.


13. What is the role of the zip operator in RxJS?

Answer:
The zip operator combines the emissions of multiple observables together. It takes one value from each input observable and emits an array of these values.


14. How can you cancel an observable in RxJS?

Answer:
You can cancel an observable by unsubscribing from it. This can be done using the unsubscribe method, which is returned when you subscribe to an observable.


15. What is the purpose of the retry operator?

Answer:
The retry operator resubscribes to the source observable if it encounters an error. You can specify the number of times it should retry or use it without any arguments for infinite retries.


16. Explain the concept of hot and cold observables.

Answer:
Cold observables start producing values when they are subscribed to, and each subscriber gets its own set of values. Hot observables, on the other hand, produce values regardless of subscriptions, and all subscribers share the same set of values.


17. How do you create a custom observable in RxJS?

Answer:
You can create a custom observable using the Observable constructor. This allows you to define the behavior of the observable, including when and how values are emitted.


18. What is the purpose of the debounceTime operator?

Answer:
The debounceTime operator is used to filter out rapid, successive emissions from an observable. It waits for a specified amount of time after the last emission before allowing the value to pass through.


19. Explain the role of the distinctUntilChanged operator.

Answer:
The distinctUntilChanged operator filters out consecutive duplicate values emitted by an observable. It only allows values that are different from the previous emitted value.


20. How does the pluck operator work in RxJS?

Answer:
The pluck operator is used to extract a specific property from the emitted objects of an observable. It takes the property name or key as an argument and emits only that property’s value.


21. What is the purpose of the share operator?

Answer:
The share operator allows multiple subscribers to share the same subscription to the source observable. This is useful when you have side effects in your observable and want to avoid redundant operations.


22. How can you handle errors in RxJS?

Answer:
You can use the catchError operator to handle errors emitted by an observable. It allows you to replace an error with another observable or value, or perform any other error handling logic.


23. Explain the concept of multicasting in RxJS.

Answer:
Multicasting in RxJS is the process of sharing a single source observable among multiple subscribers. This allows multiple subscribers to receive the same set of values emitted by the source observable.


24. What is the purpose of the finalize operator?

Answer:
The finalize operator is used to perform a specific action when the source observable completes, whether due to a successful completion or an error.


25. How do you create a custom operator in RxJS?

Answer:
You can create a custom operator by defining a function that takes an observable as input and returns a new observable with the desired transformation or side effect.


26. What is the purpose of the switchMap operator?

Answer:
The switchMap operator is used for scenarios where you want to switch to a new observable whenever a new value is emitted by the source observable. It unsubscribes from the previous inner observable and subscribes to the new one.


27. Explain the difference between forkJoin and combineLatest in RxJS.

Answer:

  • forkJoin: It waits for all the observables provided as arguments to complete and then emits an array of their last values. It’s used when you want to wait for multiple observables to complete before taking an action.
  • combineLatest: It emits an array containing the latest values of all observables whenever any observable emits a value. It’s used when you want to react to changes in any of the observables.

28. What is the purpose of the zip operator in RxJS?

Answer:
The zip operator combines the emissions of multiple observables together, emitting an array of corresponding values. It only emits when all observables have emitted a value.


29. How can you create a custom operator in RxJS?

Answer:
You can create a custom operator by defining a function that takes an observable as input and returns a new observable with the desired transformation or side effect.


30. Explain the concept of backpressure in RxJS.

Answer:
Backpressure occurs when a faster producer of data tries to push data onto a slower consumer. RxJS provides strategies like buffering, dropping, or erroring to handle backpressure situations.


31. What is the purpose of the retry operator in RxJS?

Answer:
The retry operator resubscribes to the source observable when it encounters an error. It takes a parameter specifying the number of times to retry or a predicate function to determine if a retry should occur.


32. How does the throttleTime operator work in RxJS?

Answer:
The throttleTime operator emits the first value emitted by the source observable and then ignores all subsequent values for a specified amount of time.


33. Explain the role of the partition operator in RxJS.

Answer:
The partition operator splits the source observable into two separate observables based on a provided predicate function. Values that satisfy the predicate go to one observable, while others go to the second observable.


34. What is the purpose of the debounceTime operator in RxJS?

Answer:
The debounceTime operator waits for a specified amount of time after the last emitted value from the source observable, and then emits that value. It is often used for scenarios like handling user input in search fields.


35. Explain the concept of multicasting in RxJS.

Answer:
Multicasting in RxJS allows you to share the execution of an observable among multiple subscribers. This means that the source observable is only subscribed to once, and the emitted values are broadcasted to all subscribers.


36. How can you handle errors in RxJS observables?

Answer:
Errors in RxJS observables can be handled using operators like catchError and retry. The catchError operator allows you to handle errors by returning a new observable, while retry retries the observable a specified number of times on error.


37. What is the purpose of the distinctUntilChanged operator in RxJS?

Answer:
The distinctUntilChanged operator filters out consecutive duplicate values emitted by the source observable. It only emits a value if it is different from the previous value.


38. Explain the role of the pluck operator in RxJS.

Answer:
The pluck operator is used to extract a specific property value from each emitted object in the source observable. It takes the property name as an argument.


39. How does the skip operator work in RxJS?

Answer:
The skip operator ignores the first n values emitted by the source observable and then emits the remaining values. It is often used when you want to ignore an initial set of values.


40. What is the purpose of the auditTime operator in RxJS?

Answer:
The auditTime operator periodically emits the most recent value emitted by the source observable within a specified time interval, regardless of how quickly the source emits values.


41. Explain the concept of cold and hot observables in RxJS.

Answer:

  • Cold Observable: A cold observable starts producing values when it is subscribed to, and each subscription gets its own independent stream of values.
  • Hot Observable: A hot observable produces values regardless of whether there are any subscribers. Subscribers receive values that were emitted before they subscribed.

42. What is the purpose of the combineLatest operator in RxJS?

Answer:
The combineLatest operator takes multiple observables as inputs and emits an array of the most recent values from each observable whenever any of the input observables emits a value.


43. How does the zip operator work in RxJS?

Answer:
The zip operator combines the values emitted by multiple observables together in a way that corresponds to their index. It waits for all observables to emit at least one value and then emits an array of the combined values.


44. Explain the concept of a scheduler in RxJS.

Answer:
A scheduler in RxJS is an object that defines how the execution of operations like subscription, emission, and disposal will be handled. It provides a way to control the concurrency and timing of operations.


45. What is the purpose of the share operator in RxJS?

Answer:
The share operator is used to multicast the source observable, allowing multiple subscribers to receive the same set of emitted values without causing redundant subscriptions to the source observable.


46. How can you cancel an observable in RxJS?

Answer:
In RxJS, you can cancel an observable by unsubscribing from it. This is done using the unsubscribe method, which stops the observable from emitting further values to its subscribers.


47. Explain the use of the finalize operator in RxJS.

Answer:
The finalize operator is used to perform a specified action when the source observable completes, whether by emitting a value or by encountering an error.


48. What is the purpose of the concatMap operator in RxJS?

Answer:
The concatMap operator maps each value emitted by the source observable to a new observable and subscribes to it. It ensures that the observables are subscribed to and completed in order.


49. How does the race operator work in RxJS?

Answer:
The race operator takes an array of observables and subscribes to all of them. It then emits the first value from the observable that emits, and unsubscribes from the others.


50. Explain the role of the distinct operator in RxJS.

Answer:
The distinct operator filters out duplicate values emitted by the source observable. It ensures that only values that have not been previously emitted are passed through.


51. What is the purpose of the debounceTime operator in RxJS?

Answer:
The debounceTime operator is used to filter out values emitted by the source observable that occur in quick succession. It waits for a specified amount of time after the last emitted value before allowing it to pass through.


52. Explain the concept of a subject in RxJS.

Answer:
A subject in RxJS is a special type of observable that allows both emitting values and subscribing to them. It serves as a bridge between the non-observable code and the observable code, allowing for easy multicasting.


53. How does the bufferTime operator work in RxJS?

Answer:
The bufferTime operator collects values emitted by the source observable into an array, and then emits that array after a specified time interval. It provides a way to batch values over time.


54. What is the purpose of the pluck operator in RxJS?

Answer:
The pluck operator is used to extract a specific property from each emitted object in the source observable. It simplifies working with nested data structures.


55. Explain the use of the partition function in RxJS.

Answer:
The partition function allows you to split the source observable into two separate observables based on a predicate function. One observable emits values that satisfy the predicate, while the other emits those that don’t.


56. How can you handle errors in RxJS observables?

Answer:
Errors in RxJS observables can be handled using the catchError operator or the try...catch block within the subscribe callback. catchError allows for specific error handling logic.


57. What is the purpose of the startWith operator in RxJS?

Answer:
The startWith operator emits a specified value as the first item in the output observable sequence, before it begins emitting values from the source observable.


58. How does the windowTime operator work in RxJS?

Answer:
The windowTime operator is similar to bufferTime, but instead of emitting arrays of values, it emits observables. Each emitted observable contains values collected within a specified time window.


59. Explain the concept of backpressure in RxJS.

Answer:
Backpressure occurs when a fast-producing observable is overwhelming a slow-consuming one. RxJS provides strategies like buffering, dropping, or erroring to handle this situation.


60. What is the purpose of the multicast operator in RxJS?

Answer:
The multicast operator allows you to share a single source observable among multiple subscribers by using a subject to handle the multicasting.


61. How does the switchMap operator differ from mergeMap in RxJS?

Answer:

  • switchMap only emits values from the most recent inner observable. It cancels any previous subscriptions once a new value is emitted by the source observable.
  • mergeMap, on the other hand, maintains subscriptions to all inner observables and emits all their values, potentially leading to overlapping emissions.

62. Explain the concept of hot and cold observables in RxJS.

Answer:

  • Cold Observable: Produces values only when there is a subscriber. Each subscriber gets its own independent sequence of values.
  • Hot Observable: Produces values regardless of subscribers. Subscribers receive values that are emitted after they subscribe.

63. What is the purpose of the share operator in RxJS?

Answer:
The share operator converts a cold observable into a hot one. It shares a single subscription to the source observable among multiple subscribers, ensuring that the source is only subscribed to once.


64. How does the race operator work in RxJS?

Answer:
The race operator takes multiple observables and emits the values from the first observable that emits. It effectively competes the observables against each other, and the winner’s values are emitted.


65. What is the purpose of the throttleTime operator in RxJS?

Answer:
The throttleTime operator ensures that the values emitted by the source observable are only passed on to subscribers at a specified interval. It discards values that occur too quickly.


66. Explain the concept of the concatMap operator in RxJS.

Answer:
The concatMap operator projects each value from the source observable into an inner observable, subscribes to them sequentially, and emits their values in order. It maintains order while handling concurrency.


67. How does the distinctUntilChanged operator work in RxJS?

Answer:
The distinctUntilChanged operator filters out consecutive duplicate values emitted by the source observable. It compares each value with the previous one and only allows distinct values to pass.


68. What is the purpose of the zip operator in RxJS?

Answer:
The zip operator combines the values emitted by multiple observables together in a way that each value from one observable corresponds to a value from another observable with the same index.


69. Explain the use of the auditTime operator in RxJS.

Answer:
The auditTime operator emits the most recent value emitted by the source observable within a specified time interval. It effectively throttles the observable, ensuring a minimum time gap between emissions.


70. How can you create a custom operator in RxJS?

Answer:
A custom operator in RxJS is created by defining a function that returns an observable, often using existing operators. This function can then be used in a pipe.


71. Explain the purpose of the buffer operator in RxJS.

Answer:
The buffer operator groups values emitted by the source observable into arrays based on a specified condition. It emits these arrays as a new observable.


72. How does the debounceTime operator work in RxJS?

Answer:
The debounceTime operator waits for a specified amount of time after the last emitted value from the source observable. It then emits that value. It is often used for handling user input or events.


73. What is the purpose of the partition operator in RxJS?

Answer:
The partition operator separates the values emitted by the source observable into two separate observables based on a specified condition. One observable emits values that meet the condition, while the other emits those that don’t.


74. Explain the concept of multicasting in RxJS.

Answer:
Multicasting in RxJS involves sharing a single subscription to the source observable among multiple subscribers. This ensures that the source is only subscribed to once, even if there are multiple subscribers.


75. How does the pluck operator work in RxJS?

Answer:
The pluck operator is used to extract a specific property from each emitted object in the source observable. It takes the property name as an argument and emits only that property’s value.


76. What is the purpose of the retry operator in RxJS?

Answer:
The retry operator resubscribes to the source observable when it encounters an error, allowing it to potentially complete successfully on subsequent attempts.


77. Explain the concept of backpressure in RxJS.

Answer:
Backpressure occurs when a fast-producing observable overwhelms a slow-consuming observable. Strategies like buffering, dropping, or emitting only the latest value can be used to handle backpressure.


78. How does the mapTo operator work in RxJS?

Answer:
The mapTo operator transforms each emitted value from the source observable into a constant value. It emits the specified constant for every value.


79. What is the purpose of the takeUntil operator in RxJS?

Answer:
The takeUntil operator allows values to be emitted by the source observable until a second observable emits a value. After that, it completes, and no more values are emitted.


80. Explain the concept of the exhaustMap operator in RxJS.

Answer:
The exhaustMap operator projects each value from the source observable into an inner observable. It only subscribes to the new inner observable if the previous one has completed.


81. How does the auditTime operator work in RxJS?

Answer:
The auditTime operator periodically takes the latest value emitted by the source observable over a specified time interval. It then emits that value, regardless of how many values were emitted in that interval.


82. What is the purpose of the mergeMap operator in RxJS?

Answer:
The mergeMap operator maps each value emitted by the source observable to an inner observable. It then flattens these inner observables into a single observable, which emits the values from all of them.


83. Explain the concept of hot and cold observables in RxJS.

Answer:

  • Hot Observables: These emit values regardless of whether there are any subscribers. Subscribers receive values that are emitted after they subscribe.
  • Cold Observables: These start emitting values when a subscriber subscribes. Each subscriber gets its own stream of values.

84. How does the concatMap operator work in RxJS?

Answer:
The concatMap operator projects each value from the source observable into an inner observable. It subscribes to these inner observables in order and emits their values sequentially.


85. What is the purpose of the distinctUntilChanged operator in RxJS?

Answer:
The distinctUntilChanged operator ensures that consecutive values emitted by the source observable are distinct. It filters out values that are equal to the previous one.


86. Explain the concept of multicasting operators in RxJS.

Answer:
Multicasting operators allow multiple subscribers to share the same source observable, ensuring that the source is only subscribed to once. Examples include publish, share, and multicast.


87. How does the pluck operator differ from the map operator in RxJS?

Answer:

  • pluck: It extracts a specific property from each emitted object in the source observable.
  • map: It applies a given function to each emitted value, allowing for more complex transformations.

88. What is the purpose of the zip operator in RxJS?

Answer:
The zip operator combines the latest values emitted by multiple observables. It emits an array of these combined values whenever any of the source observables emits a value.


89. Explain the concept of error handling in RxJS.

Answer:
Error handling in RxJS involves using operators like catchError or retry to handle errors emitted by the source observable. This ensures the observable continues to emit values even in the presence of errors.


90. How does the combineLatest operator work in RxJS?

Answer:
The combineLatest operator combines the latest values emitted by multiple observables. It emits an array of these combined values whenever any of the source observables emits a value.


91. Explain the purpose of the debounceTime operator in RxJS.

Answer:
The debounceTime operator waits for a specified period of inactivity after the last emitted value from the source observable. It then emits that value, effectively filtering out rapid-fire emissions.


92. What is the role of the retry operator in RxJS?

Answer:
The retry operator resubscribes to the source observable if it encounters an error, up to a specified number of times. This can be useful for handling temporary network issues or recoverable errors.


93. How does the forkJoin operator work in RxJS?

Answer:
The forkJoin operator combines the latest values emitted by multiple observables. It waits for all observables to complete and then emits an array of their combined values.


94. Explain the concept of the switchMap operator in RxJS.

Answer:
The switchMap operator maps each value emitted by the source observable to an inner observable. It then flattens these inner observables into a single observable. If a new value is emitted by the source, it switches to the new inner observable.


95. What is the purpose of the bufferTime operator in RxJS?

Answer:
The bufferTime operator periodically gathers emitted values into an array and emits that array at specified time intervals. It ensures that no values are lost, even if there is no activity.


96. Explain the concept of backpressure in RxJS.

Answer:
Backpressure occurs when the rate of emissions from a source observable exceeds the rate at which the observer can process those emissions. This can lead to issues if not handled appropriately.


97. How does the distinctUntilKeyChanged operator differ from distinctUntilChanged in RxJS?

Answer:

  • distinctUntilChanged: Compares consecutive values emitted by the source observable directly.
  • distinctUntilKeyChanged: Compares consecutive values emitted by the source observable based on a specified key or property.

98. What is the purpose of the delay operator in RxJS?

Answer:
The delay operator introduces a specified delay before emitting values from the source observable. This can be useful for simulating network latency or scheduling emissions.


99. Explain the concept of multicasting in RxJS.

Answer:
Multicasting involves sharing a single source observable among multiple subscribers. This ensures that the source is only subscribed to once, reducing unnecessary duplication of work.


100. How does the pairwise operator work in RxJS?

Answer:
The pairwise operator groups consecutive emissions from the source observable into pairs. It emits an array where the first element is the previous value and the second element is the current value.