10 RxJS Interview Questions and Answers in 2023

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As the JavaScript ecosystem continues to evolve, so too does the need for developers to stay up to date on the latest technologies. RxJS is one of the most popular libraries for reactive programming, and it's becoming increasingly important for developers to understand its core concepts. In this blog post, we'll be taking a look at 10 of the most common RxJS interview questions and answers for 2023. We'll cover topics such as observables, operators, and more, so that you can be prepared for any RxJS-related questions that may come up in your next interview.

1. How would you explain the concept of Observables and how they are used in RxJS?

Observables are a core concept of RxJS, and they are used to represent asynchronous data streams. An Observable is a collection of values over time, and it can be subscribed to in order to receive notifications when new values are emitted. Observables are used to represent any source of data, such as user input, network requests, or data from a database.

RxJS provides a number of operators that can be used to manipulate and transform Observables. These operators allow developers to filter, map, reduce, and combine Observables in order to create complex data flows. RxJS also provides a number of utility functions that can be used to create Observables from existing data sources, such as Promises, Arrays, or Iterables.

Observables are a powerful tool for managing asynchronous data flows, and they are used extensively in modern web applications. RxJS provides a comprehensive library of operators and utilities that make it easy to create and manipulate Observables, allowing developers to create complex data flows with minimal effort.


2. What is the difference between a Subject and an Observable in RxJS?

The main difference between a Subject and an Observable in RxJS is that a Subject is both an Observer and an Observable. This means that a Subject can both emit values and subscribe to values. An Observable, on the other hand, is only able to emit values.

Subjects are useful when you need to multicast a value to multiple Observers. This is because a Subject can subscribe to a single Observable and then emit the same value to multiple Observers.

Subjects are also useful when you need to manually emit values. This is because Subjects have a .next() method that allows you to manually emit values.

In addition, Subjects are useful when you need to create a custom Observable. This is because Subjects can be used as the source of an Observable.

In summary, the main difference between a Subject and an Observable in RxJS is that a Subject is both an Observer and an Observable, while an Observable is only able to emit values.


3. How would you create an Observable from an array of values in RxJS?

To create an Observable from an array of values in RxJS, you can use the of operator. The of operator takes an array of values and returns an Observable that emits each item in the array sequentially. For example, if you have an array of numbers:

const numbers = [1, 2, 3, 4, 5];

You can create an Observable from this array using the of operator like this:

const observable = Rx.Observable.of(...numbers);

The of operator will emit each item in the array sequentially, so the resulting Observable will emit 1, then 2, then 3, and so on.

You can also use the from operator to create an Observable from an array. The from operator takes an array and returns an Observable that emits each item in the array as a separate emission. For example, if you have the same array of numbers:

const numbers = [1, 2, 3, 4, 5];

You can create an Observable from this array using the from operator like this:

const observable = Rx.Observable.from(numbers);

The from operator will emit each item in the array as a separate emission, so the resulting Observable will emit 1, then 2, then 3, and so on.


4. What is the purpose of the map operator in RxJS?

The map operator in RxJS is used to transform the items emitted by an Observable. It applies a projection function to each item emitted by the source Observable and emits the resulting value. This operator is commonly used to transform data from one form to another, such as converting a stream of strings to a stream of numbers. It can also be used to perform calculations on the data, such as calculating the average of a stream of numbers. The map operator is a powerful tool for transforming data and can be used to create complex data pipelines.


5. How would you create a custom operator in RxJS?

Creating a custom operator in RxJS is a relatively straightforward process. First, you need to create a function that takes an Observable as its input and returns an Observable as its output. This function should contain the logic for your custom operator.

Next, you need to use the RxJS pipeable operator API to create a higher-order function that takes the custom operator function as its argument. This higher-order function should return a function that takes an Observable as its argument and returns an Observable as its output.

Finally, you need to use the RxJS create operator to create an Observable from the higher-order function. This Observable will be the custom operator that you can use in your code.

For example, if you wanted to create a custom operator that filters out values that are greater than a certain number, you could do the following:

// Create the custom operator function const filterGreaterThan = (threshold) => (source) => source.pipe( filter((value) => value <= threshold) );

// Create the higher-order function const filterGreaterThanOperator = (threshold) => pipe(filterGreaterThan(threshold));

// Create the custom operator const filterGreaterThan$ = create(filterGreaterThanOperator);

// Use the custom operator const source$ = of(1, 2, 3, 4, 5); const result$ = source$.pipe(filterGreaterThan$(3));

// Output: 1, 2, 3


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

The switchMap operator in RxJS is used to flatten inner observables and switch to a new inner observable each time the source observable emits a new value. It is a combination of the switch and map operators, and it is used to map each value to an inner observable, then switch to the new observable when the source emits a new value.

The switchMap operator is useful when you want to cancel any in-flight inner observables and switch to a new one each time the source observable emits a new value. This is especially useful when dealing with user input, such as when a user types in a search box. The switchMap operator will cancel any in-flight requests and switch to the new request each time the user types a new character. This prevents multiple requests from being sent for the same search query.


7. How would you handle errors in an RxJS stream?

When handling errors in an RxJS stream, it is important to understand the type of error that is occurring. Depending on the type of error, there are different approaches to handling it.

For example, if the error is a runtime error, such as an exception thrown by an operator, then the best approach is to use the catchError operator. This operator allows you to catch any errors that occur in the stream and handle them accordingly. You can either log the error, or you can use the retry operator to retry the stream until it succeeds.

If the error is a user-generated error, such as an invalid input, then the best approach is to use the filter operator. This operator allows you to filter out any invalid inputs before they reach the stream. This way, you can ensure that only valid inputs are processed by the stream.

Finally, if the error is a system-generated error, such as a network error, then the best approach is to use the retryWhen operator. This operator allows you to retry the stream until it succeeds, or until a certain number of attempts have been made.

In summary, when handling errors in an RxJS stream, it is important to understand the type of error that is occurring and use the appropriate operator to handle it.


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

The combineLatest operator in RxJS is used to combine the output of multiple Observables so that the resulting output is a combination of the most recent values emitted by each Observable. It is a useful operator for combining multiple sources of data into a single stream of values.

The combineLatest operator takes an array of Observables as its argument and returns an Observable that emits an array of the most recent values emitted by each Observable. The order of the values in the array is the same as the order of the Observables in the array.

The combineLatest operator is useful for combining multiple sources of data into a single stream of values. For example, if you have two Observables that emit the current temperature and humidity, you can use combineLatest to combine them into a single stream of values that contains both the temperature and humidity.

The combineLatest operator is also useful for combining multiple asynchronous operations into a single stream of values. For example, if you have two asynchronous operations that each return a value, you can use combineLatest to combine them into a single stream of values that contains both the values.

In summary, the combineLatest operator in RxJS is used to combine the output of multiple Observables so that the resulting output is a combination of the most recent values emitted by each Observable. It is a useful operator for combining multiple sources of data into a single stream of values.


9. How would you create a hot Observable in RxJS?

Creating a hot Observable in RxJS is a relatively straightforward process. First, you need to create a Subject, which is a type of Observable that allows values to be multicasted to multiple Observers. You can do this by using the RxJS Subject constructor, which takes an optional SubjectConfig object as an argument. This object can be used to configure the Subject, such as setting the initial value or enabling/disabling certain features.

Once the Subject is created, you can then use the Subject's next() method to emit values to the Observers. This method takes a single argument, which is the value that will be emitted.

Finally, you can use the Subject's subscribe() method to create Observers that will receive the values emitted by the Subject. This method takes a callback function as an argument, which will be called whenever a new value is emitted.

Once all of these steps are completed, you will have a hot Observable that can be used to multicast values to multiple Observers.


10. How would you create a cold Observable in RxJS?

Creating a cold Observable in RxJS is a relatively straightforward process. First, you need to create a new Observable object using the Observable constructor. This constructor takes a function as its argument, which is the function that will be executed when the Observable is subscribed to. This function is known as the "subscribe function".

The subscribe function is responsible for creating the data that will be emitted by the Observable. It can do this by using the RxJS operators such as of, from, interval, timer, etc. For example, if you wanted to create an Observable that emits a sequence of numbers, you could use the of operator like this:

const observable = Rx.Observable.of(1, 2, 3, 4, 5);

Once the Observable has been created, you can then call the subscribe method on it to start the data emission process. This is what makes the Observable a cold Observable, as the data emission process only starts when the Observable is subscribed to.

Finally, you can also add operators to the Observable to transform the data that is emitted. For example, if you wanted to transform the sequence of numbers emitted by the Observable above into a sequence of strings, you could use the map operator like this:

const observable = Rx.Observable.of(1, 2, 3, 4, 5).map(num => num.toString());

This would transform the sequence of numbers emitted by the Observable into a sequence of strings.

In summary, creating a cold Observable in RxJS involves creating a new Observable object using the Observable constructor, calling the subscribe method on the Observable to start the data emission process, and adding operators to the Observable to transform the data that is emitted.


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