5 RxJS Interview Questions (With Sample Answers And Tips)

Updated 16 March 2023

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Reactive programming allows developers to work with asynchronous data streams in web applications, enabling them to handle a large number of users and data. They use Reactive Extensions for JavaScript (RxJS), which is a JavaScript extension, to implement reactive programming. If you are interviewing for a web application engineer or a similar role, learning about frequently asked questions and knowing how to answer them can help you during your interview preparation. In this article, we discuss RxJS interview questions and their sample answers and share tips to help you prepare for an interview.

RxJS Interview Questions With Sample Answers

RxJS interview questions may include a mix of general and technical questions to help interviewers assess if you are a suitable candidate for the role. Interviewers use these questions to evaluate your understanding of concepts, professional background and experience. Here are example questions and responses you can use as a guide:

1. What are the differences between imperative, functional and reactive programming?

The knowledge of various programming styles is necessary for writing quality code. This question helps interviewers assess your basic understanding of different programming styles available, along with their unique features. Define the three terms and list key differences between them in your answer. You can also give examples of each programming style.

Example answer: 'Imperative programming is procedural, requiring programmers to define step-by-step instructions for the computer. The languages are very specific and the operation is system-oriented. In imperative programming, the order of execution and state changes are important. Some examples include C++ and Java. Functional programming uses mathematical functions, conditional operations and recursion to perform computation. The order of execution is of low importance and states changes are non-existent. Some examples include Python and Scala.

Reactive programming focuses on asynchronous operations, callbacks and event-driven operations. Its responsiveness ensures consistent response times. Being resilient ensures it continues to function in case of hardware or software failures. Because of its elasticity, it can handle a variety of workloads.'

2. What are the advantages of using RxJS?

It is important for programmers to have a basic understanding of RxJS and its features to use them efficiently. This question helps interviewers assess your understanding of important RxJS features when working with asynchronous processes. When mentioning the benefits of RxJS, consider including points relevant to its ease of usage, the various functionalities it provides to work with asynchronous streams of data and its compatibility with other frameworks. You can also discuss its community support and documentation, which helps users resolve their questions and simplify the learning process.

Example answer: 'RxJS offers an extensive range of features that make it easy for users to implement reactive programming. This library provides Observables to handle all asynchronous, event-based and callback programs. With its predefined operators, users can transform, query and compose asynchronous streams of data using mathematical, transformation, utility, conditional and filtering operations. It also provides error handling operators that help debug issues in code.

It provides good optimisation and memory leak protection. You can use this library extension with popular frameworks such as Angular or React. The RxJS library is self-sufficient, making it less dependent on third parties compared to other libraries. The library offers community support where users can interact with other programmers to resolve their queries. It also has detailed documentation and resources for users to get started.'

3. What are the differences between Observables and Promises?

Programmers use Observables and Promises to handle asynchronous operations in JavaScript. There are some advantages of using Observables over Promises. Interviewers ask this question to assess your understanding of the objects and how they compare. Answer this question by mentioning how Observables and Promises respond differently to one or more events. Describe how both objects handle callbacks. You can also mention how each one behaves when invoked. By including these points in your answer, you can demonstrate that you understand how and when to use them in your code.

Example answer: 'You can use Promise and Observable to handle asynchronous operations. A Promise object can handle a single event in case an async operation fails or completes. Observables can handle one or multiple events. A Promise is not cancellable, whereas Observables are cancellable. For example, if there is an expensive async operation that is no longer necessary, the Subscription object of Observable allows you to cancel the subscription, whereas a Promise waits for a successful or failed callback. This results in waiting for a callback even when you no longer need the notification or result of the operation.

A Promise is eager because a function immediately invokes when you define it. Observables are lazy because the functions do nothing unless subscribed.'

4. What are RxJS operators? List some examples

Operators are functions that modify a value emitted by an Observable before passing them through the data stream. Interviewers ask this question to evaluate your understanding of the operators available in RxJS, along with your practical experience with it. Begin your answer by defining what an operator is and mentioning its categories.. Make sure to state an example from each category. This demonstrates your proficiency in working with operators and a good understanding of RxJS libraries.

Example answer: 'An operator is a pure function whose inputs and outputs are Observables. An Observable pipe chains multiple operators together. The categories of operators include Pipeable and Creation operators. Pipeable operators are functions that, when called, do not change the state of the Observable. They return a new Observable whose subscription logic is the same as the original Observable. Creation operators are stand-alone functions you can use to create a new Observable.

The of operator is a creation operator that can create an Observable stream from a source. The from operator is a creation operator that converts an array or Promise into an Observable. The map operator is a transformation operator that fetches values from one Observable, transforms it and creates a new Observable to emit the new value. You can use tap, which is a utility operator, to perform various actions or steps on Observables without alerting the original stream.'

5. What is back pressure? How does RxJS handle it?

Interviewers may ask this question to check if you know how to control and manage an overload of data in a communication stream. Interviewers expect you to start your answer by defining the concept. In your answer, define what back pressure is and how it affects system performance. It is important to mention the back pressure strategy and how you can handle it using lossy and lossless operations. State various approaches you can take to implement the back pressure strategy in your code and describe each one in detail. For each of these approaches, categorise them as lossy or lossless operations.

Example answer: 'Back pressure is a scenario that occurs when Observable outputs large amounts of data at a rate faster than the consumers can use. This results in the collection of unconsumed data and adds stress to the system. A back pressure strategy is a mechanism through which the system handles the load instead of collapsing. You can handle back pressure through lossy and lossless operations. In lossy operations, the consumer is not interested in all the transmitted data. In lossless operations, a loss of data may cause errors in a system.

Debounce, throttle, buffer count and buffer time are some ways RxJS handles back pressure. In the debounce approach, the Observable emits an item only if there is no trigger within a specified period. The throttle approach works by omitting all data emitted within a specified interval, resulting in data loss. A buffer count approach is a lossless approach in which the Observable buffers the data streams until a particular count before emitting them to subscribers. In the buffer time approach, which is lossless, the Observable buffers the data streams up to a specified time interval before transmitting them to subscribers.'

Tips To Prepare For An RxJS Interview

Here are tips that can help you prepare for your next interview:

Review important concepts

Reviewing before an interview helps you identify areas of improvement. When preparing for an RxJS interview, go through the following:

Understand the differences between imperative, functional and reactive programming.
Read the advantages and limitations of using RxJS.
Learn how RxJS works.
Learn the different operators available in RxJS.
Learn the concept of back pressure in reactive programming.
Go through the concept of RxJS Observables, Observers and Schedulers.

After reviewing the key concepts, go through your past projects and detail your essential tasks and implementation details.

Learn the STAR technique

The STAR technique, where STAR stands for situation, task, action and result, can help you be prepared to handle real-time situations related to the job. This approach allows you to prove your experience and skills for the job by providing real-life examples. For example, in response to behavioural questions, you can share relevant details, your responsibilities, the steps you took and the situation's outcome. Sharing your experience helps hiring managers decide if you are suitable for the job.

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