React JS Interview Questions and Answers from Our Experts

20 min read
Jun 18, 2021
React JS Interview Questions and Answers from Our Experts
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The best React interview questions are those that explore what developers achieve using the platform, rather than simply what they know. However, before delving into the React JS test questions, we would consider some background into this much-talked-about platform. ReactJS (also known simply as React) is a Javascript library. It is used to develop web applications. It is open-source and primarily for front-end UI development.

It shouldn’t be confused with React Native, which is a framework for mobile app development. Although, this is possible because it can also function as a base to build mobile apps and single-page applications. By the way, most ReactJS coding interview questions dwell upon its usefulness as a web application builder.

react

It was initially released by Facebook 8 years ago as an open-source Javascript library. Before it became open-source, Facebook already implemented a version in its news feed. Later it was introduced to Instagram’s feed in 2012 after the acquisition of the photo-sharing app. It is currently in its 17th version, which signifies the rapid growth of the platform since it was released.

By the way, interview questions for React developer candidates may also explore some features from each version. Because of how efficient apps built with the library are, it has gained widespread adoption among developers of all experience levels. Many global corporations have the best websites made with React. Developers also get the advantage of massive community support. React JS basic interview questions should focus on what one can do with it.

React.js developers are in great demand

However, since there are so many people around who use this platform and so many React JS interview questions to be asked, it might be difficult for companies to find a definite person who is skilled, understand their company’s peculiar needs, and has the right mix of experience as well as attitude for a project.

The following series of top React interview questions and answers are curated to help business leaders and recruiters to hire better developers. Most of the React developer interview questions shared here can be used to determine how skilled a developer is.

The Best React Interview Questions for Beginners

Here you’ll find some of the best React JS basic interview questions. Even junior developers are integral to the software development project. They help to handle routine tasks as well as assist in debugging while also writing basic code. Primarily though, is that they are being groomed for an active charge of development projects in the future.

Homer-is-a-trainee

It is a future-focused platform; therefore, your junior developers would be an important part of your team. You should ensure that they have React’s basic ideas, its concepts, and so on, even if they lack in-depth knowledge yet, including their overall result in React JS advanced interview questions and answers.

Why React?

React is officially defined as a “Javascript library for building user interfaces.” Basically, the idea is to build web applications with rich UI elements. It facilitates smoother communication between developers and web page DOMs (Document Object Models), therefore eliminating the need for the DOM API.

Developers achieve this via composable UIs which are more efficient than traditional ideas of operating the backend of web applications. Using the Model-View-Controller (MVC) architectural model, React represents the ‘View’ section.

What are components?

No set of React JS interview questions for beginners or any level would be complete without components. Components are the main building blocks and it is impossible to not talk about them.

react js interview should include components

When building an app, the entire app is divided into components and writing code instructs the components on interaction and so on. Each component is a functional UI element that forms a building block of an application. Further, the two main features of components; they are:

  • Composable – can be reused and new components can be iterated from former ones.
  • Independent – each component, though linked with others, still stands apart. So, when you fix the code in one section, you still preserve the rest…

What are the main features of React?

This section of React JS test questions explores some popular concepts on the platform. For developers trying to know how to prepare for React interview, be able to explain the core functions of these main features.

  • Declarative UI: as against the traditional imperative mode of operating DOM APIs, this library uses a declarative model. Its components are declarative in that the developer does not have to define step-by-step instructions on how the application must operate. Instead, by declaring each visual element of the UI and its contextual data, the app responds by building a dynamic, interactive UI.
  • State: the state is the contextual data of a component. Developers can manipulate the state by changing a component’s data properties. Components can also create and manage their own data using this State feature. The setState method is used to modify a component’s data properties.
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  • JSX: this stands for ‘Javascript XML’ and it is what makes it possible to write HTML. It is a Javascript syntax extension that’s used to produce elements. Some React front end developer interview questions may include live JSX coding online test.
  • Redux: Redux is a JavaScript state management tool most commonly used with React. It’s described as “a predictable state container for JavaScript apps.” Redux often proves useful whenever a developer hits some limitation with the component state.
  • Events: these are Reactions triggered by a user’s action in interacting with the UI such as clicking a button, pressing a key, etc. Alternatively, events may be system-generated.

During your own set of interview questions on ReactJS for experienced developers, in particular, you may choose to explore each of these features even further.

What are the advantages of React?

Every developer must have answers to these basic React JS interview questions. To develop on a platform, you need to understand the pros of using that platform.

  • Flexibility: it allows developers to build upon any desired technology stack. Therefore, unlike other frameworks, developers are free of restrictions to rigid architectures and workflows.
  • Performance optimization: declarative programming enhances performance by creating a virtual DOM by allowing the developer to write HTML directly into JavaScript files. The virtual DOM is much easier to manipulate than the real DOM and so that improves efficiency. Answers to ReactJS coding interview questions such as this should explain how it enables building rich user interfaces for mobile and web applications.
Need for speed
  • SEO Efficiency: it is used for building web applications; therefore, SEO is a strong factor in considering it. Thankfully, it is SEO-friendly. This is despite the fact that search engines commonly have problems reading apps that are heavy on JavaScript. The virtual DOM feature makes this less of a challenge. This may also feature in the React JS advanced interview questions and answers section.
  • Productivity Improvement: developers commonly describe as the best feature the ability to reuse system components. This helps them to deal with issues concerning the library’s complex logic. For companies, the ability to reuse code assets helps in setting internal coding standards and app consistency across the board.
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  • Learn once – write anywhere: while React is popular for web development, there is the React Native framework, which enables building web applications. Again, this improves cross-platform consistency and efficiency. It also improves the work of developers, since, for all platforms, they are working with similar tools. This is an important part of React frontend developer interview questions.

The answers to React JS basic interview questions may invite follow-up questions in the case of senior and intermediate developers, for instance, in being asked how using the library improves SEO.

What is the difference between ReactJS and React Native?

For non-technical interviewers, it is easy to confuse ReactJS coding interview questions for React Native. However, even for entry-level developers, basic React JS interview questions such as this should not be overlooked.

In simple terms, ReactJS is a library, while React Native is a framework. Specifically, the latter is a mobile framework that enables cross-platform mobile development. It is commonly used to build apps meant for Android and iOS. Apps that use it include Skype, Pinterest, Uber Eats, etc.

react mem

On the other hand, ReactJS is a library for building the user interface (UI) of web applications. Keep in mind that when people simply use ‘React’, they are most often referring to ReactJS. The ‘native’ factor stems from the fact that it uses native APIs to render components on the app. In the end, each has its pros and cons.

What are the differences between React and AngularJS?

Another fact that should be covered at the basic React JS interview questions stage is how it compares with similar applications such as AngularJS.

First, while AngularJS is a JavaScript framework, React is a JavaScript library. Therefore, because of the concept of inversion of control, the latter enables developers to have a greater charge of the application flow.

react.js vs angular

AngularJS uses an actual DOM while React uses the more recent Virtual DOM technology. Therefore, the latter is more suitable for applications requiring dynamic content and regular updates. That’s why some popular apps that use React include Facebook, Netflix, WhatsApp, Discord, etc. More so, a smaller library size makes its apps more lightweight than AngularJS.

In general, in answering basic React JS interview questions, more programmers tend to prefer React over AngularJS. The latter uses a two-way binding approach, which seems easier at first glance, but it is inefficient in the face of code-heavy applications. Its one-way binding allows developers to efficiently manage large code.

Why did you start working with React?

This is one of the very good non-technical React JS test questions to ask rookie developers. It helps to recover individual motivation and drive to have begun working with React, as well as what has kept them going. Particularly, look out for candidates who mention innovation and growth among their motivations.

one more react js developer

However, don’t overlook candidates who admit that they started learning due to expanding opportunities. That kind of answer is honest and practical enough. It could lead to a follow-up question as to how they must stay on top of the industry and regularly adapt to the changes and remain relevant. Other useful ones among the best React interview questions for beginners, plus answers exist, but these are the most relevant and are enough.

Top React Interview Questions and Answers for More Experienced Professionals

Mid-level developers mainly are responsible for implementing standard coding practices, maintaining the codebase, adapting the technical requirements of a project, and so on. They also execute tests and debugging. A mid-level React developer should have at least 2 years of experience; unlike junior developers, they need only limited supervision.

A guy is asking about difficult things

Therefore, the best React front end developer interview questions for this category of developers are those that try to determine how well they can work autonomously and how they can effectively implement the technical requirements of a project and answer some ReactJS experienced interview questions. Many of these interview questions on ReactJS for experienced professionals expound on components and how they work.

What are React Component life cycles?

The three main phases of a lifecycle: mounting, updating, and unmounting. Each features different methods, although they can be overridden by the programmer.

  • Mounting: this happens when elements are entered into the DOM. That is when the component is initially created. The most common methods here are constructor(), render(), and componentDidMount().
  • Updating: when one triggers a change to state, the component is re-rendered. Common methods are render() and componentDidUpdate().
  • Unmounting: this is removing a component. The componentWillUnmount() method applies here.

What differentiates the class component from the functional component?

ReactJS coding interview questions can be quite practical. The preeminent difference between both types of components is syntax. A functional component accepts properties in plain Javascript functions and returns HTML in the form of JSX elements. As such, they are also known as stateless components. More so, they do not use any render method.

However, class components, also called stateful components, create render functions that extend React’s component classes. While the former is straightforward and easy to read, class components use complex UI logic. Moreover, functional components do not support lifecycle hooks though class components do.

Explain Controlled vs Uncontrolled Components in React?

Top React interview questions and answers like this are aimed at providing a short direct response to a practical inquiry. One can have a form handled by a React component or alternatively by the DOM directly. Forms implemented in the former manner are called controlled components, while for the latter, they are uncontrolled components.

you have to feel forms

Uncontrolled components are similar to traditional HTML in that the state is stored internally, although you can still retrieve the current value from the DOM by using a ref. Moreover, controlled components maintain their internal state and allow for validation control, unlike uncontrolled components, which do neither. However, React mostly recommends controlled components for implementing forms.

How do you render elements in React?

In React, elements are the ‘smallest building blocks’ of apps. Elements are the plain objects returned from components and rendered into the DOM. Elements determine what you see on the browser screen but they themselves are not seen; rather they are stored in the app’s memory.

i will find you and i will render you

Therefore, every component has an element that determines the properties, type, etc. of that component. Basically, one renders elements into the root DOM node by using ReactDOM.render(). In addition, since React elements cannot be changed neither can their attributes be updated, one has to use the same code function to create a new element in order to implement an update to the UI.

What is the difference between props and state?

Properties (props) are read-only. They determine the configuration of a component as received from the parent. If React components are functions, you may think of props are arguments for a function. Props remain immutable throughout the lifecycle of a component.

On the other hand, the state is dynamic. Components don’t have a default state passed down from parents. Hence, the state of a component is mutable, representing a data value that can be updateable when certain conditions are fulfilled. Thus, the state is essentially what makes dynamic rendering possible.

What are some lifecycle methods of React components?

  • render() – the most commonly used lifecycle method and the only one required in a class component.
  • constructor() – a method called before mounting a component. This method is not necessary if you didn’t initialize state yet or bind methods.
  • componentDidMount() – the first method called after a component is mounted.
  • componentDidUpdate() – this method is implemented once a component is updated, though not at initial rendering.
  • componentWillUnmount() – to be implemented before you umount a component is unmounted and also used to perform due cleanups.

Those are the common lifecycle methods. Some are rarer, including shouldComponentUpdate(), static getDerivedStateFromProps(), getSnapshotBeforeUpdate(), static getDerivedStateFromError(), and so on. This question may also easily fall under ReactJS experienced interview questions for senior developers if you need to establish their understanding of components.

ReactJS Experienced Interview Questions for Senior Developers

ReactJS experienced interview questions for senior developers dwell upon the role of senior developers as team overseers who also review code for accuracy, functionality, and consistency. In addition, they handle the most complicated coding assignments and may also organize tutorials for their juniors.

The Rock faced a challenge

Therefore, the top React experienced interview questions for senior developers should be geared towards practical inquiries. This section aims to cover some React front end developer interview questions in this regard.

How does Virtual DOM Work?

Full understanding of the virtual DOM is one of the most complex advanced React questions for senior developer candidates.

React allows developers to write HTML into JavaScript files. This empowers developers to maintain control of their HTML code. The Document Object Model (DOM) is an API that works with HTML and XML files. Therefore, it is very useful in building web applications. However, React deviates from the traditional approach to using DOM.

react technical questions should cover all pross of react

With a virtual DOM, one can keep a representation of the DOM in memory. Thus, virtual DOM is lightweight than the real DOM and does not demand as many resources to maintain. However, through the process of reconciliation, the virtual DOM may be synced with the real DOM via a library.

Unlike a real DOM, one can use a virtual DOM to directly interact with the interface, even though it shares properties with a real DOM. many JavaScript framework operations require rapid updates to the DOM. this can slow down the application a lot. However, with virtual DOM, the time cost is greatly reduced.

What is Higher-Order Component (HOC)?

Another one of the top React interview questions and answers, Higher-Order Components are used to add sophisticated functionalities to existing components. The purpose of the HOC technique, which is common in third-party React libraries, is to reuse component logic.

look i'm doing react js

Therefore, the use of every Higher-Order component function returns a new component. HOCs in React are related to higher-order functions in JavaScript. Higher-order components find implementation in Redux connect (which is a higher-order function), state abstraction, and so on.

How does conditional rendering work in React?

Conditional rendering is the dynamic output of UI markups depending on the state of a condition. It works similarly to JavaScript conditions. With conditional rendering, one can toggle certain application functions, determine permission levels, show or hide elements, handle authentication, render API data, and so on.

There are various ways to implement conditional rendering in React. Some of the expected answers for this particular one of the React questions for senior developer candidates are:

  • Using the if-else conditional logic (suitable for smaller and medium-sized apps)
  • Using element variables, which actually enables writing cleaner code
  • Using ternary operators, which take some complication away from if-else statements, etc.

What are the drawbacks of React?

Interview questions on ReactJS for experienced developers such as this help you to determine that a person knows what they are doing. For someone to claim expertise over a platform, they need to be well acquainted with its limitations and challenges, so as to know how to get around them.

JSX: although it is well-established that JSX has its benefits such as enhancing app performance, developers find it difficult to learn and use. Many developers don’t have JSX foundation as a syntax extension. Hence, they find it complex.

programming pro tip

The rapid pace of development in React can also be a disadvantage because it means developers have to continually relearn new concepts as updates are released. It also makes it difficult to keep up with documentation if updates always result in significant changes. However, there has been some improvement on this end recently, React’s core API is now much stable than before so it’s indeed getting better.

Also included is how the platform is highly flexible and the lack of conventional standards can make things complex particularly when onboarding new developers. Every React development team seems to have its own separate standards that developers have to learn to work with. In any case, this challenge can be mitigated through proper onboarding, as we do at ProCoders.

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React is rich enough as a library, but developers coming from platforms such as Angular might miss some of the features associated with frameworks. For instance, on the Model-View-Controller structure algorithms, React represents only one part: the ‘View’ part. This gap can make React poorly structured and poorly managed.

What are React Hooks?

React Hooks are still a relatively new feature. That’s why this question is in the mid-level section. Many people haven’t gotten the hang of hooks completely. React Hooks were introduced in version 16.8, which was released in February 2019. For context, the latest version of React is version 17. So, it obviously has not been much longer than hooks were introduced. In any case, hooks are backward-compatible, so that’s one potential problem avoided.

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In simple terms, hooks allow a programmer to manipulate the state of stateless functional components without having to convert them into stateful class components. Essentially, with hooks, one no longer needs to write a class to use state and other React features. Some common hooks are:

  • setState – return a function to update a stateful value
  • useEffect – implementing side effects in function components
  • useContext – return the current context value.

Users can also create their own custom hooks, for instance, by using higher-order components. The two rules of hooks are that one must only call hooks at the top level and that one can only call hooks from function components.

Business Benefits of React?

An expert should be able to answer such advanced React questions for senior developer candidates. These React JS advanced interview questions and answers allow them to offer some thoughtful perspective around the prospects of React for web development in your company and industry.

Because of reusable components, React developers don’t have to write new code every time for different features that have similar functionalities. This improves the speed of the development process and reduces the time-to-market, essentially making the business more competitive in the industry.

business

React enables building SEO-friendly apps, and the benefits of SEO to marketing and business growth in this day cannot be overstated. By boosting page performance and lowering loading time, a business can improve the performance and user experience of its apps. This is a plus for SEO since website performance is a critical ranking factor of search engines.

React apps are easy to maintain. This lowers incidents of downtime, which may affect customer experience. In addition, React is a cost-optimized solution for building web apps.

Finally, to add to this, when answering these interview questions on ReactJS for experienced developers, they should be able to paint a picture of the future of React and its continued viability in the development stage.

How to Prepare For React Interview as a Developer?

Beyond knowing how to answer React JS test questions, you need to be equipped in areas such as soft skills and cultural fit. The best React interview questions are the ones that do not just aim to uncover an individual’s technical skills through React developer interview questions. Instead, these best React interview questions for beginners and experts alike explore other non-technical skills.

In answering React developer interview questions, you need to understand the peculiarities of the company’s industry. It is one thing to be a highly skilled developer; yet, it is another thing to be knowledgeable about building functional solutions for a certain audience.

An experienced guy

Therefore, during the interview, you want to demonstrate that you have researched the company and understand some of the challenges and opportunities present. This is particularly important when answering advanced React questions for senior developer candidates.

During the phone interview questions for React developer, which is often the first stage in the interview process, be prepared to give a little insight into your React journey, your learning experience, and other interests picked along the way.

Basically, why React? After all, there are other good web application development platforms. Answer these personal questions honestly and confidently. Bluffing might make you come across as insincere, which may negatively impact your chances of winning the job.

advanced react js interview questions should cover why developer want to use react

Further, you may be required to answer situational React JS basic interview questions or experts notwithstanding, but about hypothetical issues in the development process. Say, you may be asked to explain how you would overcome a particular challenge in React.

These practical instances are more common with advanced React questions for senior developer candidates. Although, it may apply to junior developers too. Before your interview, think up a wide range of complex situations and how you would solve them practically as a React developer.

When you are asked to compare certain technologies, say, ReactJS versus AngularJS, avoid talking down too much on the other technology because you want to extoll React. This particularly applies to emerging technologies. One of the hallmarks of a good developer is the openness to learn and the willingness to innovate. Therefore, focus more on the positive potentials of technologies rather than dwell on your own difficulty with using them.

How to Prepare For React Interview as an Employer?

The interview questions for React developer shared here are mainly technical. Therefore, this section would cover other important areas to quiz developers about beyond the technical aspects.

Consider outsourcing the project to an offshore team. Offshore teams from countries like Ukraine, Poland, China, Mexico, etc. are much cheaper to hire. More importantly, many offshore development companies, such as ProCoders, usually have a team on the ground already. All you need to do as an employer is to choose the developers you want to work with and get the project running.

Of course, even when hiring developers through an offshore agency, you should still interview the individual developers as well as the agency’s management. The two most important qualities to look out for are passion and teamwork. Of course, skills are important, but a highly skilled person who lacks drive or the ability to work well with a team would derail the project; you don’t want that.

Therefore, hiring even underdeveloped programmers who have a passion to learn and are great teammates might serve you much better in the long run, never mind the short term.

Check for cultural fit. Ensure that they fit within your team structure and engage your business model. Therefore, they need to have a cultural fit for your business.

communication the key is

This also includes the aspect of understanding the language. For instance, many businesses outsource to Ukraine because Ukrainian developers, besides their skills, are fluent in English language, which is the most global language in the world. In essence, cultural fit enables smoother communication and contributes to the success of a project.

If hiring a software development agency, even before delving into the React interview questions for beginners or otherwise, be careful to look at their track record of projects. Particularly, choose an agency that has extensive experience in React, including building apps related to your own business and industry. Such understanding is useful for building the right apps to serve your customers.

These are the main areas on how to prepare for React interview as an employer in order to find the best candidates, even beyond just asking React developer interview questions.

Conclusion

These tips and interview questions for React developer must have definitely been helpful for you, just as it has been for us. At ProCoders, all our developers are screened and interviewed before they join the team. We also encourage clients to interact with our developers and ask them useful and even tricky interview questions for React developer when we set up development teams.

One of the foundations of a successful development project is good communication, which must begin even before the project starts. This is achieved by staging top React JS advanced interview questions and answers session. We provide some of the best software development outsourcing services, operating from Ukraine. We can help you build your next React software development team.

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