20+ Critical React Interview Questions and Answers

With a component-based approach and one of the best possible rendering performances, React provides everything you need to create rich and engaging web apps. Companies like Facebook, Dropbox, and Instagram have long using it to solve user interface issues.

This open-source JavaScript library can be easily labeled as a bestseller in terms of web app development. According to statistics, React ranked second in the Developer Survey on Stack Overflow Top Web Frameworks (2019).

Choosing React has many benefits, what about finding developers with great skills? As a product manager with little or no technical background, you may face challenges with conducting job interviews.

From interview advice to technical questions, this article contains everything you need to hire the best talents.

Things to Look When Hiring React Developer

Before diving into React interview questions and answers, it is necessary to clarify the essential hard and soft skills. As a matter of fact, React represents a library, so good professionals should also know coding languages and other instruments as well.

Your primary focus should be on checking candidates’ understanding of HTML, CSS, JSX, and  JavaScript. It is then mandatory to check skills on ES6, Node, npm, Redux, Git, and more. No less important point is the general theory of computer science (you can check these fundamentals using software developer interview questions).

Beyond technical skills, it is not superficial to determine the personal characteristics of a developer you are looking for. After all, you will communicate with a team a lot, so it is better to choose people who share the same vision or even interests. So, don’t forget to make a list of soft skills.

22 React Interview Questions to Clarify

When it comes to the technical side of a job interview, it may be challenging to determine candidates’ level of proficiency. To check the breadth and depth of knowledge, we suggest the following list of questions with comprehensive answers.

1. What is React, and how does it work?

A quite obvious yet no less important question. A popular front-end library, React focuses on building a UI of a single page web app (SAP) and mobile app. Firstly created by Facebook to handle the internal tasks of the company in 2011, it became open-source and available for everyone in 2013.

It is a server-side templating language with a set of callback-functions. More specifically, it is powered by JSX – a programming language that has a similar syntax to HTML.

The philosophy behind React is to separate UI (aka Presentation layer, or View) into components and work with them separately in real-time. Once these parts are created, you can easily modify a needed element without intervening in the whole structure.

So, it is an ideal tool for those who anticipate visually appealing user interfaces and need a stable platform to handle it.

2. What is the difference between library and framework?

React is often mentioned along with other frameworks like Angular, Node.js, or Vue.js. And although placing all of them together is quite logical, React relates to another specific category since it is a library.

Both frameworks and libraries represent a code written by other programmers to facilitate performing some tasks. The whole difference is that framework runs the code and controls all other environments, whereas the library is something developers use additionally (and control environment somewhere else).

To make this question more complex, ask a candidate to compare Angular and React. For instance, Angular is a full-fledged framework that includes a presentation layer and holds all the control over the program. React, in turn, is an additional instrument that doesn’t interfere with the flow of other processes of development.

3. Name advantages of using React over other libraries/frameworks

Although React is not a universal solution like other frameworks (Angular or Ember), it gives an impression of a tool that does one thing but does it well. You will not find a ‘default router’ or other pre-imposed DI dependencies there. Instead, you can reap a variety of other benefits. Here are some of the most important ones you should expect to hear from a candidate:

• Component-based approach. Everything connected with the user interface can be treated as a separate component, or in other words, JavaScript function. This is salvation in many cases because you can divide everything into classes, modules, or other elements and conduct more comfortable development. A component is a kind of building block that forms the entire user interface. With just a snippet of source code, developers can recognize how each component will eventually look like. You can also perform various manipulations with components like reusing, separating, or arranging them as you want, etc. This approach is especially useful for large-scale projects where there is a need to share code with other developers. Such structure of a web page may look like this:

Image source: Medium

• React Hooks. It is a relatively new feature that came up with the 16.8 version of React. The whole point is that it provides more convenient control of the state of objects. For instance, they can separate the logic of the state mode of a particular element without interacting with the whole architecture. This, in turn, enables to build web applications with a large number of objects faster.
• Isomorphic approach. One more advantage that React provides is the ability to build isomorphic applications. This means you can use the same code for the client and the server-side tasks as well. Consequently, you can use less code, make SEO friendly pages, and render them much faster. On top of that, this advantage allows reducing the costs and time needed for development.
• Virtual DOM. Usual DOM is a tree with thousands of elements, which makes great trouble when there is a need to update only a few items. Virtual DOM solves this problem in seconds since it eliminates the need to interact with the entire document. In simple terms, it is the way to optimize updates and interactions with objects in HTML, XHTML, and XML documents.

All of these benefits make React a great tool for collaboration with other developers and making progress together.

4. What about some limitations of React? Are there any?

If a candidate has a clear idea of limitations, it can be a great sign you are interviewing the right person. As far as this library deals with the presentation layer, potentially it is suitable for all kinds of tasks for web development. However, there is no opportunity to work with AJAX, get an event system (other than native DOM events), get some data layer or promises, and more.

5. What kind of tasks it is possible to do with React? Give examples

Bearing in mind, above all, it is great to cover all tasks connected with front-end development. It gives users a complete experience as far as it allows to add more dynamic qualities for a site and ensures uninterrupted operation.

If diving into more detail, here is a list of tasks you can do using React:

• responsive navigation menu;
• dropdown lists with additional buttons for navigation (like on YouTube left side dropdown list);
• autocomplete (when we need to change something in real-time after pressing the button);
• different charts and graphs;
• admin panels with an opportunity to add/ edit/ change something after clicking;
• drag-and-drop feature and various lists;
• sliders & more.

6. What is React Native?

Other important points you should clarify are React Native interview questions. So, it is another well-used framework that is mainly connected with mobile apps. It allows you to write the code once and run it on all popular platforms like iOS and Android. Therefore, you need only one team of developers to cover all the needs for mobile development.

7. How would you compare React and React Native?

Both increase the efficiency of development, but in different directions. React Native is a whole framework that uses React as the main library and supplements its opportunities. While React is used for creating user interfaces of web apps, React Native is mainly focused on developing mobile apps.

8. When you should use Gatsby? 

It is a fast static generator mainly used to develop static sites, PWAs (Progressive Web Apps), and blog sites that should come with all React benefits. Put it more simply, Gatsby is like a compromise between web development and full-fledged CMS, like WordPress.

9. How would you explain JSX to a non-technical person?

At its core, it is an extension of JavaScript syntax, and it is as simple a thing as plain HTML. The main opportunity it provides is the ability to write code using ready-made components. Put it more simply, it gives templating right in the programming language and makes development more convenient.

10. Contrast class and functional components

Among other interview questions on React.js, one more good point to clarify is to ask a candidate to describe these two components and tell when it is better to choose each of them. When it comes to the answer, expect to hear two differences: syntax and purpose. Moreover, the first component is a class that has a state and allows developers to access stores, whereas the second type is a function that returns some element.

11. Contrast uncontrolled and controlled components

These are two more types of components supported you can face in React. The main feature between them is the way they manage data forms. Uncontrolled elements do it through DOM, whereas controlled elements use other components in React.

One more difference is the internal state: the first type of elements maintain it, whereas the controlled type does not, which makes manipulation of controlled elements much easier.

12. How many components are there in Redux? Explain each of them

It is not superficial to oak a candidate some of the React Redux interview questions since this library is also well-used in web development. So, answering this question, there are four components of Redux:

• Store. It is a JavaScript object that brings Action and Reducer together. It can be considered as the heart of an app since it stores all information.
• Reducer. It is a function, which determines how a state of an app will change to action.
• Action. An action is a JavaScript object that holds information about what happened with an app, actions can be sent to a store using store.dispatch().
• Middleware. It is a way to use custom functionality to extend Redux.

13. Is there a difference between state and props?

They are typically used in tandem to control data flows, and their particular value is to help automatically render and update UI elements. However, they are not interchangeable because there is a distinct difference in how and where they are arranged. For instance, props come from the parent component, whereas the state is originated locally. Apart from this, you can reuse props components, but the state cannot be instantly changed within the component.

14. Can you tell the significance of Firebase?

Firebase is another platform to conduct mobile app development. You can also find other descriptions of Firebase like Backend as a Service (BaaS), which means it is an entire toolset to craft mobile apps of all kinds. All this happens through features it provides: authentication, real-time database, cloud messaging, crash reporting and analytics, and more.

15. What is Flux and why do we need it?

It is a set of programming templates for performing various tasks connected with user interfaces. Its main purpose is to help create dynamic applications faster.

Flux augments the capabilities of React. While the latter one is only responsible for the presentation layer or View, Flux holds the part of Model in the MVC paradigm, which aim is to maintain an application’s behavior and data.

16. Name components of Flux architecture

Standard architecture consists of four main elements that interact with each other:

• Actions. They possess two features: name and payload.
• Dispatcher. This element receives data from Actions and transmits its further to subordinate elements. It is like a specific repository, where callbacks are registered and then we can determine further connections with other components.
• Stores. These elements act as containers that store information about the app’s logic.
• Views. This is the endpoint of the data route. These elements are responsible for showing content to a user.

Image credit: GitHub

17. Compare Redux and Flux

Unlike Redux, which represents just a library, Flux is a whole pattern. Both of them have actions, but they treat them differently. That’s why it is important to keep in mind the following differences:

18. How code base differs for iOS and Android devices in React Native?

A simple yer tricky question at the same time. As a matter of fact, there are no differences since React Native provides an advantage to use the same code base for all platforms.

19. Give an example of how to use synthetic events

This type of events is created opposite to real events. Their main purpose is to normalize events and make sure they have the same properties across different browsers and platforms.

20. What is meant by the process of reconciliation?

It is a widespread process in React, so it is necessary to clarify whether your candidate knows how it works. It is primarily linked to updating data and making changes more predictable.

If to delve into more details, reconciliation reproduces the following procedure: once a component’s props or state undergoes changes, React compares it with the previous alteration. If these two changes are different, we get reconciliation.

21. Describe the purpose of React Fiber

At its core, React offers several ways to store a state. Basically, it is a tree of internal elements like DOM nodes. But since these elements did not always cope excellently with increased performance tasks (like animation), starting from version 16, React presented a more advanced algorithm for this purpose called Fiber.

Its main advantage is the ability to divide render performance into units and distribute them across multiple frames. Using Fiber, developers can pause or cancel the playback of DOM tree updates at any time. It is also possible to configure the prioritization of different updates.

22. How to conduct re-rendering of a component?

This may be a final point you can touch on while preparing the list of React.js interview questions and answers. The most comfortable way to invoke rerendering is to change the state and props. Moreover, it is possible to use methods like this.setState and this.forceUpdate.

Project Success Starts With A Team

Every specialist in your team can be viewed as a puzzle piece in a much larger canvas. Your final result will mainly depend on how your team collaborates and whether everyone shares a common goal. That’s why it is so crucial to approach the task of hiring and forming a team correctly. Be sure you have picked the person who will add new experiences and bring inspiration to your company culture.

Struggling to build a team? Hire React developers with Bridge. Since we have access to a tremendous pool of talents worldwide, we can quickly find the best fit for your project’s requirements and budget.

We are more than global recruitment because we are focused on organizing a perfect team for your idea. That’s why we manage teams and care about your workers’ comfort so that they can be a valuable asset to your mission.

If you are curious about the pricing, run numbers of needed specialists’ salary using our smart calculator.

Have something to ask? Don’t hesitate to contact us.