Top 20 React Interview Questions & Answers You Need To Know in 2021

React is one of the fastest-growing JavaScript frameworks in the market today. If you are an aspiring front end developer, we have compiled some important React interview questions that will help you learn all the major concepts. 

Building user interfaces for single-page or mobile applications becomes comfortable with React. And you are most likely to be quizzed about this tool in job interviews. React certifications and crash courses are also in high demand due to this very reason. 

So, here are the top React interview questions to help you make an excellent first impression. 

Top React Interview Questions and Answers 

1. Compare real DOM and virtual DOM

Although real DOM updates slow, it can update HTML directly. It creates a new DOM if an element updates. However, DOM manipulation is costly in this case and can cause substantial memory wastage.

Virtual DOM can update faster and updates the JSX if element updates. It cannot directly update the HTML. But DOM manipulation is easy in this programming concept. And there is no issue of memory wastage. 

2. Explain React in simple terms

React is a JavaScript library developed by Facebook in the year 2011. It became open-source in 2015, gaining popularity among a community of individual developers and companies. React comes handy in the development of complex and interactive mobile and web UI. It uses a component-based approach for building reusable components. 

3. List some features of React

React uses virtual DOM and server-side rendering. Also, it follows the principle of data binding, which is a unidirectional data flow. These three features provide a clear overview of React.

4. What are the advantages of React? Also, list some of its limitations.

React is not difficult to integrate with other JavaScript frameworks like Meteor, Angular, etc. Writing test cases of UI becomes convenient with this tool. It is an easy tool to use for both the client and on the server-side. Another advantage of React is that it improves app performance. Also, the code has high readability due to JSX.

Now let’s look at some disadvantages.

Firstly, React is a library and not a full-fledged framework. Inline templating and JSX can make the coding complex, which can be a task for novice programmers to grasp. Since the React library is vast, understanding it can be a time-consuming process.  

5. What is React JSX?

JSX is the short form of JavaScript XML. It converts HTML tags into React elements, capturing the expressiveness of JavaScript with a syntax like HTML. JSX converts HTML tags into react elements. Such a type of file is easy to understand and results in robust and high-performance applications. 

6. Can browsers read JSX?

No, browsers can only read JavaScript objects. So, we need to enable the browser to read the JSX. This exercise involves converting the JSX file into a JavaScript object before passing it on to the browser. Transformers like Babel can be used for the same. 

7. Explain the working of virtual DOM

A virtual DOM is initially a lightweight copy of the real DOM. It is a JavaScript object in the form of a node tree. The render function in React creates a node tree from the React components containing the elements, attributes, and object properties. Various actions by the user or system cause mutations in the data model and update this tree. The three-step process in virtual DOM works in the following way:

  1. The entire UI gets re-rendered when the underlying data changes
  2. Then, the new virtual DOM representation is compared with the previous one, and the difference is calculated
  3. The real DOM is updated considering the actual change or difference

8. Differentiate between React and Angular.

Angular uses the real DOM and two-way data binding as compared to the virtual DOM and one-way data binding in React. It has run-time debugging and not compile time debugging like React. Also, Angular is maintained by Google, while React is a Facebook product. 

9. “Everything is a component in React.” Do you agree?

The user interface of a React application is made up of building blocks called components. The components break the entire UI into independent, reusable pieces. These fragments are then rendered independently of the rest of the UI.  

10. What is the purpose of render() in React?

Each component in React has a render(), which returns a single element representing the native DOM component. Elements are grouped together when more than one HTML element has to be rendered. The elements are grouped inside closing tags like <group>, <form>, <div>, etc. Whenever invoked, this function must return the same result.

These were some general React interview questions and answers to assist you in your preparation. Let us cover some more to develop more understanding of the concepts. 

11. What are props in React?

Properties are called ‘props’ in React. These are read-only, and immutable components passed down from the parent to the child throughout the application. It is integral to maintain the unidirectional flow of the data, especially when it is generated dynamically. Therefore, the child component cannot send props back to the parent component. 

12. Explain state in React

State is the heart of a component in React that is accessed using this.state(). States are basically the source of data that determine the components’ rendering and behavior. Unlike props, states are mutable objects that create interactive components. 

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13. What are the phases in the lifecycle of a React component?

The lifecycle of a React component has three main phases. These are:

  • Initial Rendering: The component makes its way to the DOM.
  • Updating: The component updates or re-renders as props or state changes occur.
  • Unmounting: Component is destroyed and removed from the DOM. 

14. How are refs used in React?

We can return references to a particular element returned by render(). Refs attribute makes this possible. So, refs store a reference to a React component to be returned by the render configuration function. We generally use refs to add methods to components or measurements to DOM. 

15. Controlled vs. Uncontrolled components

The primary difference between controlled and uncontrolled components is that the former get their current values through props and the latter through refs. Controlled components do not keep their own state as changes can be notified via callbacks. The parent component controls their data. On the other hand, uncontrolled components maintain their state, and the DOM controls their data. 

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16. Explain events in React

In React, specific reactions like the keypress, mouse hover, clicks, etc. trigger reactions known as events. An event argument contains its own set of properties and behavior, capable of being accessed by the event handler alone. Events are passed as functions and named using the camelCase.

17. What do you mean by Flux?

Flux is an architectural pattern that provides stability to the application by reducing run-time errors. It uses a central ‘store’ to enable communication between different components, maintaining authority over the data. All updates throughout the application must occur here only.

18. What is Redux? What are its components?

Redux is a predictable state container used for state management of JavaScript applications. It uses a single state tree or ‘Store’ to stock the entire application’s state in one place.

Redux is composed of the following parts:

  • Action: An object describing what happened
  • Reducer: Determines how the state will change
  • Store: Tree of the entire application comprising objects and states
  • View: Displays the data given by the Store

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19. What is the significance of Reducers?

Reducers specify how the application’s state would change in response to specific actions. Based on the type of activity, reducers determine what updates are required and then return new values. They return the same previous state if no change is necessary.

20. What is a React Router?

React Router is a routing library that helps add new screens and flows to JavaScript applications. You add this library to your app to create multiple routes, each leading to a unique page. The URL matches what is being displayed on the web page. 

With this, we have covered most of the React interview questions that you will face in any interview. Being thorough on all these themes will help land that front-end development job!

If you’re interested to learn more about React, check out upGrad & IIIT-B’s PG Diploma in Full-stack Software Development which is designed for working professionals and offers 500+ hours of rigorous training, 9+ projects, and assignments, IIIT-B Alumni status, practical hands-on capstone projects & job assistance with top firms.

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