Top 40 React Interview Questions and Answers

React is in high demand in the job market due to its numerous advantages and widespread adoption. Possessing React skills can help you land a job as a mobile app developer, front-end developer, or full-stack developer. Here are the top 40 React interview questions and answers for freshers and experienced candidates. Get started with our React course syllabus.

React JS Interview Questions and Answers for Freshers

Here are the React JS interview questions and answers for freshers:

1. What is ReactJS?

An open-source JavaScript package called React is used to create user interfaces, mostly for single-page apps. It is renowned for its effective rendering through the use of a virtual DOM and component-based design. 

2. What’s the purpose of ReactJS?

The uses of ReactJS are:

Build UIs: Developers may construct user interfaces for web and mobile applications with the aid of ReactJS.

Creates Reusable Components: With ReactJS, developers can make reusable elements like product cards, navigation bars, and buttons.

Enhances the User Experience: Without re-rendering the entire page, ReactJS can display portions of the user interface that are changing.

Build Scalable Applications: Developers can create scalable, dependable, and quick online apps with ReactJS.

3. What distinguishes JavaScript from ReactJS?

The primary programming language used to create websites is JavaScript. A JavaScript package called ReactJS makes creating graphical user interfaces easier. 

4. When does one use ReactJS?

• Applications with rapidly changing and dynamic data are best suited for ReactJS.
• Both single-page and multi-page interfaces can be created with ReactJS. 

5. Explain the concept of JSX.

With the help of the syntactic extension JSX (JavaScript XML), you can construct HTML-like code inside of JavaScript. It facilitates the writing and comprehension of UI components and enhances readability.

Hone your skills with our JavaScript training in Chennai.

6. What is the Virtual DOM in React?

A lightweight in-memory representation of the real DOM is called the Virtual DOM. Every time data changes, React updates the Virtual DOM, determines the fewest necessary adjustments, and then effectively updates the actual DOM to improve performance. 

7. What are components in React?

The fundamental units of React applications are called components. 

They are reusable UI components that combine display and logic. Class-based (stateful) or functional (stateless) components are also possible. 

There are two types of components such as functional components and class components.

8. What is the difference between functional and class components?

JavaScript functions that return JSX are called functional components in React, whereas JavaScript classes that extend React.Component are called class components. 

9. Describe this concept of props.

Read-only values known as “properties” (short for “properties”) are transferred from parent components to child components. They enable data and configuration information to be sent between components. 

10. What is the state in React?

A component’s state is an internal data store that may be modified and re-rendered. Dynamic data that changes over time is managed with it.

• The constructor initializes the state object.
• The state object can be modified using the this.setState() function.
• A shallow merge between the new and old states is carried out by the this.setState() function.

11. How do you handle events in React?

Event handlers are special properties applied to elements in JSX that are used to handle events in React. The on prefix is commonly used to name event handlers (e.g., onClick, onChange).

Steps to handle events in React:

• Create a function that handles events.
• Assign an event attribute to the event handler function.
• Give an element the event handler function as a prop.

Example:

onClick={handleClick}

Recommended: React JS Online Course Program.

12. What are keys in React lists?

Each element in a list is given a unique identifier called a key. When data changes, they enable React to track changes effectively and re-render the list. Within the list, keys ought to be consistent and distinct.

13. What are hooks in React?

You can “hook into” React state and lifecycle aspects from functional components using hooks. Examples: UseState, useEffect, useContext, and useReducer.

14. What are the functions of hooks?

The functions of hooks in Reacts are:

• State management: Developers can monitor data (state) within function components by using hooks. 
• Lifecycle features: When a component loads or updates, hooks let developers take action. 
• Reusable logic: To reuse stateful functionality across components, developers can design their own Hooks.

15. How do Hooks work? 

• Functions that begin with the word “use” are known as hooks.
• Function components are the sole way to call hooks.
• Only at a component’s top level can hooks be called.
• Conditional hooks are not possible.

16. List some examples of hooks in React.

• useState(): Enables components to store information and modify their display in response to memory usage.
• useEffect(): Assists developers with tasks like data retrieval and subscription setup.
• useMemo(): Allows programmers to store the outcome of a costly computation.
• useCallback(): Before sending a function definition to an optimized component, callback() enables developers to cache it. 

17. Explain the concept of context API.

Sharing data between components is made possible by the Context API, which eliminates the need to pass props down through several tiers of the component tree.  

By essentially creating a “global” state that can be accessed by any component regardless of its nesting depth, the Context API in React. 

It solves the problem of “prop drilling” by enabling components at any level of a React application to access and share data without explicitly passing props through every component in the hierarchy.

18. What is a higher-order component (HOC)?

A function that accepts a component as input and outputs a new, improved component is known as a HOC. HOCs allow components to share functionality without duplicating code.

19. Describe the React Router concept.

A well-liked library for managing routing and navigation in React applications is called React Router. It enables you to construct a single-page application experience, match URLs to components, and establish routes.

20. What is the difference between controlled and uncontrolled components?

Suggested: Web Development Training in Chennai.

React JS Interview Questions for Experienced

Here are the React JS Interview Questions for Senior Developer with Answers:

1. What are lifecycle methods in class components?

Lifecycle methods, such as componentDidMount, componentDidUpdate, and componentWillUnmount, are unique methods that are invoked at various points during a component’s lifecycle.

Common lifecycle methods:

• constructor
• static getDerivedStateFromProps
• render
• componentDidMount
• shouldComponentUpdate
• componentDidUpdate
• componentWillUnmount

Example:

class MyComponent extends React.Component {

  constructor(props) {

    super(props);

    this.state = { data: null };

  }

  componentDidMount() {

    fetch(‘https://api.example.com/data’) 

      .then(response => response.json())

      .then(data => this.setState({ data }));

  }

  render() {

    return (

      <div>

        {this.state.data && <p>{this.state.data}</p>}

      </div>

    );  }

}

2. What is code splitting in React?

Code splitting is a strategy that entails dividing the code of your application into manageable sections. By loading only the required code when required, this reduces initial load time and enhances overall performance.

How it operates

• Split code: The code of the application is divided into more manageable sections or modules.
• Load on demand: Only when necessary, like when a user switches to a new page, are the chunks loaded.
• Enhance performance: The program scales better and the initial load time is shortened.

Advantages of Code Split: 

• Improved user experience: The user experience has been enhanced, particularly on sluggish or unstable networks. 
• Shorten load times: The application’s load times have been greatly shortened. 
• Enhance scalability: The program scales more effectively. 

Ways to Implement Code Split:

• Apply the syntax for dynamic import().
• Make use of built-in features such as Suspense and React.lazy().
• Make use of programs like Browserify, Rollup, and Webpack.

Explore our full-stack developer training in Chennai.

3. Explain the concept of lazy loading and suspense.

Lazy Loading: Lazy loading reduces the initial load time by loading components as needed.

Important details regarding lazy loading

• Enhances performance: Lazy loading shortens a web page’s initial load time by postponing the download of elements that might not be immediately apparent to the user, which makes the user experience seem faster.
• Code splitting is a common technique that divides your program into smaller bundles that are loaded only when necessary.
• Use Cases: Perfect for lengthy lists where only a section is initially visible, enormous image galleries, or content that may not be needed on every page load. 

Suspense: A feature of React that lets you “wait” for a component to load before rendering it is called suspense. 

• Fallback UI: Suspense’s primary purpose is to visually alert the user that something is loading by displaying a “fallback” component while a lazy-loaded component is being retrieved.
• React Integration: To wrap a lazy-loaded component in React and specify the fallback content to be displayed while loading, use the Suspense component.
• Dynamic Imports: Frequently used to load components as needed in combination with dynamic imports (import() function).  

How they collaborate:

• Using suspense while loading slowly: Usually, a Suspense component is used to encapsulate a lazy-loaded component in a React application.
• Loading indicator: The Suspense component will render the designated fallback user interface (such as a loading spinner) until the lazy-loaded component is prepared for display while it is being fetched.

4. What are some performance optimization techniques in React?

Utilizing a Content Delivery Network (CDN), optimizing images, eliminating pointless re-renders, and memoization. The goal is to load only necessary components when needed, minimize needless re-renders, and effectively manage large data sets. 

Some important React performance optimization strategies:

• Lazy loading (code splitting).
• Using React.memo for memoization.
• Optimizing images.
• Virtualizing lists.
• Avoiding needless re-renders.
• Using React Fragments.
• Throttling and debouncing events.
• Using the React Profiler to identify performance bottlenecks.
• Avoiding inline functions.
• Optimizing state management.
• Using useMemo and useCallback hooks.
• PureComponents.
• Key prop usage.

Enroll in our MERN Stack training in Chennai.

5. How do you test React components?

React components are usually tested using a testing library like “React Testing Library” and a test runner like Jest. 

This ensures that your components work as intended when users interact with them by rendering the component in a simulated environment, interacting with it as a user would, and confirming that the rendered output meets your expectations. 

This approach focuses on testing the component’s behavior from the user’s perspective rather than implementation details.  

To make sure your components operate as intended, you can create unit and integration tests using libraries like Jest and Enzyme.

• Testing Library: React Testing Library is the recommended method for the majority of React component testing.
• Rendering and Interaction: After rendering the component in a virtual DOM using functions like render, you employ methods like screen.either screen or getByText.obtainByRole.
• Assertions: You utilize Jest’s assertion methods to confirm that the rendered output satisfies your expectations following your interaction with the component.

Example:

import { render, screen } from ‘@testing-library/react’;

import UserGreeting from ‘./UserGreeting’; // Your component

test(‘renders the correct greeting message’, () => {

    render(<UserGreeting name=”John” />);

    const greetingElement = screen.getByText(‘Hello, John!’);

    expect(greetingElement).toBeInTheDocument(); 

});

Explore what is the TCS salary in Chennai and enhance your skills accordingly.

6. What are shallow rendering and full rendering in React testing?

The “shallow rendering” refers to rendering a component only at its top level, concentrating only on its own structure and props. The “full rendering” entails rendering a component entirely, including all of its child components, enabling testing of lifecycle methods and interactions between the component and its children.

Shallow Rendering: 

Shallow rendering doesn’t render any child components; it just renders the component itself.

• Focus on the component itself: Only the component’s immediate output is tested; the behavior of its descendant components is not. 
• Faster testing: It is typically faster than full rendering because it does not render child components. 
• Ideal for unit tests: It is helpful for independently confirming a component’s properties and structure. 

Full Rendering: 

Rendering the component and all of its descendant components is known as full rendering. 

• Complete component tree: Enables testing of inter-component interactions by rendering all child components. 
• Lifecycle methods: Enables testing of component lifecycle methods like componentDidMount and componentWillUnmount. 
• More complex scenarios: Ideal for testing situations in which the behavior of a component is dependent on its children or DOM interactions. 

When to use which:

• When you want to rapidly test a component’s fundamental structure and props without worrying about its child components, use shallow rendering.
• when testing a component’s handling of user events, interactions with its children, or reliance on lifecycle methods is necessary.

7. How do you fetch data in React?

Handling responses, doing API calls, and utilizing the useEffect hook with fetch. Using libraries like SWR or Axios for more complex data retrieval requirements.

Steps to fetch data in React:

• Make an application using React.
• Modify the Project Directory.
• Use yarn or npm to install the Axios Library.
• Go to the API Endpoint.
• Set the useState() Hook to Hold Data after importing the Axios.
• To retrieve and store data, create a callback function called FetchInfo(). 

8. What is data fetching with async/await?

Utilizing the async/await paradigm to write data fetching code that is more readable and succinct.

Making asynchronous HTTP queries in JavaScript is made easier by using fetch in conjunction with async/await, which also makes your code easier to read and maintain. It enables you to transmit data to APIs, retrieve data from other sources, and gracefully manage errors.

9. What is Redux?

Redux is a JavaScript application container with predictable states. It facilitates effective and centralized application state management.

An open-source JavaScript package called Redux is used to centralize and manage application state. It is employed in the development of user interfaces for applications in which components exchange data.  

• It keeps track of and controls every condition of the application.
• It offers APIs for altering the current state and retrieving the application’s current state.
• It helps in creating apps that are easy to test, behave consistently, and function across a variety of contexts. 

Uses of Redux:

• Redux is most frequently used with libraries like React or Angular, however it may be used with any library or framework.
• Redux can be utilized in the backend (server side).
• Redux can be used to save the state of browser-based multiplayer games. 

10. Explain the core concepts of Redux: store, actions, reducers.

• Store: Preserves the complete state of the application.
• Actions: Simple JavaScript objects that provide a description of the event.
• Reducers are pure functions that return a new state after receiving an action and the current state. 

11. What is the difference between Redux and React Context API?

Redux offers a more reliable and scalable way to manage global application state, albeit being more complicated. For smaller applications, the React Context API is more straightforward and frequently adequate.

Our AngularJS training in Chennai helps you gain the basic understanding.

12. What is the purpose of middleware in Redux?

The Redux store’s capabilities are expanded by middleware. Before an action reaches the reducer, it enables you to intercept and alter it.

• In Redux, middleware serves as an extension point that lets you intercept actions before they get to the reducer. 
• This lets you do other things like handle asynchronous requests, log actions, modify actions, or add custom logic to your state management flow.
• It gives you a way to improve the dispatch function’s behavior beyond simply updating the state.  

Example use cases of middleware:

• Redux-Thunk: It is a well-liked middleware that lets you handle asynchronous activities in your actions by using dispatch functions rather than simple objects.
• Redux-Saga: Another middleware that offers a more reliable method of handling asynchronous side effects with generators and sagas is Redux-Saga.
• Redux-Logger: For debugging reasons, Redux-Logger is a middleware that records every action that is delivered to the console.

13. How do you deploy a React application?

To make your React application available online, you must first use npm run build to create production-ready static files. Next, you must select a hosting platform such as Netlify, Heroku, AWS Amplify, or GitHub Pages, and push your created files to that platform. This usually entails connecting your repository and configuring automatic deployment for smooth updates.  

• Deploying to systems such as AWS, Vercel, Heroku, or Netlify.
• Using yarn build or npm run build, the application is built for production.

14. Which security best practices apply to applications built with React?

Some of the best practices to build React applications:

• Validating user input.
• Avoiding direct DOM manipulation.
• Using secure authentication techniques.
• Updating React and its dependencies.
• Sanitizing HTML.
• Using secure server-side rendering.
• Avoiding insecure libraries.
• Routinely scanning for vulnerabilities. 

In other words, making sure that input is validated correctly, handling data securely, and staying current with security patches for React and its dependencies. 

15. What does React’s accessibility mean?

Accessibility in the context of React refers to creating web applications that are usable by everyone, including those with disabilities using features like 

• Proper semantic HTML.
• Keyboard navigation.
• Screen reader compatibility.
• Enough color contrast. 

This ensures that users can access and interact with the application efficiently regardless of their visual, auditory, motor, or cognitive abilities.  

It is about ensuring that those with impairments can use your React application and it is utilizing semantic HTML, ARIA features, and WCAG accessibility requirements.

16. If you had to optimize a React component that was rendering slowly, how would you respond?

React components can be broken up into smaller pieces, memoized with `React. memo`, or optimized with `shouldComponentUpdate`. `useCallback` and `useMemo` help stop needless re-renders for state management bottlenecks. Re-profile after making modifications to guarantee gains.

• Use profiling tools to find the bottlenecks.
• To cut down on pointless re-renders, use memoization.
• Make pictures better.
• Put lazy loading into practice.
• For lengthy lists, think about using virtualized lists. 

17. Explain how you go about creating an advanced React application.

The steps to create advanced React application:

• Divide the application into more manageable, smaller parts.
• Create the state management plan (e.g., Context API or Redux).
• Create unit tests for every part.
• Use linting tools and maintain a consistent code style.

18. What would you do if a React application had errors?

The first step in resolving faults in a React application is to use the browser developer tools console to examine the error message and stack trace. 

Then, closely examine the pertinent code to determine the problem, possibly with the help of debugging tools like console.log statements to monitor variable values.

Depending on the type of error, you may need to correct syntax, deal with logic problems in your components, manage asynchronous operations appropriately, verify user input, or apply error boundaries to handle runtime errors gracefully and give the user informative feedback.  

• To catch errors, use try-catch blocks.
• Show comprehensible error messages.
• To detect and manage faults within a subtree of the component tree, use error boundaries.

19. Describe the methodology you would use for a React project that calls for regular data updates.

• To manage side effects and get data, using the useEffect hook.
• Use optimization strategies for data fetching (e.g., caching, debouncing).
• For effective data fetching and caching, think about utilizing frameworks like SWR or React Query. 

20. In a React application, how would you go about incorporating a third-party library?

In order to integrate a third-party library into a React application, you usually use a package manager such as npm (Node Package Manager) to install the library, import the required components or functions into your React components, and then utilize the library’s functionality in your JSX code.

• Carefully read the library’s documentation.
• Use yarn or npm to install the library.
• Import and utilize the library’s features or components as required.

Explore all our software training programs for your bright career in IT.

Conclusion

From basic concepts to advanced strategies, this extensive list of React interview questions and answers covers a lot of ground. You can show that you understand React and improve your chances of getting hired by carefully preparing for these questions. Utilize our React JS training in Chennai for a bright career in web development.