React JS Interview Questions in Top 30

React JS continues to dominate the world of frontend development in 2025, powering dynamic, high-performance web applications. If you’re preparing for a React JS interview, being well-versed in the latest trends and core concepts is crucial. This blog post lists the Top 30 React JS Interview Questions that will help you prepare effectively and stand out from the competition.

1. What is React JS, and why is it so popular?

React JS is a JavaScript library for building user interfaces, developed and maintained by Facebook. Its popularity stems from its virtual DOM, component-based architecture, and efficient rendering, enabling developers to create fast, scalable, and reusable UI components.

2. What are the key features of React?

JSX: A syntax extension for JavaScript that allows embedding HTML in JS.
Virtual DOM: Optimizes UI updates by comparing previous and current DOM states.
Component-Based Architecture: Encourages code reusability and modularity.
Unidirectional Data Flow: Ensures predictable state management.
Hooks: Introduced in React 16.8, allowing functional components to manage state and side effects.

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

Class Components are ES6 classes with render() methods and lifecycle methods.
Functional Components are simpler and can manage state and lifecycle using React Hooks like useState and useEffect.

4. What is the virtual DOM? How does it work?

The virtual DOM is a lightweight representation of the actual DOM. React updates the virtual DOM, compares it with the previous version (using a process called reconciliation), and applies only the necessary changes to the real DOM for optimal performance.

5. Explain React’s lifecycle methods.

React class components have lifecycle methods like:

Mounting: constructor, render, componentDidMount
Updating: shouldComponentUpdate, render, componentDidUpdate
Unmounting: componentWillUnmount
In functional components, lifecycle behavior can be replicated using hooks like useEffect.

6. What are React Hooks? Name a few commonly used hooks.

Hooks are functions introduced in React 16.8 that let you use state and lifecycle features in functional components. Common hooks include:

useState
useEffect
useContext
useReducer
useRef

7. What is JSX?

JSX (JavaScript XML) is a syntax extension that allows writing HTML-like code in JavaScript. It is transpiled to React.createElement() calls, making it easy to describe UI structures.

8. How does state differ from props?

State: Managed within a component and can change over time.
Props: Passed from parent to child components and are immutable.

9. What is the purpose of the key attribute in React?

The key attribute helps React identify and track elements efficiently during rendering. It must be unique among siblings to prevent unnecessary re-renders.

10. What is Context API, and how is it used?

The Context API allows sharing state across components without prop drilling. It is often used with React.createContext and useContext.

11. What is React Router? Why is it used?

React Router is a library for routing in React applications, enabling navigation without full-page reloads. Features include dynamic routing, nested routes, and URL parameters.

12. Explain lazy loading in React.

Lazy loading delays the loading of components until they are needed, improving performance. It is implemented using React.lazy() and Suspense.

13. What is the difference between useEffect and useLayoutEffect?

useEffect: Runs after rendering and is non-blocking.
useLayoutEffect: Runs synchronously after DOM mutations, blocking visual updates until complete.
You can check our own guide about useEffect here!

14. How is state management handled in React?

State management can be done with:

React’s built-in state (useState, useReducer)
Context API
State libraries like Redux, MobX, or Zustand

15. What is Redux? How does it relate to React?

Redux is a state management library that centralizes app state, enabling predictable state changes. React integrates Redux via libraries like react-redux.

16. What are higher-order components (HOCs)?

HOCs are functions that take a component and return an enhanced component. They are used for reusing component logic, e.g., authentication or theming.

17. What is Prop Drilling, and how can it be avoided?

Prop Drilling refers to passing props through multiple layers of components unnecessarily. It can be avoided using the Context API or state management libraries like Redux.

18. What is the useCallback hook? Why is it used?

useCallback memoizes callback functions, preventing them from being recreated during re-renders. It improves performance by reducing unnecessary computations.

19. How does React handle forms?

React handles forms using controlled components (stateful inputs) or uncontrolled components (inputs using refs).

20. What is the difference between useRef and createRef?

useRef: Used in functional components, maintaining the same reference across renders.
createRef: Used in class components, creating a new reference each time.

21. Explain the role of Error Boundaries.

Error Boundaries are components that catch JavaScript errors in their child components and display fallback UIs. They are implemented using componentDidCatch and getDerivedStateFromError.

22. What are React Portals?

Portals allow rendering children outside the parent DOM hierarchy, useful for modals and tooltips. Created using ReactDOM.createPortal.

23. How does React handle reconciliation?

Reconciliation is React’s process of updating the DOM efficiently. It uses a diffing algorithm to compare virtual DOM trees and apply minimal updates.

24. What is memoization in React?

Memoization optimizes performance by caching computations or component renders. React provides React.memo and hooks like useMemo and useCallback for this purpose.

25. What is Server-Side Rendering (SSR) in React?

SSR generates HTML on the server instead of the client, improving SEO and load time. Frameworks like Next.js simplify implementing SSR.

26. Explain React’s event handling mechanism.

React’s events are normalized across browsers via SyntheticEvent, ensuring consistency and preventing memory leaks.

27. What are fragments in React?

Fragments allow grouping multiple elements without adding extra nodes to the DOM. Created using or shorthand .

28. How does React differ from frameworks like Angular?

React is a library focused on UI, while Angular is a full-fledged framework.
React uses a virtual DOM, while Angular uses a real DOM.
React emphasizes flexibility with third-party integrations, while Angular has a more opinionated structure.

29. What is the significance of React’s StrictMode?

StrictMode identifies potential problems in an application by enabling additional checks during development.

30. What are some best practices for React development in 2025?

Use functional components and hooks over class components.
Optimize performance with memoization and lazy loading.
Write reusable and modular components.
Adopt a state management strategy based on app complexity.
Leverage TypeScript for type safety.

Tips to Prepare for a React JS interview questions

1. Master the Basics of JavaScript

React is built on JavaScript, so having a solid foundation in core JavaScript concepts is essential. Topics such as closures, promises, ES6 features (like arrow functions, destructuring, and modules), and async/await are frequently asked in interviews.

Pro Tip: Practice writing small JavaScript functions and understand how they work under the hood. Websites like LeetCode and Codewars are great for improving your problem-solving skills.

2. Build Real-World Projects

Practical experience speaks louder than theoretical knowledge. Create small React applications to showcase your skills. Some popular project ideas include:

A to-do list with CRUD operations.
A weather app using an API like OpenWeatherMap.
A portfolio website showcasing your resume and projects.
Pro Tip: Deploy your projects on platforms like Netlify or Vercel to demonstrate your deployment skills.

3. Learn State Management Libraries

While React’s built-in state management with hooks is powerful, many interviewers expect candidates to know tools like Redux, Zustand, or MobX. Understanding Redux Toolkit and concepts like reducers, actions, and selectors can be a big plus.

Pro Tip: Build a project with Redux Toolkit to understand its advantages, such as simpler syntax and less boilerplate code.

4. Explore Advanced React Concepts

To excel in interviews, go beyond the basics. Understand concepts such as:

React Context API
Server-Side Rendering (SSR) with frameworks like Next.js
React Suspense and Concurrent Mode for optimizing performance
Pro Tip: Spend time exploring the React documentation to solidify your understanding of these advanced topics.

5. Prepare for System Design Questions

For senior or mid-level roles, you may face questions about architecting a React application. Be prepared to discuss:

How to structure your components.
When to use state management libraries.
Best practices for managing large-scale React applications.
Pro Tip: Practice explaining your project structure to a friend or mentor. Clarity in communication is as important as technical expertise.

6. Understand Testing in React

Testing is a critical skill in modern React development. Learn how to write:

Unit tests using Jest
Component tests using React Testing Library
End-to-end tests using Cypress
Pro Tip: Focus on writing simple, clear test cases to cover common scenarios. Avoid overcomplicating your test logic.

7. Stay Updated with the Latest React Trends

React evolves constantly. New features like React Server Components and improvements in the React 18 concurrent rendering model are hot topics for 2025.

Pro Tip: Follow the official React Blog and prominent React developers on platforms like Twitter or LinkedIn to stay informed.

8. Brush Up on Problem-Solving Skills

Many companies include a coding round where you might need to solve general programming problems using JavaScript or React. Practice solving algorithms and understanding data structures like arrays, objects, and trees.

Pro Tip: Use platforms like HackerRank or AlgoExpert for focused practice.

9. Learn Deployment and CI/CD Basics

Employers appreciate developers who can deploy applications. Understand how to:

Deploy a React app on platforms like AWS, Vercel, or Netlify.
Use CI/CD tools like GitHub Actions to automate deployment.
Pro Tip: Practice deploying your projects and document the steps for quick reference during interviews.

10. Practice Mock Interviews

Mock interviews can help you identify your weak points and improve your confidence. You can:

Pair up with a friend for a peer interview.
Use platforms like Pramp or Interviewing.io for professional mock interviews.
Pro Tip: After each mock interview, take notes on areas where you struggled and revisit those topics.

Conclusion

By reading this article, you have seen the most asked React JS interview questions. Mastering these questions will prepare you for React JS interviews in 2025. Stay updated on the latest React features and trends, and practice implementing them in real-world projects. Good luck with your interview and with your React JS interview questions learning!

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