Top 30 ReactJS Interview Questions and Answers for Experienced Developers
React JS has become the de facto standard for building dynamic and scalable front-end applications. If you’re preparing for a React JS interview as an experienced developer, you can expect questions ranging from React fundamentals to advanced concepts such as hooks, state management, performance optimization, and integration with other technologies.
Here’s a curated list of 30 commonly asked React JS interview questions with answers to help you ace your next interview.
1. What are the main features of React?
React is a JavaScript library for building user interfaces. Key features include:
- Virtual DOM for efficient rendering.
- Component-based architecture.
- One-way data binding.
- Support for hooks and functional components.
- Strong ecosystem and community support.
2. What is the difference between functional and class components?
- Class components: Use
classsyntax, support lifecycle methods, can hold local state. - Functional components: Use plain functions, rely on hooks (
useState,useEffect, etc.) for state and lifecycle. Preferred in modern React for simplicity and performance.
3. What is JSX?
JSX (JavaScript XML) is a syntax extension that allows you to write HTML-like code inside JavaScript. It improves readability and makes component creation more intuitive.
4. How does Virtual DOM work?
React maintains a virtual copy of the DOM:
- Updates are first applied to the virtual DOM.
- React compares (diffing algorithm) the new virtual DOM with the previous one.
- Only the changed nodes are updated in the real DOM (reconciliation).
5. Explain React’s component lifecycle.
For class components, phases are:
- Mounting (
constructor,componentDidMount). - Updating (
shouldComponentUpdate,componentDidUpdate). - Unmounting (
componentWillUnmount).
In functional components, lifecycle logic is handled using hooks (useEffect).
6. What are React hooks?
Hooks let you use state and lifecycle features in functional components. Common hooks:
useState– manage state.useEffect– side effects.useContext– consume context.useMemo,useCallback– performance optimization.useReducer– alternative state management.
7. What are controlled vs uncontrolled components?
- Controlled components: Form inputs managed by React state (
valueandonChange). - Uncontrolled components: Use
refto directly access DOM values, less React involvement.
8. What is Redux, and why use it?
Redux is a state management library based on:
- Single source of truth (store).
- State is read-only (immutable).
- Changes via pure functions (reducers).
It’s used for predictable state management across complex apps.
9. How does React Context differ from Redux?
- Context API: For prop drilling avoidance, best for small to medium apps.
- Redux: More powerful, offers debugging tools, middleware, and complex state handling.
10. What is React Fiber?
Fiber is the new reconciliation engine in React (from v16). It enables incremental rendering, prioritization of tasks, and smooth UI updates.
11. How do you optimize React app performance?
- Use React.memo and PureComponent.
- Lazy load components with
React.lazyandSuspense. - Use
useMemoanduseCallback. - Code splitting with Webpack.
- Avoid unnecessary re-renders.
- Optimize lists with
react-windoworreact-virtualized.
12. What is code splitting in React?
Code splitting allows you to split code into smaller bundles that can be loaded on demand, reducing initial load time. Achieved using React.lazy and Suspense or tools like Webpack.
13. What are higher-order components (HOCs)?
HOCs are functions that take a component and return a new component with additional functionality. Example: withRouter, connect from Redux.
14. What is the difference between useEffect and useLayoutEffect?
- useEffect: Runs after the render is committed.
- useLayoutEffect: Runs synchronously after all DOM mutations but before the browser paints. Useful for measurements and DOM manipulations.
15. What is reconciliation in React?
The process React uses to compare virtual DOM trees and update only the necessary parts of the real DOM efficiently.
16. What is the significance of keys in React lists?
Keys help React identify which items have changed, been added, or removed. They improve rendering efficiency and should be unique.
17. What is React Suspense?
A React feature that lets you defer rendering part of the UI until asynchronous data is ready. Often used with code splitting and data fetching libraries.
18. What are React fragments?
Fragments let you group multiple children without adding extra DOM nodes.
Syntax: <></> or <React.Fragment></React.Fragment>.
19. What are render props?
A technique for sharing code between components using a prop whose value is a function. Example:
<DataProvider render={(data) => <Chart data={data} />} />
20. Difference between React.memo and useMemo?
- React.memo: Prevents re-rendering of functional components if props haven’t changed.
- useMemo: Memoizes a calculation result.
21. How do you handle error boundaries in React?
Use an Error Boundary (a class component with componentDidCatch and getDerivedStateFromError). It catches errors during rendering and displays fallback UI.
22. What is Prop Drilling?
When props are passed down multiple levels to reach deeply nested components. Solved with Context API or state management libraries.
23. What is the difference between state and props?
- Props: Passed from parent to child, read-only.
- State: Managed within a component, can change over time.
24. How do you handle forms in React?
Forms can be controlled (with state) or uncontrolled (using refs). Controlled components are most common in React.
25. What is Server-Side Rendering (SSR) in React?
SSR renders React components on the server and sends HTML to the client, improving SEO and initial page load performance. Frameworks like Next.js implement SSR.
26. What is hydration in React?
Hydration is the process of attaching React’s event listeners to existing server-rendered HTML. Common in SSR apps.
27. How does React handle routing?
React uses libraries like React Router for client-side routing. It maps URL paths to components without full page reloads.
28. What is the difference between useReducer and Redux?
- useReducer: Local state management inside a component.
- Redux: Global state management across the app.
29. What are React portals?
Portals allow rendering children into a DOM node outside the parent DOM hierarchy. Useful for modals, tooltips, etc.
30. How do you secure a React application?
- Use HTTPS.
- Validate inputs to prevent XSS.
- Use JWT tokens securely.
- Avoid storing sensitive data in localStorage.
- Keep dependencies updated.
Final Thoughts
Mastering React JS interview questions requires not just memorizing answers but understanding why certain patterns and practices are used. As an experienced developer, focus on advanced topics like hooks, performance optimization, SSR, and state management. With this list of Top 30 React JS interview questions and answers, you’ll be well-prepared for your next interview.