Here’s a comprehensive list of the Top 30 React.js Interview Questions and Answers for Senior Web Developers specializing in React.js, categorized by concept for clarity:
🔹 1. Core React.js Concepts
1. What are the key features of React.js?
Answer:
- JSX (JavaScript XML)
- Virtual DOM
- Unidirectional Data Flow
- Component-Based Architecture
- Lifecycle Methods
- React Hooks (from 16.8+)
2. What is the Virtual DOM, and how does it improve performance?
Answer:
Virtual DOM is a lightweight in-memory representation of the real DOM. React updates the Virtual DOM first, then compares it with the previous version (diffing), and finally updates only the changed parts in the real DOM, reducing performance overhead.
3. Explain the reconciliation process in React.
Answer:
Reconciliation is the process of comparing the previous Virtual DOM with the new one to determine the minimal number of operations needed to update the real DOM efficiently.
4. What are React fragments and why are they used?
Answer:<React.Fragment> or <> is used to group multiple elements without adding extra nodes to the DOM.
5. What is the significance of keys in React lists?
Answer:
Keys help React identify which items have changed, are added, or removed. This improves rendering performance and avoids bugs in dynamic lists.
Top 30 React.js Interview Questions and Answers
🔹 2. React Hooks (Advanced)
6. What is the difference between useEffect, useLayoutEffect, and useInsertionEffect?
Answer:
useEffect: Runs after paint (non-blocking)useLayoutEffect: Runs before paint (blocking)useInsertionEffect: Runs before DOM mutations, used for styling libraries (React 18+)
7. How do custom hooks work? Give an example.
Answer:
Custom hooks are JavaScript functions that use other hooks.
Example:
function useCounter(initial = 0) {
const [count, setCount] = useState(initial);
const increment = () => setCount(c => c + 1);
return { count, increment };
}
8. How does useCallback differ from useMemo?
Answer:
useCallback(fn, deps): Returns memoized functionuseMemo(fn, deps): Returns memoized value
9. Why should you avoid using hooks inside loops or conditions?
Answer:
Hooks must be called in the same order on every render. Loops or conditions can break that consistency, leading to bugs.
10. How can you mimic componentDidMount using hooks?
Answer:
useEffect(() => {
// Code here
}, []); // Empty dependency array
Top 30 React.js Interview Questions and Answers
🔹 3. State Management & Context
11. When would you use Context API over Redux?
Answer:
Use Context API for light, global state (e.g., theme, user info). Use Redux (or Zustand, Recoil, etc.) for complex state with middleware, async actions, and structured architecture.
12. How do you prevent unnecessary re-renders with Context API?
Answer:
- Split context into multiple providers
- Use
React.memo - Use
useContextSelector(fromuse-context-selectorlibrary)
13. What are some Redux alternatives you’ve used in React?
Answer:
- Zustand
- Recoil
- Jotai
- MobX
- React Query (for server state)
14. How does React Query differ from Redux?
Answer:
React Query manages server state (e.g., caching, background sync, pagination) while Redux is more suited for client state.
15. How do you implement lazy loading of components in React?
Answer:
const LazyComp = React.lazy(() => import('./MyComponent'));
<React.Suspense fallback={<Spinner />}><LazyComp /></React.Suspense>
Top 30 React.js Interview Questions and Answers
🔹 4. Performance Optimization
16. How do you handle performance optimization in large React apps?
Answer:
- Code splitting
- Memoization (
React.memo,useMemo,useCallback) - Virtualization (e.g.,
react-window) - Debouncing/throttling
- Avoid anonymous functions in JSX
17. What causes unnecessary re-renders and how can you fix them?
Answer:
Causes:
- Changing props/state unnecessarily
- Not memoizing callbacks
Fix: - Use
React.memo,useCallback,useMemo - Split components
18. How do you implement debouncing in React?
Answer:
Use useEffect and setTimeout or use a library like lodash.debounce.
Example:
useEffect(() => {
const handler = setTimeout(() => doSearch(query), 300);
return () => clearTimeout(handler);
}, [query]);
19. What is React Profiler and how do you use it?
Answer:
React Profiler is a dev tool to measure rendering performance.
Enable it in React DevTools and wrap components using:
<Profiler id="MyComponent" onRender={callback}>...</Profiler>
20. How would you prevent re-renders in child components?
Answer:
React.memo- Pass stable props using
useCallbackoruseMemo - Use key prop carefully
Top 30 React.js Interview Questions and Answers
🔹 5. Advanced Patterns & Techniques
21. What is Higher-Order Component (HOC)?
Answer:
A function that takes a component and returns a new component.
const withLogger = (Component) => (props) => {
console.log(props);
return <Component {...props} />;
};
22. What is Render Props?
Answer:
A pattern where a component’s child is a function.
<DataProvider render={(data) => <Chart data={data} />} />
23. What is compound component pattern?
Answer:
Encapsulating multiple related components under one main component to share implicit state.
24. What is controlled vs uncontrolled component in React?
Answer:
- Controlled: React controls state via props
- Uncontrolled: DOM manages state via refs
25. What is React Server Components?
Answer:
React Server Components (RSC) allow rendering components on the server without sending their code to the client, improving performance and load times.
Top 30 React.js Interview Questions and Answers
🔹 6. Testing and TypeScript
26. How do you test React components?
Answer:
- Unit Testing:
Jest,React Testing Library - E2E Testing:
Cypress,Playwright - Example:
render(<Button label="Click" />);
expect(screen.getByText(/click/i)).toBeInTheDocument();
27. What are best practices for using TypeScript in React?
Answer:
- Use
interfacefor props - Use generics with hooks
- Avoid
any, use proper types - Use
React.FCor type components explicitly
28. How do you type a function component with props in TypeScript?
Answer:
interface Props {
label: string;
}
const Button: React.FC<Props> = ({ label }) => <button>{label}</button>;
Top 30 React.js Interview Questions and Answers
🔹 7. Ecosystem & Tools
29. What tools do you use in your React workflow?
Answer:
- ESLint + Prettier
- Webpack/Vite
- Storybook
- GitHub Actions
- React DevTools
- TypeScript
- Jest + RTL
30. How do you stay updated with the latest in React?
Answer:
- Follow React RFCs on GitHub
- Read blog posts from Kent C. Dodds, Dan Abramov
- Join React communities on Reddit, Twitter, Discord
- Attend conferences like React Conf
Top 30 React.js Interview Questions and Answers
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