list of 25 essential React.js interview questions and answers for experienced developers, optimized for clarity and SEO:
Core React Concepts
1. What is the Virtual DOM, and how does it improve performance?
Answer:
The Virtual DOM is a lightweight copy of the real DOM. React compares the Virtual DOM with the previous version (diffing) and updates only the changed parts (reconciliation), reducing expensive DOM operations.
2. Explain the difference between class and functional components.
Answer:
- Class components use
this.stateand lifecycle methods (e.g.,componentDidMount). - Functional components use hooks (e.g.,
useState,useEffect) and are now preferred for simplicity and performance.
3. What are React Fragments?
Answer:
Fragments (<></> or <Fragment>) let you group elements without adding extra DOM nodes.
Hooks
4. How does useState work?
Answer:useState returns a stateful value and a setter function. Example:
const [count, setCount] = useState(0);5. When would you use useMemo vs. useCallback?
Answer:
useMemo: Memoizes a computed value (e.g., filtered list).useCallback: Memoizes a function (e.g., event handler to prevent re-renders).
6. What is the purpose of useEffect’s dependency array?
Answer:
It controls when the effect runs. Empty array ([]) = runs once (mount), no array = runs on every render.
State Management
7. Compare Redux and Context API.
Answer:
- Redux: Predictable state container for large apps (middleware, dev tools).
- Context API: Lightweight for prop drilling avoidance (no middleware).
8. How does Redux middleware like redux-thunk work?
Answer:redux-thunk allows async actions by dispatching functions (not plain objects).
9. What is the React Context API?
Answer:
A way to share state globally without prop drilling. Uses createContext, Provider, and useContext.
Performance Optimization
10. How do you optimize React rendering?
Answer:
- Use
React.memofor component memoization. - Avoid inline functions/objects in props.
- Code-splitting with
React.lazy.
11. What is code-splitting in React?
Answer:
Splitting code into smaller chunks (e.g., with React.lazy and Suspense) to reduce initial load time.
12. Explain the key prop’s importance in lists.
Answer:key helps React identify which items changed, were added, or removed (prevents re-rendering entire lists).
Advanced Patterns
13. What are Higher-Order Components (HOCs)?
Answer:
Functions that take a component and return a new component (e.g., for reuse logic like withAuth).
14. What is render prop pattern?
Answer:
A component prop that returns a function to share state/behavior (e.g., <DataProvider render={(data) => <Child data={data} />}).
15. How do you handle errors in React?
Answer:
- Error boundaries (class components with
componentDidCatch). try/catchfor hooks/async code.
React Router
16. How does React Router v6 differ from v5?
Answer:
- Routes are nested with
<Routes>and<Route>. useNavigatereplacesuseHistory.- No
exactprop (routes match partially by default).
17. What is lazy loading in React Router?
Answer:
Loading components only when needed (e.g., const Home = React.lazy(() => import('./Home'))).
Testing
18. How do you test React components?
Answer:
- Jest: Test runner.
- React Testing Library: Render and assert components.
- Cypress: E2E testing.
19. What is the act function in testing?
Answer:
It ensures updates related to React state changes are processed before assertions.
Miscellaneous
20. What are controlled vs. uncontrolled components?
Answer:
- Controlled: Form data handled by React state (e.g.,
<input value={name} onChange={...}>). - Uncontrolled: Form data handled by DOM (e.g.,
useRef).
21. How does React handle server-side rendering (SSR)?
Answer:
Frameworks like Next.js render React on the server for SEO/performance, hydrating it client-side.
22. What is React StrictMode?
Answer:
A tool that highlights potential problems (e.g., unsafe lifecycle methods, legacy APIs) in development.
23. Explain the React Fiber architecture.
Answer:
A reimplementation of React’s core algorithm for better rendering performance (incremental rendering, prioritization).
24. What are portals in React?
Answer:
Render children outside the parent DOM hierarchy (e.g., modals, tooltips) with ReactDOM.createPortal.
25. How do you optimize a slow React app?
Answer:
- Profile with React DevTools.
- Memoize components (
React.memo). - Virtualize long lists (
react-window). - Avoid unnecessary re-renders.
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