React.js Junior to Mid: The 50+ Question Masterclass (2026)

Part 1: Core React Concepts & Fundamentals (15 Questions)

Component Architecture

1. Explain the difference between class components and functional components with hooks. When would you choose one over the other in 2026?

2. What are React Fragments and why are they useful? Provide practical examples of when to use <></> vs <Fragment>.

3. Describe the component lifecycle in modern React with hooks. How do useEffect, useLayoutEffect, and cleanup functions map to traditional lifecycle methods?

4. What are Higher-Order Components (HOCs) and custom hooks? Compare their use cases and when to prefer custom hooks in modern React development.

5. Explain the concept of compound components. How would you implement a TabSwitcher component with this pattern?

State Management

6. What is the difference between state and props? When should you lift state up versus using local component state?

7. Compare useState, useReducer, and state management libraries (Redux, Zustand). When is each appropriate for a medium-sized application?

8. Explain the concept of derived state. How would you implement it efficiently to prevent unnecessary recalculations?

9. What are the common pitfalls with useState in arrays and objects? How can you avoid them?

10. Describe how React's state batching works. What happens with multiple state updates in event handlers vs asynchronous code?

Performance Optimization

11. What triggers re-renders in React components? How can you identify unnecessary re-renders?

12. Explain React.memo, useMemo, and useCallback. When should you use each, and what are the common performance pitfalls?

13. What is code splitting in React? How would you implement route-based and component-based code splitting?

14. How does the virtual DOM work, and what's the reconciliation process? Explain with a diffing algorithm example.

15. What are concurrent features in React 18+? How do useTransition, useDeferredValue, and Suspense work together?

Part 2: Hooks Deep Dive (10 Questions)

Core Hooks Mastery

16. Explain the rules of hooks and why they exist. What happens if you break these rules?

17. Create a custom useFetch hook that handles loading, data, and error states with abort capabilities.

18. How does the dependency array in useEffect work? What are common mistakes and how to avoid them?

19. Compare useEffect vs useLayoutEffect. Provide specific use cases for each.

20. Implement a useLocalStorage hook that synchronizes state with localStorage.

Advanced Hooks Patterns

21. What is the useReducer hook pattern? When is it preferable over useState for complex state logic?

22. Explain the useRef hook. Beyond DOM references, what are other creative uses for useRef?

23. How would you implement a usePrevious hook to track previous prop/state values?

24. What are the limitations of hooks? When might you still need class components or different patterns?

25. Create a useIntersectionObserver hook for lazy loading images or infinite scrolling.

Part 3: Advanced Patterns & Architecture (10 Questions)

Render Patterns

26. Explain render props pattern vs custom hooks. Which is more appropriate in modern React?

27. What are controlled vs uncontrolled components? Provide examples of form inputs for both patterns.

28. How would you implement a Context-based theming system with support for system preferences?

29. Describe the Provider pattern with Context API. What are performance considerations?

30. What is prop drilling and how can you avoid it? Compare Context API, component composition, and state management libraries.

Application Architecture

31. Design a folder structure for a medium-sized React application. Explain your reasoning for component organization.

32. How would you implement error boundaries in a production application? Create a comprehensive error handling strategy.

33. What are the principles of clean component design? How do you decide what should be a separate component?

34. Explain the container/presentational component pattern. Is it still relevant with hooks?

35. How would you structure a feature-based architecture with shared UI components?

Part 4: React Ecosystem & Tooling (10 Questions)

Routing & Data Fetching

36. Compare React Router v6+ with alternative routing solutions. What are the key features and when to choose each?

37. How would you implement authentication-protected routes? Include route guards and token refresh logic.

38. Explain different data fetching strategies: SSR, SSG, ISR, and client-side fetching with React Query/SWR.

39. What is React Query/TanStack Query? How does it simplify server-state management compared to manual useEffect fetching?

40. How would you implement optimistic updates in a React application?

Build Tools & Testing

41. Compare Vite, Webpack, and Next.js build systems. What are the trade-offs for different project sizes?

42. What testing strategies would you use for React components? Explain unit, integration, and E2E testing approaches.

43. How would you set up Jest and React Testing Library for testing hooks?

44. Explain the importance of TypeScript in React development. Provide examples of useful generic patterns.

45. What are the key considerations for React application deployment? Discuss environment variables, build optimization, and CI/CD.

Part 5: Real-World Scenarios & Problem Solving (10 Questions)

Practical Implementation

46. How would you implement an accessible modal/dialog component? Include focus trapping, ARIA attributes, and keyboard navigation.

47. Design a real-time chat component with WebSocket integration and optimistic UI updates.

48. Create a virtualized list component for displaying 10,000+ items efficiently.

49. Implement a drag-and-drop interface for reordering items with proper accessibility.

50. Build a form with complex validation, multi-step workflow, and submission handling.

Debugging & Optimization

51. A component is re-rendering excessively. What's your debugging process?

52. How would you debug a memory leak in a React application? What are common causes?

53. Explain how to profile React performance using DevTools. What metrics matter most?

54. How would you handle backward compatibility when upgrading React versions?

55. What strategies would you use to reduce bundle size in a large React application?

Bonus: Emerging Trends (5+ Questions)

56. What are React Server Components and how do they change React architecture?

57. How would you implement a React Native-like "write once, run anywhere" solution for web and mobile?

58. Explain the role of AI/ML in modern React development (2026 perspective).

59. What are the security best practices for React applications? Include XSS, CSRF, and dependency vulnerabilities.

60. How would you design a React micro-frontend architecture? Compare different implementation approaches.

Answer Strategy Guidelines

• Junior-level answers: Focus on syntax, basic usage, and simple examples

• Mid-level answers: Include architectural considerations, performance implications, and real-world trade-offs

• Senior consideration: Discuss team scalability, maintainability, and business impact

This masterclass covers 55 core questions with bonus questions extending the total. Each question is designed to assess depth of understanding, practical experience, and the ability to make architectural decisions—key differentiators between junior and mid-level React developers in 2026.

Study Approach:

1. Master the fundamentals (Parts 1-2) thoroughly

2. Build practical projects using patterns from Part 3

3. Explore tooling and ecosystem (Part 4) through hands-on experimentation

4. Solve real-world problems (Part 5) to bridge theory and practice

This progression ensures not just theoretical knowledge, but the practical skills needed to advance from junior to mid-level React developer.