Top 100+ Most Asked React.js Interview Questions

Comprehensive collection of the most frequently asked React JS interview questions covering fundamentals, hooks, routing, testing, and advanced concepts. Each answer is concise and interview-ready.

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1.What is React, and what are its main features?

2.What is JSX and how does it work?

3.Explain the concept of the Virtual DOM in React.

4.How does virtual DOM in React work? What are its benefits and downsides?

5.What is the difference between React Node, React Element, and React Component?

6.What are React Fragments used for?

7.What is the purpose of the key prop in React?

8.What is the consequence of using array indices as keys in React?

9.What are props in React? How are they different from state?

10.What is the difference between React's class components and functional components?

11.When should you use a class component over a function component?

12.What is React Fiber?

13.What is reconciliation?

14.What is the difference between Shadow DOM and Virtual DOM?

15.What is the difference between Controlled and Uncontrolled React components?

16.How would you lift the state up in a React application, and why is it necessary?

17.What are Pure Components?

18.What is the difference between createElement and cloneElement?

19.What is the role of PropTypes in React?

20.What are stateless components?

21.What are stateful components?

22.What are the recommended ways for type checking of React component props?

23.Why does React recommend against mutating state?

24.What are the benefits of using hooks in React?

25.What are the rules of React hooks?

26.What is the difference between useEffect and useLayoutEffect in React?

27.What does the dependency array of useEffect affect?

28.What is the useRef hook in React and when should it be used?

29.What is the purpose of callback function argument format of setState() in React class components?

30.What is the useCallback hook in React and when should it be used?

31.What is the useMemo hook in React and when should it be used?

32.What is the useReducer hook in React and when should it be used?

33.What is the useId hook in React and when should it be used?

34.Can you explain how to create and use custom hooks in React?

35.What does re-rendering mean in React?

36.What is forwardRef() in React used for?

37.What are error boundaries in React for?

38.What is React Suspense?

39.Explain what React hydration is?

40.What are React Portals used for?

41.What is React strict mode and what are its benefits?

42.What is code splitting in a React application?

43.How would one optimize the performance of React contexts to reduce rerenders?

44.What is the Flux pattern?

45.Explain one-way data flow of React

46.What are some pitfalls of using context in React?

47.What are some React anti-patterns?

48.How do you decide between using React state, context, and external state managers?

49.Explain what happens when setState is called in React?

50.Explain prop drilling

51.Describe lazy loading in React

52.Discuss synthetic events in React

53.Explain the React component lifecycle methods in class components.

54.What is Concurrent Mode in React, and how does it improve rendering performance?

55.How does React handle concurrent rendering with multiple updates and prioritize them?

56.How would you handle long-running tasks or expensive computations in React applications without blocking the UI?

57.Explain server-side rendering of React applications and its benefits

58.Explain static generation of React applications

59.What are higher-order components in React?

60.Explain the presentational vs container component pattern in React

61.What are render props in React?

62.Explain the composition pattern in React.

63.How do you re-render the view when the browser is resized?

64.How do you handle asynchronous data loading in React applications?

65.What are some common pitfalls when doing data fetching in React?

66.What is a React Router?

67.How does React Router work, and how do you implement dynamic routing?

68.How do you handle nested routes and route parameters in React Router?

69.What is the difference between BrowserRouter and HashRouter?

70.How React Router is different from the history library?

71.What are the <Router> components of React Router v6?

72.What is the purpose of the push and replace methods of history?

73.How do you navigate programmatically in React Router?

74.How would you implement route guards or private routes in React?

75.How do you manage the active route state in a multi-page React application?

76.How do you handle 404 errors or page not found in React Router?

77.How to get query parameters in React Router?

78.How do you perform an automatic redirect after login in React Router?

79.How do you pass props to a route component in React Router?

80.How do you localize React applications?

81.What is react-intl?

82.What are the main features of react-intl?

83.What are the two ways of formatting in react-intl?

84.How to use FormattedMessage as a placeholder using react-intl?

85.How to access the current locale with React Intl?

86.How to format date using react-intl?

87.How do you test React applications?

88.What is Jest and how is it used for testing React applications?

89.What is React Testing Library and how is it used for testing React components?

90.How do you test React components using React Testing Library?

91.How do you test asynchronous code in React components?

92.How do you mock API calls in React component tests?

93.How do you test React hooks in functional components?

94.How do you test custom hooks in React?

95.What is Shallow Renderer in React testing?

96.What is Snapshot Testing in React?

97.How do you test React components that use context?

98.How do you test React components that use Redux?

99.What are the key differences between shallow rendering and full DOM rendering in React tests?

100.What is the TestRenderer package in React?