Top 50 react interview questions with answers.
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Certainly, here's a list of 50 React interview questions along with their answers:

1. What is React, and why is it used?

• Answer: React is an open-source JavaScript library for building user interfaces. It's used to create reusable UI components for web applications.

2. What is the Virtual DOM in React?

• Answer: The Virtual DOM is a lightweight in-memory representation of the actual DOM. React uses it to improve performance by minimizing direct manipulation of the real DOM.

3. Explain JSX in React.

• Answer: JSX (JavaScript XML) is a syntax extension that allows developers to write HTML-like code within JavaScript files. It's transformed into JavaScript code by transpilers like Babel.

4. What is a React component?

• Answer: A React component is a self-contained, reusable UI element that can have its own state and properties.

5. What's the difference between a class component and a functional component?

• Answer: Class components have state and lifecycle methods, while functional components are stateless and primarily for rendering UI based on props.

6. How do you define a component in React?

• Answer: A component can be created using a class (class component) or a JavaScript function (functional component).

7. What is state in React, and how do you initialize it?

• Answer: State is an object that represents data that can change over time. In a class component, you initialize it in the constructor using this.state = { ... }.

8. How can you update state in a React component?

• Answer: You use this.setState() to update the state in a class component. In functional components, you can use React hooks like useState to manage state.

9. What is a prop in React?

• Answer: A prop (short for property) is a way to pass data from a parent component to a child component.

10. What is the significance of keys in React lists?

•  Answer: Keys are used to give React a hint about which elements have changed, been added, or removed in a list. They help improve performance and avoid re-rendering issues.

11. What is the purpose of the render() method in a class component?

•  Answer: The render() method returns the JSX that should be rendered on the screen. It's called whenever the component needs to be re-rendered.

12. What are React hooks, and why were they introduced?

•  Answer: React hooks are functions that allow functional components to have state and lifecycle features. They were introduced to simplify and encourage the use of functional components.

13. What is the useState hook, and how do you use it?

•  Answer: useState is a hook used to add state to functional components. It returns an array with the current state and a function to update it. For example: const [count, setCount] = useState(0);

14. Explain the purpose of the useEffect hook in React.

•  Answer: useEffect is used for handling side effects in functional components, such as data fetching, DOM manipulation, and subscription management.

15. What is a prop drilling issue in React? How can it be solved?

•  Answer: Prop drilling occurs when you need to pass props through multiple intermediary components. To solve it, you can use context, Redux, or other state management solutions.

16. What is a higher-order component (HOC) in React?

•  Answer: A higher-order component is a function that takes a component and returns a new component with enhanced features. It's used for code reuse and separation of concerns.

17. Explain the concept of React Router.

•  Answer: React Router is a library for routing in React applications. It allows you to define and navigate between different views or pages within a single-page application.

18. What is Redux, and why might you use it in a React application?

•  Answer: Redux is a state management library for managing application state in a predictable way. It's used for complex applications where state needs to be shared among multiple components.

19. Describe the Flux architecture and its role in React.

•  Answer: Flux is a design pattern that involves unidirectional data flow. It complements React by providing a clear and predictable way to manage application state.

20. What is the Context API in React, and how does it work?

•  Answer: The Context API provides a way to pass data through the component tree without having to pass props manually at every level. It consists of a Provider and a Consumer component.

21. How do you make an HTTP request in React?

•  Answer: You can use the fetch API, Axios, or other HTTP client libraries to make HTTP requests in React.

22. What is server-side rendering (SSR) in React, and why might you use it?

•  Answer: SSR is a technique that renders a React application on the server and sends the initial HTML to the client. It can improve SEO and initial load times.

23. Explain the concept of code splitting in React and how it improves performance.

•  Answer: Code splitting involves breaking your code into smaller chunks that are loaded on demand. It improves performance by reducing the initial bundle size and loading only what's needed.

24. What are some best practices for optimizing the performance of a React application?

•  Answer: Some best practices include code splitting, using shouldComponentUpdate or PureComponent, optimizing render methods, and minimizing unnecessary re-renders.

25. What is error boundaries in React?

•  Answer: Error boundaries are special React components that catch JavaScript errors during rendering, in lifecycle methods, and in constructors of the whole component tree.

26. How can you handle forms in React?

•  Answer: You can manage forms in React by using controlled components, which bind the form input's value to the component's state.

27. What are React Portals, and when might you use them?

•  Answer: React Portals allow you to render components outside the current parent component's DOM hierarchy. They are useful for modal dialogs, popovers, and similar components.

28. What is memoization in React, and how can you implement it?

•  Answer: Memoization is a technique for optimizing performance by caching the results of expensive function calls. You can use the useMemo hook or libraries like reselect for memoization.

29. How can you handle authentication in a React application?

•  Answer: Authentication can be handled by using tokens (JWT), OAuth, or other authentication mechanisms. You may also use libraries like Auth0 or Firebase for authentication.

30. Explain the difference between React's class and className attributes.

•  Answer: In React, you should use className instead of class when specifying CSS classes for HTML elements because class is a reserved word in JavaScript.

31. What is the role of Redux Thunk in a React-Redux application?

•  Answer: Redux Thunk is a middleware that allows you to write asynchronous logic in Redux actions. It helps manage side effects like data fetching in a Redux application.

32. Describe the use of localStorage and sessionStorage in a React application.

•  Answer: localStorage and sessionStorage are web storage mechanisms used to store data locally in a user's browser. They can be used to store and retrieve data in a React application.

33. How can you prevent cross-site scripting (XSS) attacks in a React application?

•  Answer: You should sanitize and escape user-generated content, use libraries like DOMPurify, and be cautious with dangerouslySetInnerHTML and innerHTML.

34. What is the purpose of the key attribute in React?

•  Answer: The key attribute is used to give React a hint about which elements have changed, been added, or removed in a list. It helps React efficiently update the DOM.

35. What is a pure component in React, and when should you use it?

•  Answer: A pure component is a class component that automatically implements shouldComponentUpdate to optimize rendering. Use pure components when performance is critical.

36. Explain the concept of the useReducer hook in React.

•  Answer: useReducer is a hook used for managing complex state logic. It's similar to useState but more suitable for managing state transitions in complex data structures.

37. What is the purpose of the getDerivedStateFromProps lifecycle method in React? 

• Answer: getDerivedStateFromProps is used to update the state of a component in response to changes in props. It's a static method and should return an object to update the state.

38. What is the role of the shouldComponentUpdate method in a class component?

•  Answer: shouldComponentUpdate is called before rendering a component and allows you to control whether the component should re-render based on changes in state or props. It can be used for performance optimization.

39. Explain the difference between localStorage and cookies for data storage in a React application.

•  Answer: localStorage stores data locally in the user's browser, while cookies are small pieces of data sent to and received from a server. Cookies are often used for session management and server-side storage.

40. How can you pass data between sibling components in React?

•  Answer: You can lift the shared state to a common ancestor component (lifting state up) or use a state management library like Redux or the Context API.

41. What is the dangerouslySetInnerHTML attribute in React, and when should you use it?

•  Answer: dangerouslySetInnerHTML allows you to insert HTML into a component, but you should use it with caution because it can open your application to cross-site scripting (XSS) attacks.

42. Explain the concept of React Fragments. - Answer: React Fragments allow you to group multiple elements without adding an extra node to the DOM. They are useful when you don't want to introduce an additional parent element.

43. How do you handle routing in a React application, and what is the role of React Router?

•  Answer: React Router is a library for handling routing in React applications. It allows you to define routes and navigate between different views within a single-page application.

44. What is the purpose of the memo function in React?

•  Answer: memo is a higher-order component that memoizes a functional component, preventing unnecessary re-renders when its props haven't changed.

45. Explain the concept of server-side rendering (SSR) in React.

•  Answer: SSR is a technique that renders a React application on the server and sends the initial HTML to the client. It can improve SEO and initial load times.

46. What is the difference between componentWillUnmount and useEffect cleanup functions in React?

•  Answer: componentWillUnmount is a lifecycle method in class components used for cleanup, while useEffect cleanup functions serve the same purpose in functional components using hooks.

47. How do you pass data from a child component to a parent component in React?

•  Answer: You can pass data from a child component to a parent component by defining a callback function in the parent and invoking it from the child component with the data.

48. What is the significance of the key prop in React lists?

•  Answer: The key prop helps React efficiently update the DOM by giving hints about which elements have changed, been added, or removed in a list.

49. Explain the concept of code splitting in React.

•  Answer: Code splitting involves breaking your code into smaller, separate bundles that are loaded on demand. It improves performance by reducing the initial bundle size and loading only what's needed.

50. How can you optimize the performance of a React application?

•  Answer: You can optimize performance by implementing code splitting, using React.memo, optimizing rendering methods, minimizing re-renders, and reducing the size of the application bundle.

These questions and answers cover a wide range of React concepts and best practices, from fundamental to advanced topics. Interviewers may choose questions based on the specific role and experience level they are hiring for.