React Interview Questions and Answers for freshers

1. What is React?

   • Answer: React is a JavaScript library for building user interfaces (UIs), specifically single-page applications. It's maintained by Meta and a community of individual developers and companies. React focuses on components, making UIs easier to build and maintain.

2. Explain JSX.

   • Answer: JSX (JavaScript XML) is a syntax extension to JavaScript. It allows you to write HTML-like code within your JavaScript code. It makes React code more readable and easier to understand, but it needs to be transpiled into plain JavaScript before it can be run in a browser.

3. What are components in React?

   • Answer: Components are reusable building blocks of React applications. They can be either functional components (simple JavaScript functions) or class components (ES6 classes that extend React.Component). They encapsulate UI elements and their logic, making code modular and maintainable.

4. What is the difference between functional and class components?

   • Answer: Functional components are simpler, often just functions that return JSX. Class components use classes, providing features like lifecycle methods (e.g., componentDidMount, componentDidUpdate) and state management via `this.state`. Functional components are preferred now with the introduction of hooks.

5. Explain props in React.

   • Answer: Props (short for properties) are data passed from a parent component to a child component. They are immutable – you cannot change them within the child component. They are a way to configure and customize child components.

6. Explain state in React.

   • Answer: State is an internal data structure of a component that can change over time. When the state changes, the component re-renders to reflect the update. It is used to manage data that is specific to a component.

7. What are React hooks?

   • Answer: React Hooks are functions that let you "hook into" React state and lifecycle features from within functional components. Examples include `useState`, `useEffect`, `useContext`, etc. They allow functional components to have state and other capabilities previously only available to class components.

8. Explain `useState` hook.

   • Answer: `useState` is a hook that lets you add state to a functional component. It takes an initial state value as an argument and returns an array with two elements: the current state value and a function to update it. For example, `const [count, setCount] = useState(0);`.

9. Explain `useEffect` hook.

   • Answer: `useEffect` is a hook that lets you perform side effects in functional components. Side effects are things like data fetching, subscriptions, and manually changing the DOM. It takes a function as an argument that will run after the component renders (and optionally a dependency array to control when it runs).

10. What is virtual DOM?

    • Answer: The virtual DOM is a lightweight representation of the real DOM. React uses it to efficiently update the actual DOM only when necessary. This significantly improves performance compared to directly manipulating the real DOM.

11. Explain reconciliation in React.

    • Answer: Reconciliation is the process by which React compares the previous virtual DOM with the updated virtual DOM and determines the minimal changes needed to update the real DOM. This minimizes expensive DOM manipulations.

12. What are keys in React lists?

    • Answer: Keys are unique identifiers given to each element in a list or array when rendering it in React. They help React efficiently update lists by identifying which items have changed, been added, or removed. They should be stable and unique within the list.

13. How to prevent re-rendering in React?

    • Answer: Use `React.memo` for functional components or `shouldComponentUpdate` for class components to prevent unnecessary re-renders. `useMemo` and `useCallback` can also be used to memoize expensive calculations and callbacks respectively.

14. What is `React.Fragment`?

    • Answer: `React.Fragment` lets you group a list of children without adding extra nodes to the DOM. It's useful when you need to return multiple elements from a component but don't want an extra div or other element in the output.

15. Explain event handling in React.

    • Answer: Event handling in React is similar to standard JavaScript, but with a few differences. Events are named using camelCase (e.g., `onClick` instead of `onclick`). Event handlers are functions passed as props to components.

16. What are controlled and uncontrolled components?

    • Answer: Controlled components have their values managed entirely by React state. Uncontrolled components rely on the DOM's internal state to manage values. Controlled components are generally preferred for better control and predictability.

17. What are refs in React?

    • Answer: Refs provide a way to directly access DOM elements or React components. They are useful for tasks like directly manipulating the DOM, focusing elements, or integrating with third-party libraries.

18. Explain context API in React.

    • Answer: The Context API provides a way to pass data through the component tree without having to pass props down manually at every level. This is useful for sharing global state or theme information.

19. What is the difference between `setState` and `forceUpdate`?

    • Answer: `setState` is the standard way to update state in React; it efficiently updates the state and re-renders the component. `forceUpdate` forces a re-render, bypassing React's optimization, and should generally be avoided.

20. What are higher-order components (HOCs)?

    • Answer: HOCs are functions that take a component as an argument and return a new enhanced component. They are a way to reuse component logic and add functionality without modifying the original component.

21. Explain render props.

    • Answer: Render props are a technique where a component receives a function prop (the render prop) that it uses to return JSX. It's a way to share code between components by passing a rendering function.

22. What are some common React libraries/tools?

    • Answer: Popular libraries include Redux (state management), React Router (routing), Axios (HTTP requests), styled-components (CSS-in-JS), and Material-UI (UI component library).

23. What is Redux?

    • Answer: Redux is a predictable state container for JavaScript apps. It helps manage the application's state in a centralized and organized way, making it easier to debug and maintain large applications.

24. Explain the concept of immutability in React.

    • Answer: Immutability means that once an object is created, its values cannot be changed. In React, this is important because it allows React to efficiently track changes and update the UI only when necessary.

25. What is the purpose of `create-react-app`?

    • Answer: `create-react-app` is a tool that sets up a new React project with all the necessary configurations and dependencies, simplifying the initial setup process.

26. How to handle errors in React applications?

    • Answer: Use try-catch blocks to handle errors during asynchronous operations. Use error boundaries (components that catch errors in their child component tree) to gracefully handle errors without crashing the entire application.

27. What are lifecycle methods in class components? (Give examples)

    • Answer: Lifecycle methods are functions that are called at specific points in a component's life. Examples include `componentDidMount` (runs after the component mounts), `componentDidUpdate` (runs after an update), and `componentWillUnmount` (runs before the component unmounts).

28. Explain how routing works in React.

    • Answer: React Router is a popular library that allows you to implement navigation and routing in React applications. It provides components like ``, ``, and `` to create single-page applications with multiple views.

29. What is a forwardRef?

    • Answer: `forwardRef` is a higher-order component that lets you forward a ref from a higher-order component to a wrapped component. This is essential when using refs with custom components.

30. What are some best practices for writing React code?

    • Answer: Use functional components with hooks, keep components small and focused, use prop types for validation, write clean and readable code, use a consistent coding style, and test your components thoroughly.

31. How do you optimize performance in a large React application?

    • Answer: Use techniques like memoization (`React.memo`, `useMemo`, `useCallback`), code-splitting, lazy loading, and efficient state management to optimize performance.

32. Explain how to use `useReducer` hook.

    • Answer: `useReducer` is similar to `useState` but is better suited for managing more complex state logic. It takes a reducer function and an initial state value as arguments and returns the current state and a dispatch function to update it.

33. What is a React developer tool?

    • Answer: The React developer tools are a browser extension that provides tools to inspect the React component tree, view props and state, and debug React applications more effectively.

34. How do you test React components?

    • Answer: Popular testing libraries include Jest and React Testing Library. Tests can be written at different levels, including unit tests (testing individual components), integration tests (testing interactions between components), and end-to-end tests (testing the entire application).

35. What is the difference between shallow and deep comparison?

    • Answer: Shallow comparison checks only if the references of two objects are the same. Deep comparison checks if the values of all properties of two objects are the same. React uses shallow comparison by default for performance reasons.

36. How to handle asynchronous operations in React?

    • Answer: Use promises, async/await, or libraries like Axios to handle asynchronous operations. Manage loading states to provide feedback to the user while waiting for data.

37. Explain conditional rendering in React.

    • Answer: Conditional rendering involves rendering different UI elements based on certain conditions. This can be achieved using `if` statements, ternary operators, or the `&&` operator.

38. What are portals in React?

    • Answer: Portals render children into a different part of the DOM than where the component is rendered. This is useful for modals or tooltips that need to appear outside of their parent component's layout.

39. How do you handle form submissions in React?

    • Answer: Prevent default form submission behavior using `preventDefault` on the form's `onSubmit` event handler. Handle input changes using controlled components and update the state accordingly.

40. What are some common React anti-patterns to avoid?

    • Answer: Avoid overusing state, using unnecessary class components, neglecting prop types, writing overly complex components, and neglecting code optimization.

41. Explain server-side rendering (SSR) in React.

    • Answer: SSR renders the React application on the server, sending fully rendered HTML to the client. This can improve SEO and initial load times.

42. What is Next.js or Gatsby?

    • Answer: Next.js and Gatsby are popular React frameworks that provide features like SSR, routing, and static site generation, simplifying the development of complex React applications.

43. Explain code splitting in React.

    • Answer: Code splitting divides your application's code into smaller chunks, loading only the necessary code on demand. This reduces the initial bundle size and improves load times.

44. What is lazy loading in React?

    • Answer: Lazy loading loads components only when they are needed, improving initial load times and overall application performance.

45. How to handle user authentication in React?

    • Answer: Use libraries like Firebase, Auth0, or create a custom backend solution to handle user authentication. Store authentication state securely and use it to control access to different parts of the application.

46. What is a build process in React?

    • Answer: The build process transforms your React code (including JSX and other assets) into optimized files for production deployment. Tools like Webpack are commonly used.

47. Explain the concept of component composition in React.

    • Answer: Component composition is the process of creating complex UI elements by combining simpler, reusable components. This enhances code reusability and maintainability.

48. What is a custom hook? Give an example.

    • Answer: A custom hook is a function that starts with `use` and can use other hooks within it. It's a way to extract reusable stateful logic from functional components.

49. Explain how to use CSS modules in React.

    • Answer: CSS modules allow you to scope CSS classes to a specific component, preventing naming conflicts. They typically use a build process to generate unique class names.

50. What is Styled-Components?

    • Answer: Styled-Components is a library that enables writing CSS directly within your React components, using tagged template literals, providing CSS-in-JS capabilities.

51. How to deploy a React application?

    • Answer: Deploy your built React app to services like Netlify, Vercel, AWS, Heroku, or other hosting providers. The process often involves configuring a build step and connecting it to a server.

52. Explain the concept of prop drilling in React and how to avoid it.

    • Answer: Prop drilling is the process of passing props down multiple levels of a component tree. It's often an indicator of needing a better state management solution like Context API, Redux, or Zustand.

53. What is the difference between a library and a framework?

    • Answer: A library is a collection of reusable code that you can use within your own application. A framework dictates the structure and flow of your application; you build within its constraints.

54. What are some common React patterns for fetching data?

    • Answer: Use `useEffect` to fetch data when a component mounts, manage loading and error states, and implement data caching to improve performance.

55. How would you approach building a reusable component?

    • Answer: Identify common functionality, abstract logic into reusable functions, use props to make it configurable, and write thorough tests.

56. What are some common debugging techniques in React?

    • Answer: Use `console.log` to check values, use the React developer tools, use the browser's debugging tools, and write unit tests to catch errors early.

57. Describe your experience with version control (e.g., Git).

    • Answer: [Describe your experience with Git commands like `commit`, `push`, `pull`, `branch`, `merge`, etc. and any experience with platforms like GitHub or GitLab.]

58. Explain your understanding of accessibility in React applications.

    • Answer: [Describe your understanding of making applications accessible to users with disabilities, using ARIA attributes, semantic HTML, and providing alt text for images.]

59. How do you stay updated with the latest advancements in React?

    • Answer: [Mention following the official React blog, reading articles on React related websites, attending webinars, and participating in the React community.]

60. Walk me through your process for tackling a new React project.

    • Answer: [Outline a structured approach: planning, setting up the project, defining components, implementing functionality, testing, and deployment.]

61. What are your strengths as a React developer?

    • Answer: [Highlight relevant skills and experiences, focusing on areas like problem-solving, component design, performance optimization, and teamwork.]

62. What are your weaknesses as a React developer?

    • Answer: [Choose a genuine weakness, but frame it positively. For example, mention a specific area you are actively working on improving and what steps you're taking.]

63. Why are you interested in this React developer position?

    • Answer: [Express genuine enthusiasm, mentioning specific aspects of the role or company that appeal to you. Connect your skills and goals to the opportunity.]

64. Where do you see yourself in 5 years?

    • Answer: [Show ambition but be realistic. Mention specific skill development, career progression, and contributions you hope to make.]

Thank you for reading our blog post on 'React Interview Questions and Answers for freshers'.We hope you found it informative and useful.Stay tuned for more insightful content!