React.js Interview Questions and Answers for freshers

1. What is React.js?

   • Answer: React.js is a free and open-source front-end JavaScript library for building user interfaces based on UI components. It's maintained by Meta (formerly Facebook) and a community of individual developers and companies.

2. Explain JSX.

   • Answer: JSX is a syntax extension to JavaScript. It allows you to write HTML-like code within your JavaScript code. React uses JSX to describe the structure of the user interface. It makes the code more readable and easier to understand.

3. What are components in React?

   • Answer: Components are the building blocks of React applications. They are independent, reusable pieces of code that encapsulate specific UI elements and their logic. They can be functional components (simple functions) or class components (using ES6 classes).

4. Explain the difference between functional and class components.

   • Answer: Functional components are simpler and easier to write. They are just JavaScript functions that accept props as input and return JSX. Class components, on the other hand, are ES6 classes that extend React.Component. They offer features like lifecycle methods and internal state, which are not directly available in functional components. However, with Hooks, functional components now have access to most of the functionality of class components.

5. What is props in React?

   • Answer: Props (short for properties) are a way to pass data from a parent component to a child component. They are read-only; child components cannot modify the props received from their parent.

6. What is state in React?

   • Answer: State is an object that holds the data of a component. When the state changes, the component re-renders to reflect the changes. State is mutable; you can update it using `setState` (in class components) or `useState` hook (in functional components).

7. Explain the lifecycle methods of a class component.

   • Answer: Class components have several lifecycle methods that are called at different stages of a component's existence. Key methods include `constructor`, `render`, `componentDidMount`, `componentDidUpdate`, and `componentWillUnmount`. These methods allow you to perform actions like initializing state, fetching data, updating the DOM, and cleaning up resources.

8. What are React Hooks?

   • Answer: React Hooks are functions that let you “hook into” React state and lifecycle features from within functional components. This allows functional components to have state and other features previously only available to class components.

9. Explain `useState` hook.

   • Answer: `useState` is a Hook that lets you add React state to function components. It takes an initial state value and returns an array containing the current state value and a function to update it.

10. Explain `useEffect` hook.

    • Answer: `useEffect` is a Hook that lets you perform side effects in functional components. Side effects include data fetching, subscriptions, and manually changing the DOM. It takes a function as an argument and an optional array of dependencies.

11. What is virtual DOM?

    • Answer: The virtual DOM is a lightweight in-memory representation of the actual DOM. React uses the virtual DOM to efficiently update the real DOM. Instead of directly manipulating the real DOM, React updates the virtual DOM first and then performs a diff algorithm to determine the minimal changes needed to update the real DOM, leading to improved performance.

12. What is reconciliation in React?

    • Answer: Reconciliation is the process by which React updates the real DOM based on changes in the virtual DOM. It uses a diffing algorithm to find the minimal set of changes needed, leading to efficient updates and improved performance.

13. Explain keys in React lists.

    • Answer: Keys are a special prop that React uses to identify elements in lists. They help React efficiently update lists when items are added, removed, or reordered. Using unique keys improves performance and avoids issues with re-rendering.

14. What are events in React?

    • Answer: Events in React are similar to DOM events but use camelCase naming (e.g., `onClick` instead of `onclick`). They are handled by attaching event handler functions to components.

15. How to handle forms in React?

    • Answer: Forms in React are handled using controlled components. The values of form fields are stored in the component's state, and the state is updated as the user interacts with the form. This gives you complete control over the form's data.

16. What are refs in React?

    • Answer: Refs provide a way to directly access DOM elements or components. They are useful for situations where you need to interact directly with the underlying DOM, such as focusing on an input field or measuring an element's dimensions. They can be created using `createRef()` (for class components) or the `useRef()` hook (for functional components).

17. Explain conditional rendering in React.

    • Answer: Conditional rendering is a technique used to display different UI elements based on certain conditions. You can achieve this using JavaScript's `if` statements, ternary operators, or logical AND operators within your JSX.

18. What is `Fragment` in React?

    • Answer: A Fragment is a way to group multiple elements without adding extra nodes to the DOM. It's represented by `<> ` or ``. It's useful when you need to return multiple elements from a component's render method but don't want to introduce an unnecessary parent element.

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

    • Answer: HOCs are advanced React patterns where a component takes another component as input and returns a new enhanced component. They're useful for reusing logic across multiple components (e.g., adding authentication or logging capabilities).

20. What are render props?

    • Answer: Render props are a technique where a component takes a function as a prop (the render prop) and uses it to render its children. This allows you to share code between components by passing the rendering logic as a function.

21. What is context API in React?

    • Answer: The Context API is a React feature that allows you to pass data through the component tree without having to pass props down manually at every level. It's useful for managing global state or shared data that many components need to access.

22. What is React Router?

    • Answer: React Router is a popular library that lets you add client-side routing to your React applications. It allows you to create single-page applications with multiple views or pages.

23. Explain how to handle asynchronous operations in React.

    • Answer: Asynchronous operations (like fetching data from an API) are typically handled using promises or async/await within the `useEffect` hook. The `useEffect` hook's cleanup function can be used to cancel pending requests if the component unmounts before the operation completes.

24. What are some common React libraries or tools?

    • Answer: Some popular React libraries and tools include React Router (routing), Redux or Zustand (state management), Axios or Fetch API (data fetching), styled-components or Material-UI (styling).

25. Explain the concept of data binding in React.

    • Answer: Data binding in React is primarily one-way. Changes to the component's state trigger re-rendering and updates to the UI. User input (e.g., in forms) updates the state, which in turn updates the UI. Two-way binding, while possible, is generally avoided in favor of this more controlled approach.

26. What is a controlled component?

    • Answer: A controlled component is a form element whose value is managed by the component's state rather than directly by the DOM. The value of the form element is synchronized with the component's state, ensuring consistent data management.

27. What is an uncontrolled component?

    • Answer: An uncontrolled component is a form element whose value is managed directly by the DOM. React does not track the value; instead, you access the value from the DOM when needed, usually during submission.

28. How to optimize React application performance?

    • Answer: Performance optimization involves techniques like using `memo` or `React.memo` for memoizing components, using keys in lists, avoiding unnecessary re-renders (e.g., using `useCallback` and `useMemo`), code splitting for larger apps, and using a production build for deployment.

29. What are some common debugging techniques in React?

    • Answer: React Developer Tools browser extension is invaluable for inspecting component hierarchies, state, props, and performance. Console logging, using the debugger in your browser's developer tools, and error boundary components also help identify and fix issues.

30. Explain error boundaries in React.

    • Answer: Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, and display a fallback UI instead of crashing the entire application. This improves the user experience by preventing unexpected errors from completely disrupting the app.

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

    • Answer: `setState` is the preferred method for updating a component's state. It efficiently batches updates and optimizes re-renders. `forceUpdate` forces a re-render even if the state hasn't changed; it's generally discouraged because it bypasses React's optimization mechanisms and can lead to performance issues.

32. Explain the concept of immutability in React.

    • Answer: Immutability means that once data is created, it cannot be modified. In React, it's best practice to create new objects or arrays instead of modifying existing ones when updating state. This helps React efficiently detect changes and optimize re-renders.

33. How does React handle events?

    • Answer: React synthesizes events. It creates its own event system that sits on top of the browser's native event system, offering cross-browser compatibility and extra features. This prevents direct interaction with native browser events.

34. What is the purpose of `key` prop when rendering lists?

    • Answer: The `key` prop helps React identify which items have changed, are added, or removed in a list. This allows React to efficiently update only the necessary parts of the list rather than re-rendering the entire thing, improving performance and preventing issues like state inconsistencies.

35. How can you prevent default behavior of an event?

    • Answer: Use the `preventDefault()` method on the event object in the event handler function. For example: `e.preventDefault();`

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

    • Answer: Best practices include keeping components small and focused, using functional components with Hooks, using a consistent styling solution, properly handling state and props, optimizing performance, and writing clean, well-documented code.

37. How do you handle asynchronous data fetching in React?

    • Answer: This is typically done using `useEffect` and `fetch` or a library like Axios. The `useEffect` hook fetches the data, and the component updates its state with the fetched data. Loading and error states are often handled for better user experience.

38. What is the difference between `props.children` and `this.props.children`?

    • Answer: `props.children` is used in functional components, while `this.props.children` is used in class components to access the content passed between the opening and closing tags of a component.

39. How do you style components in React?

    • Answer: Several methods exist, including inline styles (using JavaScript objects), CSS modules, styled-components, and CSS-in-JS libraries.

40. Explain what a "pure component" is.

    • Answer: A pure component is a component that renders the same output for the same input. It only re-renders if its props or state change. `React.PureComponent` (class component) or `useMemo` (functional component) can be used to create these.

41. What is the role of the `render` method in a React class component?

    • Answer: The `render` method is responsible for returning the JSX that represents the UI of the component. It should be a pure function and shouldn't directly modify the component's state.

42. What are some common state management libraries for React?

    • Answer: Redux, Zustand, Jotai, Recoil, and Context API are popular choices, each with its own strengths and weaknesses.

43. What are the benefits of using a state management library like Redux?

    • Answer: Redux provides a predictable way to manage application state, making debugging and testing easier. It promotes a clear separation of concerns and helps handle complex state updates in larger applications.

44. How would you handle errors in a React application?

    • Answer: Use try-catch blocks to handle synchronous errors. For asynchronous errors, handle rejection of promises or use error boundaries to catch errors and display graceful fallback UI.

45. What is code splitting in React?

    • Answer: Code splitting is a technique to break down a large React application into smaller chunks that are loaded on demand. This improves initial load time and overall performance.

46. Explain lazy loading in React.

    • Answer: Lazy loading is a technique to load components only when they're needed. This is often combined with code splitting to improve the initial load time of an application.

47. What is server-side rendering (SSR) in React?

    • Answer: SSR is a technique where the React application is rendered on the server instead of the client. This improves SEO and initial load time.

48. What are some common performance optimization techniques for React applications?

    • Answer: Code splitting, lazy loading, using `useMemo` and `useCallback` hooks, memoizing components with `React.memo`, optimizing re-renders, and using a production build are key techniques.

49. How do you test React components?

    • Answer: Jest and React Testing Library are popular choices. Tests can be written to verify the rendering of components, their behavior, and their interaction with state and props.

50. What is the difference between shallow and deep rendering in testing?

    • Answer: Shallow rendering only renders the component itself without rendering its children. Deep rendering renders the entire component tree. Shallow rendering is generally faster and more efficient for testing.

51. What is the purpose of the `StrictMode` component?

    • Answer: `StrictMode` is a tool for highlighting potential problems in an application. It doesn't render any visible UI but helps detect deprecated APIs, inconsistencies in state updates, etc.

52. Explain how to create a custom hook in React.

    • Answer: A custom hook is a function that starts with `use` and can use other hooks inside. It encapsulates reusable logic, making code more maintainable.

53. What are the benefits of using TypeScript with React?

    • Answer: TypeScript adds static typing to JavaScript, improving code maintainability, reducing errors, and improving developer experience.

54. How do you handle routing in a React application?

    • Answer: React Router is a commonly used library. It allows creating single-page applications with different views based on the URL.

55. What are some ways to improve the accessibility of a React application?

    • Answer: Use semantic HTML, provide alternative text for images, use ARIA attributes where needed, ensure sufficient color contrast, and follow WCAG guidelines.

56. What is the difference between `npm` and `yarn`?

    • Answer: Both are package managers for JavaScript. `npm` is the default package manager for Node.js, while `yarn` is an alternative offering some performance and workflow improvements.

57. What is a build process in React?

    • Answer: A build process transforms your React code into an optimized version for deployment. It typically involves bundling, minification, and other optimizations.

58. What are some common tools used in a React build process?

    • Answer: Webpack, Parcel, Rollup, and Vite are popular choices.

59. How do you deploy a React application?

    • Answer: Deployment methods vary depending on the hosting platform. Common options include Netlify, Vercel, AWS, Heroku, and GitHub Pages.

60. What is a React port?

    • Answer: A React port is the port number on which a development server runs during development. This allows you to access the application in a web browser.

61. What are some common reasons for re-renders in React?

    • Answer: Changes in state, changes in props, and calls to `forceUpdate` trigger re-renders.

62. How can you optimize the number of re-renders in React?

    • Answer: Techniques like `useMemo`, `useCallback`, `React.memo`, and ensuring functional components are pure can greatly reduce unnecessary re-renders.

63. What is a component's `displayName` property?

    • Answer: `displayName` is a property that can be used to set a more descriptive name for a component. It's helpful for debugging and identifying components in the React DevTools.

64. What is the difference between React and Angular?

    • Answer: React is a library focused on building UIs, while Angular is a full-fledged framework that provides a more structured approach to building applications.

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