React Interview questions with Answers- Version 2

80 React interview questions with answers

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1. What are the main features of React?

Answer: The main features of React include:

1. JSX: A syntax extension that allows mixing HTML with JavaScript.
2. Virtual DOM: A lightweight copy of the actual DOM that helps improve performance.
3. Components: Reusable and self-contained pieces of code that define how parts of an application look and behave.
4. Unidirectional Data Flow: Ensures data flows in a single direction, making it easier to understand and debug.
5. Lifecycle Methods: Hooks into different stages of a component’s lifecycle, such as mounting, updating, and unmounting.

2. Explain the concept of Virtual DOM and its benefits.

Answer: The Virtual DOM is a programming concept where a virtual representation of the UI is kept in memory and synced with the real DOM using a library like ReactDOM. Benefits include:

1. Improved performance: Updates are batched and efficient, minimizing direct manipulation of the real DOM.
2. Better user experience: Faster updates and rendering provide a smoother experience.
3. Predictable state management: Helps in managing the state of the application more predictably.

3. What is JSX?

Answer: JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows you to write HTML-like code within JavaScript. It makes the code more readable and easier to write. JSX is transpiled to React.createElement calls by tools like Babel.

4. What are components in React?

Answer: Components are the building blocks of a React application. They are reusable, self-contained pieces of code that define the structure, behavior, and rendering of parts of the UI. There are two main types of components:

1. Functional Components: Written as JavaScript functions.
2. Class Components: Written as ES6 classes.

5. What is the difference between state and props?

Answer:

State: A built-in object that holds data or information about the component. It is mutable and can be changed using the setState method. State is local to the component and is managed within it.

Props: Short for properties, props are read-only objects passed from parent to child components. They are used to pass data and event handlers down the component tree.

6. Explain the component lifecycle in React.

Answer: The React component lifecycle consists of:

Mounting: When a component is being inserted into the DOM.

1. Constructor()
2. static getDerivedStateFromProps()
3. render()
4. componentDidMount()

Updating: When a component is being re-rendered as a result of changes to props or state.

1. static getDerivedStateFromProps()
2. shouldComponentUpdate()
3. render()
4. getSnapshotBeforeUpdate()
5. componentDidUpdate()

Unmounting: When a component is being removed from the DOM.

1. componentWillUnmount()

Error Handling: When there is an error during rendering, in a lifecycle method, or in the constructor of any child component.

1. static getDerivedStateFromError()
2. componentDidCatch()

7. What are hooks in React?

Answer: Hooks are functions that let you use state and other React features in functional components. They provide a way to reuse stateful logic without changing the component hierarchy. Common hooks include:

1. useState: Allows using state in functional components.
2. useEffect: Performs side effects in functional components.
3. useContext: Accesses context in functional components.
4. useReducer: Manages complex state logic.
5. useMemo: Memoizes expensive calculations.

8. How does the useEffect hook work?

Answer: The useEffect hook lets you perform side effects in functional components. It takes two arguments: a function and an optional dependency array. The function runs after the first render and after every update if the dependencies change. If no dependencies are provided, it runs after every render. The function can return a cleanup function to run before the component unmounts or before the effect re-runs.

9. What is the purpose of the useState hook?

Answer: The useState hook allows you to add state to functional components. It returns an array with two elements: the current state value and a function to update it. You can use it to manage component-level state without needing class components.

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

Answer: Performance optimization techniques in React include:

1. Using the Virtual DOM efficiently.
2. Memoizing components using React.memo.
3. Avoiding unnecessary re-renders with shouldComponentUpdate or React.PureComponent.
4. Lazy loading components with React.lazy and Suspense.
5. Using the useMemo and useCallback hooks to memoize values and functions.
6. Code splitting with dynamic import().
7. Using the useDeferredValue and useTransition hooks for smooth rendering of complex UIs.

11. What is React Context and how do you use it?

Answer: React Context is a way to share values between components without passing props through every level of the tree. It consists of a Provider component that supplies the value and a Consumer component that accesses it. The useContext hook simplifies accessing context in functional components.

12. What is the difference between controlled and uncontrolled components in React?

Answer:

Controlled Components: Components where form data is handled by the React component. The component’s state is the single source of truth for the input values.

Uncontrolled Components: Components where form data is handled by the DOM itself. Refs are used to access the input values from the DOM.

13. How does React handle forms?

Answer: React handles forms using controlled or uncontrolled components. Controlled components use state to manage input values and event handlers to update the state. Uncontrolled components use refs to access input values directly from the DOM.

14. What are refs in React?

Answer: Refs (short for references) provide a way to access and interact with DOM nodes or React elements directly. They are created using React.createRef() or the useRef hook and can be attached to elements via the ref attribute.

15. What is 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 or components. Key components include BrowserRouter, Route, Link, Switch, and Redirect.

16. How do you handle side effects in a React component?

Answer: Side effects in React components are handled using the useEffect hook. The hook takes a function that performs the side effect and an optional dependency array to control when the effect runs. The function can also return a cleanup function to handle cleanup tasks.

17. What are Higher-Order Components (HOC)?

Answer: Higher-Order Components (HOC) are functions that take a component and return a new component with additional props or functionality. They are used to reuse component logic and are a pattern for enhancing components.

18. What is the difference between useEffect and useLayoutEffect?

Answer:

1. useEffect: Runs after the render is committed to the screen. It is useful for side effects that don’t block the browser paint, such as fetching data or subscribing to events.
2. useLayoutEffect: Runs synchronously after all DOM mutations and before the browser paints. It is useful for reading layout from the DOM and synchronously re-rendering.

19. How do you manage global state in a React application?

Answer: Global state in a React application can be managed using:

1. Context API: Shares state across the component tree.
2. State management libraries like Redux, MobX, or Recoil: Provide advanced state management solutions with features like time-travel debugging and middleware support.

20. What is Redux and how does it work with React?

Answer: Redux is a state management library that helps manage the global state of an application. It works with React by providing a predictable state container through the use of actions, reducers, and a single store. React-Redux, a binding library, connects Redux with React components.

21. Explain the concept of middleware in Redux.

Answer: Middleware in Redux is a way to extend Redux with custom functionality. It provides a third-party extension point between dispatching an action and the moment it reaches the reducer. Common use cases include logging, crash reporting, performing asynchronous tasks, and handling side effects.

22. What is the use of the connect function in React-Redux?

Answer: The connect function in React-Redux is used to connect React components to the Redux store. It provides a way to map state and dispatch to component props, allowing the component to read from the Redux store and dispatch actions.

23. How do you handle asynchronous actions in Redux?

Answer: Asynchronous actions in Redux are handled using middleware such as Redux Thunk or Redux Saga. Redux Thunk allows you to write action creators that return a function instead of an action, enabling you to perform asynchronous operations. Redux Saga uses generator functions to handle asynchronous tasks in a more declarative manner.

24. What are React Portals and when would you use them?

Answer: React Portals provide a way to render children into a DOM node that exists outside the parent component’s DOM hierarchy. They are useful for rendering modals, tooltips, or any component that needs to visually break out of its parent container while still maintaining the React component tree structure.

25. How do you handle errors in a React application?

Answer: Errors in a React application can be handled using:

1. Error boundaries: Components that catch JavaScript errors in their child component tree and render a fallback UI.
2. The componentDidCatch lifecycle method or the static getDerivedStateFromError method.
3. Using try-catch blocks in synchronous code and promise catch handlers in asynchronous code.

26. What are PropTypes and why are they used in React?

Answer: PropTypes is a type-checking library used in React to validate the props passed to a component. It helps catch bugs by ensuring that components receive the correct data types. PropTypes can be used to specify required props, default values, and custom validation functions.

27. What is the difference between React and React Native?

Answer:

React: A JavaScript library for building web applications with reusable UI components.

React Native: A framework for building native mobile applications using React. It uses native components instead of web components and provides access to native APIs.

28. What is Server-Side Rendering (SSR) and how does it work in React?

Answer: Server-Side Rendering (SSR) is the process of rendering a React application on the server and sending the fully rendered HTML to the client. This improves the initial load time and SEO. Tools like Next.js provide built-in support for SSR in React applications.

29. What is Code Splitting and how do you implement it in React?

Answer: Code splitting is a technique to split a large bundle of code into smaller chunks that can be loaded on demand. It improves the performance of the application by reducing the initial load time. In React, code splitting can be implemented using dynamic imports with React.lazy and Suspense.

30. What is the use of the useReducer hook?

Answer: The useReducer hook is an alternative to useState for managing complex state logic in functional components. It takes a reducer function and an initial state as arguments and returns the current state and a dispatch function. It is useful for handling state transitions based on specific actions.

31. Explain the concept of reconciliation in React.

Answer: Reconciliation is the process by which React updates the DOM to match the virtual DOM. When a component’s state or props change, React creates a new virtual DOM tree and compares it to the previous one. This comparison, or diffing, allows React to determine the minimal set of changes needed to update the real DOM efficiently.

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

Answer: Routing in a React application is handled using React Router. It provides components like BrowserRouter, Route, Link, Switch, and Redirect to define and manage routes, navigate between views, and handle route matching.

33. What is the difference between componentDidMount and useEffect?

Answer:

componentDidMount: A lifecycle method in class components that runs after the component is mounted to the DOM. It is used for side effects like fetching data or setting up subscriptions.

useEffect: A hook in functional components that can replicate the behavior of componentDidMount, componentDidUpdate, and componentWillUnmount. It runs after every render and can be controlled using the dependency array.

34. What are the advantages of using TypeScript with React?

Answer: Advantages of using TypeScript with React include:

1. Static type checking: Catches type-related errors at compile time.
2. Improved code quality and maintainability: Clearer type definitions and interfaces.
3. Better developer experience: Enhanced code completion, navigation, and refactoring support in IDEs.
4. Easier integration with existing JavaScript code: TypeScript is a superset of JavaScript.

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

Answer: The key attribute in React helps identify which items have changed, been added, or removed in a list of elements. It provides a way for React to optimize the rendering process by minimizing re-renders and ensuring efficient updates. Keys should be unique among siblings but stable and predictable.

36. Explain the concept of lifting state up in React.

Answer: Lifting state up is a technique in React where state is moved to the closest common ancestor of the components that need to share it. This allows multiple components to access and update the same state, ensuring data consistency and reducing duplication.

37. How do you handle authentication in a React application?

Answer: Authentication in a React application can be handled using:

1. Authentication libraries like Firebase, Auth0, or AWS Amplify.
2. Custom authentication logic with JWTs (JSON Web Tokens) and OAuth.
3. Storing authentication tokens in cookies or local storage.
4. Protecting routes using higher-order components or hooks like useAuth.

38. What is the use of the useCallback hook?

Answer: The useCallback hook memoizes a function, preventing it from being recreated on every render. It takes a function and a dependency array as arguments and returns a memoized version of the function. It is useful for optimizing performance, especially when passing callbacks to child components.

39. What is the purpose of the useMemo hook?

Answer: The useMemo hook memoizes a value, preventing it from being recalculated on every render. It takes a function that returns the value and a dependency array as arguments. It is useful for optimizing performance by avoiding expensive calculations and improving rendering efficiency.

40. How do you implement a custom hook in React?

Answer: A custom hook in React is a reusable function that encapsulates stateful logic. It starts with the prefix “use” and can use other hooks. To implement a custom hook, create a function that uses hooks like useState or useEffect and returns state and/or functions to be used by components.

41. What is the role of the shouldComponentUpdate lifecycle method?

Answer: The shouldComponentUpdate lifecycle method is used to control whether a component should re-render when its state or props change. It returns a boolean value: true to allow the update and false to prevent it. It is useful for optimizing performance by avoiding unnecessary re-renders.

42. Explain the concept of PureComponent in React.

Answer: PureComponent is a base class in React that provides a shallow comparison of props and state to determine if a component should re-render. It automatically implements the shouldComponentUpdate method with a shallow comparison, optimizing performance by preventing unnecessary re-renders.

43. What is the difference between useState and useReducer?

Answer:

useState: A hook for adding state to functional components. It is simple and straightforward, suitable for managing simple state logic.

useReducer: A hook for managing complex state logic. It uses a reducer function to handle state transitions based on actions, providing a more scalable and organized way to manage state.

44. How do you handle forms in React using Formik?

Answer: Formik is a library for building and managing forms in React. It provides components and hooks to handle form state, validation, and submission. Formik simplifies form handling by providing built-in support for common form tasks like managing input values, validating inputs, and handling form submissions.

45. What is the Context API in React?

Answer: The Context API is a feature in React that allows you to share values between components without passing props through every level of the component tree. It consists of a Provider component that supplies the value and a Consumer component or the useContext hook to access the value.

46. How do you implement lazy loading in React?

Answer: Lazy loading in React can be implemented using the React.lazy function and the Suspense component. React.lazy allows you to dynamically import components, splitting the code and loading it only when needed. The Suspense component provides a fallback UI while the lazy-loaded component is being fetched.

47. What is the purpose of the useRef hook?

Answer: The useRef hook provides a way to create a mutable reference that persists across re-renders. It can be used to access and interact with DOM elements directly or to store mutable values that do not trigger re-renders when changed.

48. How do you handle animations in a React application?

Answer: Animations in a React application can be handled using:

1. CSS transitions and animations.
2. Libraries like React Transition Group, Framer Motion, or React Spring.
3. The use of the requestAnimationFrame API for custom animations.

49. What is the difference between class components and functional components in React?

Answer:

Class Components: Written as ES6 classes, support lifecycle methods, and manage state using the this.state object and setState method.

Functional Components: Written as JavaScript functions, initially stateless but can use hooks like useState and useEffect to manage state and lifecycle.

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

Answer: Benefits of using hooks in React include:

1. Simplified component logic by using functions instead of classes.
2. Improved code readability and maintainability.
3. Reusable stateful logic with custom hooks.
4. Easier to understand and test components.
5. Reduced boilerplate code.

51. How do you implement internationalization (i18n) in a React application?

Answer: Internationalization (i18n) in a React application can be implemented using libraries like react-i18next or react-intl. These libraries provide tools and components to manage translations, handle locale changes, and format dates, numbers, and currencies based on the user’s locale.

52. What is the difference between shallow and deep rendering in testing React components?

Answer:

Shallow Rendering: Renders a component without rendering its children. It is useful for unit testing a single component in isolation.

Deep Rendering: Renders a component along with all its children. It is useful for integration testing to ensure components work together as expected.

53. How do you test React components using Jest and Enzyme?

Answer: Testing React components using Jest and Enzyme involves:

1. Writing test cases using Jest’s test, expect, and other assertion functions.
2. Using Enzyme’s shallow, mount, or render functions to render components.
3. Asserting the component’s output, state, and behavior using Enzyme’s API and Jest’s matchers.

54. What is the purpose of the getSnapshotBeforeUpdate lifecycle method?

Answer: The getSnapshotBeforeUpdate lifecycle method is called right before the DOM is updated. It allows you to capture information from the DOM, such as scroll position, before the update occurs. The value returned from this method is passed as an argument to the componentDidUpdate method.

55. How do you implement server-side rendering with Next.js?

Answer: Server-side rendering with Next.js is implemented using:

1. The getServerSideProps function to fetch data on the server for each request.
2. The getInitialProps method for data fetching in class components or custom _app.js and _document.js files.
3. Automatic static optimization and dynamic routing features provided by Next.js.

56. What is the difference between client-side rendering and server-side rendering?

Answer:

Client-Side Rendering (CSR): The application is rendered in the browser using JavaScript. The initial load is faster, but the user sees a loading spinner or blank page while the JavaScript is fetched and executed.

Server-Side Rendering (SSR): The application is rendered on the server, and the fully rendered HTML is sent to the client. The initial load is slower, but the user sees the content immediately, improving perceived performance and SEO.

57. How do you handle state management in a large-scale React application?

Answer: State management in a large-scale React application can be handled using:

1. The Context API for simple global state.
2. State management libraries like Redux, MobX, or Recoil for more complex state logic.
3. Breaking down the state into local and global state and using hooks like useState, useReducer, and useContext.

58. What is the role of selectors in Redux?

Answer: Selectors in Redux are functions that extract and compute derived data from the Redux store. They provide a way to encapsulate complex state selection logic, improve performance by memoizing results with libraries like Reselect, and ensure components only receive the data they need.

59. How do you implement caching in a React application?

Answer: Caching in a React application can be implemented using:

1. Browser APIs like localStorage, sessionStorage, and IndexedDB.
2. Service workers for caching assets and API responses.

3. Libraries like SWR or React Query for caching and synchronizing server state with the client.

60. What is the purpose of the useImperativeHandle hook?

Answer: The useImperativeHandle hook customizes the instance value that is exposed to parent components when using refs. It allows you to control which methods and properties are accessible, providing a more controlled way to interact with the component’s internal state and methods.

61. How do you handle complex animations in React?

Answer: Complex animations in React can be handled using:

1. CSS animations and transitions for simple animations.
2. Libraries like Framer Motion, React Spring, or GSAP for more complex animations.
3. The use of the requestAnimationFrame API for custom animations.

62. What is the purpose of the useDebugValue hook?

Answer: The useDebugValue hook is used to display a label in React DevTools for custom hooks. It provides a way to add custom debugging information for hooks, making it easier to understand and debug the state and behavior of custom hooks.

63. How do you implement optimistic updates in a React application?

Answer: Optimistic updates in a React application can be implemented by:

1. Updating the UI immediately after an action, before the server confirms the change.
2. Reverting the UI if the server request fails.
3. Using state management libraries like Redux or React Query to manage the optimistic state and handle success and failure scenarios.

64. What is the purpose of the Suspense component in React?

Answer: The Suspense component in React is used to handle the loading state of asynchronous components. It allows you to specify a fallback UI to be displayed while the lazy-loaded component or data is being fetched. Suspense simplifies handling asynchronous operations and improves user experience.

65. How do you handle code splitting in a React application?

Answer: Code splitting in a React application can be handled using:

1. Dynamic imports with React.lazy and Suspense for component-level splitting.
2. Tools like Webpack or Rollup for bundling and splitting code.
3. Libraries like Loadable Components for more advanced code-splitting and server-side rendering support.

66. What are error boundaries and how do you implement them in React?

Answer: Error boundaries are React components that catch JavaScript errors in their child component tree and render a fallback UI. They are implemented using the static getDerivedStateFromError and componentDidCatch lifecycle methods. Error boundaries help prevent crashes and improve user experience.

67. How do you manage side effects in a Redux application?

Answer: Side effects in a Redux application can be managed using middleware like Redux Thunk or Redux Saga. Redux Thunk allows you to write action creators that return a function for handling side effects. Redux Saga uses generator functions to handle complex asynchronous logic in a more declarative manner.

68. What is the purpose of the createSelector function in Reselect?

Answer: The createSelector function in Reselect is used to create memoized selectors for Redux state. It allows you to define functions that compute derived state, improving performance by caching the results of expensive calculations and preventing unnecessary re-renders.

69. How do you implement drag-and-drop functionality in a React application?

Answer: Drag-and-drop functionality in a React application can be implemented using:

1. The HTML5 Drag and Drop API.
2. Libraries like react-dnd or react-beautiful-dnd for more advanced and customizable drag-and-drop interactions.

70. What is the purpose of the useTransition hook in React?

Answer: The useTransition hook is used to manage transitions between different states in a React application. It allows you to specify a pending state while a transition is in progress, improving user experience by providing feedback during complex state changes or asynchronous operations.

71. How do you handle accessibility in a React application?

Answer: Handling accessibility in a React application involves:

1. Using semantic HTML elements.
2. Adding ARIA (Accessible Rich Internet Applications) attributes to improve screen reader support.
3. Ensuring keyboard navigation and focus management.
4. Using tools like React A11y, axe-core, or the Lighthouse accessibility audit.

72. What is the purpose of the useDeferredValue hook in React?

Answer: The useDeferredValue hook is used to defer updating a value until after the current render. It allows you to prioritize rendering updates and avoid blocking the main thread with expensive calculations, improving performance and user experience in complex UIs.

73. How do you handle conditional rendering in React?

Answer: Conditional rendering in React can be handled using:

1. Ternary operators or logical && operators to conditionally render elements.
2. Conditional expressions inside JSX.
3. Returning null to render nothing.
4. Using helper functions or components for more complex conditions.

74. What is the purpose of the StrictMode component in React?

Answer: The StrictMode component in React is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants in development mode, helping you identify and fix issues related to deprecated APIs, side effects, and more.

75. How do you handle forms in React using React Hook Form?

Answer: React Hook Form is a library for handling forms in React. It provides a way to manage form state, validation, and submission using hooks. It simplifies form handling by offering features like uncontrolled inputs, custom validation, and minimal re-renders.

76. What is the purpose of the useMutation hook in React Query?

Answer: The useMutation hook in React Query is used to handle asynchronous operations that modify data, such as creating, updating, or deleting records. It provides functions to trigger the mutation, manage loading and error states, and handle side effects like invalidating queries or updating the cache.

77. How do you implement virtual scrolling in a React application?

Answer: Virtual scrolling in a React application can be implemented using libraries like react-window or react-virtualized. These libraries render only the visible portion of a large list, improving performance by reducing the number of DOM elements.

78. What is the purpose of the useInfiniteQuery hook in React Query?

Answer: The useInfiniteQuery hook in React Query is used to handle paginated data fetching. It provides functions to fetch more data, manage loading and error states, and merge the results, making it easier to implement infinite scrolling or load more patterns.

79. How do you handle error boundaries in React functional components?

Answer: Error boundaries are typically implemented in class components. However, in functional components, you can use hooks like useErrorBoundary from libraries like react-error-boundary to provide similar error handling capabilities.

80. What is the purpose of the useLayoutEffect hook in React?

Answer: The useLayoutEffect hook is similar to useEffect but runs synchronously after all DOM mutations. It is useful for reading layout information or synchronously re-rendering before the browser paints the screen, ensuring the layout is updated before the user sees it.