ReactJs Interview Questions & Answers

Basics of React:

1. What is React.js?

   • Answer: React.js is a JavaScript library for building user interfaces, developed and maintained by Facebook. It allows developers to create reusable UI components and efficiently update the UI in response to changes.

2. Explain the key features of React.

   • Answer: React features a virtual DOM, component-based architecture, one-way data binding, JSX syntax for UI definition, and unidirectional data flow.

3. What is JSX in React?

   • Answer: JSX (JavaScript XML) is a syntax extension for JavaScript recommended by React. It allows writing HTML-like code within JavaScript files, making it easier to describe the UI components.

4. How does React handle components?

   • Answer: React components are modular, self-contained units that can be reused. They can be class components or functional components and are composed to build complex UIs.

5. What is the significance of the virtual DOM in React?

   • Answer: The virtual DOM is an in-memory representation of the actual DOM elements. React uses it to perform efficient updates by minimizing direct manipulation of the real DOM, leading to improved performance.

Component Lifecycle:

6. Explain the lifecycle methods of a class component in React.

   • Answer: Key lifecycle methods include componentDidMount, componentDidUpdate, and componentWillUnmount. They allow developers to perform actions at different stages of a component's existence.

7. What is the purpose of componentDidMount lifecycle method?

   • Answer: componentDidMount is invoked after a component is inserted into the DOM. It is commonly used for fetching data, setting up subscriptions, or performing other side effects.

8. How do you handle side effects in functional components in React?

   • Answer: You can use the useEffect hook to handle side effects in functional components. It combines the functionality of componentDidMount, componentDidUpdate, and componentWillUnmount in class components.

9. What is the significance of the key attribute in React lists?

   • Answer: The key attribute is used to uniquely identify elements in a list. It helps React efficiently update and re-render components when the order of items changes.

10. Explain the purpose of the shouldComponentUpdate method in React.

    • Answer: shouldComponentUpdate is called before rendering when new props or state are being received. It allows developers to control whether a component should re-render, improving performance.

State and Props:

11. What is the difference between state and props in React?

    • Answer: State is internal and controlled by the component itself, while props are external and passed to the component. State can be changed, while props cannot.

12. How do you update state in React?

    • Answer: Use the setState method to update state. It takes an object with new state values and triggers a re-render of the component.

13. Can you explain the concept of lifting state up in React?

    • Answer: Lifting state up involves moving the state to a common ancestor component when multiple components need access to the same state. This promotes better data flow and communication between components.

14. What are controlled components in React?

    • Answer: Controlled components are React components whose state is controlled by React. They take their current value through props and notify changes through callbacks.

15. Explain the purpose of defaultProps in React.

    • Answer: defaultProps is used to specify default values for props if they are not provided by the parent component. It ensures that the component has default values in case props are not passed.

Hooks:

16. What are React Hooks?

    • Answer: React Hooks are functions introduced in React 16.8 to allow functional components to use state and lifecycle features without using class components.

17. Explain the useState hook in React.

    • Answer: useState is a hook that enables functional components to manage state. It returns an array with the current state value and a function to update it.

18. How does the useEffect hook work in React?

    • Answer: useEffect is used for side effects in functional components. It takes a function as its first argument and is called after every render. It can also return a cleanup function.

19. What is the purpose of the useContext hook in React?

    • Answer: useContext allows functional components to subscribe to a context without introducing a nesting hierarchy. It takes a context object as an argument and returns the current context value.

20. How do you create a custom hook in React?

    • Answer: Custom hooks are functions that use React hooks. They must start with the word "use." For example, useCustomHook. They can encapsulate reusable logic for components.

Routing in React:

21. Explain client-side routing in React.

    • Answer: Client-side routing in React involves updating the UI based on the URL without a full page reload. Libraries like React Router help manage routing in React applications.

22. What is React Router?

    • Answer: React Router is a library for handling routing in React applications. It enables the navigation between different components while keeping the UI in sync with the URL.

23. How do you implement navigation in React using React Router?

    • Answer: React Router provides components like BrowserRouter, Route, and Link. You use these components to define routes and navigation links in your application.

24. What is the purpose of the useHistory hook in React Router?

    • Answer: The useHistory hook allows accessing the history object, providing methods for navigating, such as push and goBack.

25. Explain the concept of route parameters in React Router.

    • Answer: Route parameters are dynamic segments in the URL path, allowing components to respond to changes in the URL and display different content based on those parameters.

Forms in React:

26. How do you handle forms in React?

    • Answer: React forms can be controlled or uncontrolled. Controlled forms manage form data through React state, while uncontrolled forms rely on DOM references.

27. Explain the controlled component approach for forms in React.

    • Answer: In a controlled component, form elements like input fields have their values controlled by React state. Changes are handled through event handlers, ensuring React manages the form state.

28. What is the purpose of the onChange event in React forms?

    • Answer: The onChange event is triggered when the value of a form element changes. It is commonly used to update the state of controlled components in React forms.

29. How do you prevent the default behavior of a form submission in React?

    • Answer: Use the event.preventDefault() method in the form submission event handler to prevent the default form submission behavior.

30. What is the purpose of the useState hook in handling form input in React?

    • Answer: The useState hook is used to

manage the state of form input elements in controlled components, allowing React to track and update the input values.

State Management:

31. What is state management in React, and why is it important?

    • Answer: State management in React involves managing and sharing state across different components. It is essential for building complex applications where components need to communicate and share data.

32. Explain the Flux architecture in React.

    • Answer: Flux is an application architecture introduced by Facebook for managing state in React applications. It involves a unidirectional data flow and consists of actions, dispatchers, stores, and views.

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

    • Answer: Redux is a state management library for JavaScript applications. It works with React by providing a predictable state container that can be accessed by components, ensuring a unidirectional data flow.

34. How do you connect a React component to Redux?

    • Answer: The connect function from the react-redux library is used to connect a React component to the Redux store. It wraps the component and provides it with the necessary state and actions.

35. What is the purpose of the dispatch function in Redux?

    • Answer: The dispatch function is used to send actions to the Redux store. Actions describe changes to the state, and the reducer functions handle these actions to update the state.

Context API:

36. What is the React Context API?

    • Answer: The Context API is a feature in React that allows components to share values like themes or authentication status without manually passing props through each level of the component tree.

37. How do you create a context in React using the Context API?

    • Answer: The createContext function is used to create a new context. For example:

      const MyContext = React.createContext();
      

38. Explain the useContext hook in React.

    • Answer: The useContext hook allows functional components to consume the value from a React context. It takes the context object as an argument and returns the current context value.

39. What is the purpose of the Provider component in the Context API?

    • Answer: The Provider component is used to wrap a part of the component tree and provide the context value to all components within that subtree.

40. How can you consume a context in a class component?

    • Answer: The Context.Consumer component is used to consume a context value in a class component. It involves rendering a function as a child that receives the current context value.

Testing in React:

41. What are the popular testing libraries for React?

    • Answer: Popular testing libraries for React include Jest, Testing Library, Enzyme, and React Testing Library.

42. Explain snapshot testing in Jest for React components.

    • Answer: Snapshot testing involves capturing a snapshot of the rendered output of a component and comparing it with a stored snapshot. It helps detect unexpected changes in the UI.

43. What is the purpose of the render function in React Testing Library?

    • Answer: The render function in React Testing Library is used to render a React component into a virtual DOM, making it possible to query and interact with the component during testing.

44. How can you simulate user interactions in React tests?

    • Answer: The fireEvent utility in React Testing Library allows simulating user interactions such as clicks, changes, and submits.

45. What is the significance of the act function in React tests?

    • Answer: The act function in React Testing Library is used to explicitly run side effects and updates in a synchronous manner during tests, ensuring that asynchronous actions are properly tracked.

Error Handling and Debugging:

46. How do you handle errors in React applications?

    • Answer: React error boundaries, introduced through the ErrorBoundary component, can be used to catch JavaScript errors during rendering and log those errors.

47. Explain the concept of error boundaries in React.

    • Answer: Error boundaries are React components that catch JavaScript errors anywhere in their child component tree. They log those errors and display a fallback UI instead of crashing the component tree.

48. What is the purpose of the componentDidCatch lifecycle method in React error boundaries?

    • Answer: The componentDidCatch method is called when an error is caught by an error boundary. It receives information about the error and allows developers to log it or perform other actions.

49. How can you debug a React application?

    • Answer: Developers can use browser developer tools, the debugger statement, console.log(), and tools like React DevTools for debugging React applications.

50. Explain the concept of React DevTools.

    • Answer: React DevTools is a browser extension that provides a set of debugging tools specifically designed for React applications. It allows inspecting React component hierarchies, state, and props.