Freshers React Developer Top Interview Questions and Answers Part 1

Freshers React Developer Top Interview Questions:

Basic Questions

1. What is React?
   • Answer: React is a JavaScript library for building user interfaces, particularly single-page applications, created and maintained by Facebook.
2. What are the main features of React?
   • Answer: The main features include JSX, virtual DOM, components, one-way data binding, and the React Native for mobile app development.
3. What is JSX?
   • Answer: JSX stands for JavaScript XML. It allows you to write HTML elements in JavaScript and place them in the DOM without using functions like createElement() or appendChild().
4. What is the virtual DOM?
   • Answer: The virtual DOM is a lightweight copy of the actual DOM. React updates the virtual DOM first and then compares it with the real DOM to make efficient updates.
5. Explain the difference between state and props.
   • Answer: Props are read-only attributes used to pass data from parent to child components. State is a local data storage that can be modified within the component.

Intermediate Questions

6. What is a component in React?
   • Answer: A component is a self-contained module that renders some output. It can be a class or a function.
7. How do you create a React component?
   • Answer: You can create a component using a JavaScript class that extends React.Component or a function that returns JSX.
8. What is the lifecycle of a React component?
   • Answer: The lifecycle includes mounting, updating, and unmounting phases with methods like componentDidMount(), componentDidUpdate(), and componentWillUnmount().
9. What are hooks in React?
   • Answer: Hooks are functions that let you use state and other React features in functional components. Examples include useState, useEffect, and useContext.
10. What is the purpose of useState hook?
    • Answer: useState allows you to add state to functional components.

Advanced Questions

11. What is useEffect hook used for?
    • Answer: useEffect is used to perform side effects in functional components, such as data fetching or manually changing the DOM.
12. How do you handle forms in React?
    • Answer: Forms in React are handled by maintaining state for form inputs and updating the state on input changes.
13. What are controlled components?
    • Answer: Controlled components are components where the form data is handled by the state within the component.
14. What is Redux?
    • Answer: Redux is a state management library for JavaScript applications, often used with React for managing application state.
15. How does Redux work?
    • Answer: Redux works by maintaining a single store for the entire application state. Actions describe changes, reducers handle how the state changes, and the store holds the state.

Code and Debugging Questions

16. How do you pass data between components?
    • Answer: Data is passed from parent to child components using props.
17. What is PropTypes?
    • Answer: PropTypes is a library used for type-checking props passed to a component.
18. How do you handle errors in React?
    • Answer: Errors can be handled using error boundaries, which catch JavaScript errors in their child component tree and display a fallback UI.
19. What is the difference between componentDidMount and componentWillMount?
    • Answer: componentWillMount is deprecated and was called before the component was mounted. componentDidMount is called after the component is mounted.
20. Explain the concept of keys in React.
    • Answer: Keys help React identify which items have changed, are added, or removed. They should be unique among siblings.

Best Practices and Optimization

21. What are higher-order components?
    • Answer: Higher-order components (HOCs) are functions that take a component and return a new component with additional props or behavior.
22. How can you optimize performance in React applications?
    • Answer: Techniques include using React.memo, avoiding inline functions in JSX, lazy loading components, and optimizing rendering with shouldComponentUpdate.
23. What is React Router?
    • Answer: React Router is a library for routing in React applications, allowing navigation among views of various components.
24. How do you handle conditional rendering in React?
    • Answer: Conditional rendering can be handled using JavaScript operators like if, ternary operators, or logical &&.
25. What is context in React?
    • Answer: Context provides a way to pass data through the component tree without having to pass props down manually at every level.

Testing and Tools

26. What tools can be used for testing React applications?
    • Answer: Tools include Jest, Enzyme, React Testing Library, and Cypress.
27. How do you use React Testing Library?
    • Answer: React Testing Library provides utilities for testing React components by querying and interacting with DOM nodes.
28. What is Jest?
    • Answer: Jest is a JavaScript testing framework used for testing JavaScript and React applications.
29. How do you perform unit testing in React?
    • Answer: Unit testing in React is performed by testing individual components in isolation using tools like Jest and Enzyme.
30. What is the purpose of snapshot testing?
    • Answer: Snapshot testing is used to ensure UI does not change unexpectedly. It captures the rendered component’s output and compares it to previous snapshots.

Practical Application Questions

31. How do you fetch data in React?
    • Answer: Data can be fetched using fetch, Axios, or other libraries, typically inside useEffect or lifecycle methods.
32. What is React.lazy and Suspense?
    • Answer: React.lazy and Suspense are used for code splitting and lazy loading components.
33. How do you manage state in a large-scale React application?
    • Answer: State can be managed using Redux, Context API, or other state management libraries.
34. What are fragments in React?
    • Answer: Fragments allow you to group a list of children without adding extra nodes to the DOM.
35. How do you style components in React?
    • Answer: Components can be styled using CSS, CSS-in-JS libraries like styled-components, or inline styles.

Conceptual Questions

36. Explain one-way data binding in React.
    • Answer: One-way data binding means data flows in a single direction from parent to child components via props.
37. What is the purpose of useReducer hook?
    • Answer: useReducer is an alternative to useState for managing more complex state logic in functional components.
38. What are portals in React?
    • Answer: Portals allow you to render children into a different part of the DOM tree.
39. What is code splitting?
    • Answer: Code splitting is a technique to split code into smaller bundles which can be loaded on demand, improving performance.
40. Explain React Fiber.
    • Answer: React Fiber is a complete rewrite of the React core algorithm for rendering, optimizing for incremental rendering.

Miscellaneous Questions

41. How do you implement a responsive design in React?
    • Answer: Responsive design can be implemented using CSS media queries, CSS frameworks like Bootstrap, or libraries like Material-UI.
42. What are prop drilling and context API?
    • Answer: Prop drilling is the process of passing data through multiple levels of components. The context API provides a way to share values between components without explicit prop passing.
43. What is the difference between useCallback and useMemo?
    • Answer: useCallback returns a memoized callback function, while useMemo returns a memoized value.
44. How do you use async/await in React?
    • Answer: Async/await can be used inside useEffect or event handlers for asynchronous operations.
45. What is reconciliation in React?
    • Answer: Reconciliation is the process by which React updates the DOM to match the virtual DOM.

Project-Based Questions

46. How would you structure a React project?
    • Answer: A React project can be structured with separate folders for components, containers, services, utilities, and styles.
47. How do you handle authentication in React?
    • Answer: Authentication can be handled using libraries like Firebase, JWT tokens, or OAuth providers.
48. What is the significance of default props?
    • Answer: Default props are used to set default values for props when none are provided.
49. Explain lazy loading of images in React.
    • Answer: Lazy loading images can be done using the loading="lazy" attribute or libraries like react-lazyload.
50. What are some common performance pitfalls in React and how do you avoid them?
    • Answer: Common pitfalls include unnecessary re-renders, large bundle sizes, and inefficient state management. They can be avoided using memoization, code splitting, and efficient state updates.