Best 30+ ReactJS Interview Questions and Answers

React JS is among the leading open-source technologies that offer substantial career opportunities for developers. Familiarize yourself with these handpicked React JS interview questions and answers—essential knowledge for any aspiring React developer.

Certainly! Here are 30+ selected React JS interview questions and answers that you should prepare for if you want to build a career in React development:

1. What is React?

Answer: React is an open-source JavaScript library for building user interfaces or UI components for web and mobile applications. The maker of this language is Meta (or, formerly Facebook)

2. What is JSX?

Answer: JSX (JavaScript XML) is a syntax extension for JavaScript used in React to describe the structure of UI elements.

3. Explain the key differences between React and Angular.

Answer: React is a JavaScript library, while Angular is a comprehensive framework. React is known for its flexibility, focusing on the UI layer and using one-way data binding, primarily with JavaScript and JSX. Angular, in contrast, is a full-featured framework, employing TypeScript and two-way data binding. React boasts a larger and more diverse community, whereas Angular's community is slightly smaller. The choice between them depends on project needs and developer preferences, with React offering more freedom, while Angular provides a structured solution for complex applications.

4. What is the Virtual DOM in React?

Answer: The Virtual DOM is a virtual representation of the actual DOM in memory, which React uses to optimize and speed up UI updates.

5. What are React components?

Answer: React components are reusable building blocks for creating UI elements. They can be class-based or functional.

6. Explain the difference between class components and functional components.

Answer: Class components use ES6 classes for defining components, while functional components are JavaScript functions that return JSX.

7. What are props in React?

Answer: Props (short for properties) are inputs to React components that allow data to be passed from parent to child components.

8. What is state in React?

Answer: State is an internal data store for a component that allows it to manage and track changes in its data over time.

9. What is a controlled component in React?

Answer: A controlled component is a component whose form elements are controlled by React state, enabling dynamic and interactive forms.

10. What is the purpose of the useState hook in React?

Answer: The useState hook is used to add state management to functional components.

11. What is a React prop drilling issue, and how can it be solved?

Answer: Prop drilling is the process of passing props down through multiple levels of nested components. It can be solved using Context API or Redux for global state management.

12. Explain the React component lifecycle methods.

Answer: React component lifecycle methods include componentDidMount, componentDidUpdate, and componentWillUnmount, which allow you to hook into various stages of a component's lifecycle.

13. What is Redux, and why is it used with React?

Answer: Redux is a state management library for managing the application's global state. It is often used with React to manage complex state interactions.

14. What are React hooks?

Answer: React hooks are functions that allow you to use state and other React features in functional components.

15. Explain the useEffect hook in React.

Answer: The useEffect hook is used for side effects in functional components, such as data fetching, DOM manipulation, and more.

16. What is React Router, and why is it used?

Answer: React Router is a library for handling routing and navigation in React applications, enabling single-page application behavior.

17. What are the key differences between props and state?

Answer: Props are immutable and passed from parent to child, while state is mutable and managed within a component.

18. What is JSX and why is it used in React?

Answer: JSX is a syntax extension for JavaScript used in React to describe the structure of UI elements. It makes it easier to write and visualize the component's structure.

19. How can you optimize performance in a React application?

Answer: You can optimize performance by using the shouldComponentUpdate method, memoization, code splitting, and implementing virtualization for large lists.

20. What is the purpose of React Fragments?

Answer: React Fragments allow you to group multiple elements without adding extra nodes to the DOM.

21. What are React keys, and why are they important when rendering lists?

Answer: React keys are used to help React identify which items have changed, been added, or been removed in a list. They are crucial for efficient list rendering.

22. How can you pass data from child to parent components in React?

Answer: Data can be passed from child to parent components in React by defining callback functions in the parent and passing them as props to the child.

23. Explain the concept of Higher-Order Components (HOCs).

Answer: Higher-Order Components are functions that take a component and return a new enhanced component. They are used for code reuse and logic sharing.

24. What are the advantages of using Redux for state management in React?

Answer: Redux provides a centralized state store, predictable state updates, time-travel debugging, and better separation of concerns in larger applications.

25. What is React Context, and when is it useful?

Answer: React Context provides a way to pass data through the component tree without having to pass props manually at every level. It's useful for sharing global data.

26. Explain the concept of lazy loading in React.

Answer: Lazy loading is a technique where you load components or assets only when they are needed, improving initial page load times.

27. How do you prevent the re-rendering of a component in React?

Answer: You can prevent a component from re-rendering by using the shouldComponentUpdate method or by using React.memo for functional components.

28. What is error handling in React?

Answer: Error handling in React involves capturing and handling errors in components using error boundaries, like the componentDidCatch lifecycle method.

29. Explain the concept of portals in React.

Answer: Portals allow you to render a component's children into a DOM node that exists outside the parent component's hierarchy.

30. What is React Fiber?

Answer: React Fiber is a reimplementation of the React core algorithm, designed to improve performance and the ability to handle concurrent updates.

These questions and answers should help you prepare for React JS interviews and build a successful career as a React developer. Remember to practice coding and real-world scenarios to solidify your knowledge.