Whether preparing for a senior developer position or looking to refine your advanced React knowledge, being well-versed in a wide range of React topics is crucial.
Here’s a comprehensive list of the top 40 React interview questions to help you prepare for your next interview.

Basic React to Intermediate React Interview Questions

1. What is React?
Answer – React is a JavaScript library for building user interfaces, particularly single-page applications, developed by Facebook. It allows developers to create reusable UI components.

2. What are the main features of React?
Answer The main features of React include JSX, components, one-way data binding, virtual DOM, and the React Native platform.

3. Explain the concept of Virtual DOM.
Answer: The Virtual DOM is a lightweight copy of the real DOM. React uses it to update the UI efficiently by only re-rendering elements that have changed.

4. What is JSX?
Answer: JSX stands for JavaScript XML. It allows developers to write HTML in React, which makes the code easier to understand and debug.

5. What is the difference between state and props?
Answer: State is a mutable data structure used to store data about the component that can change over time. Props are immutable data passed from parent to child components.

6. What are React components?
Answer: Components are the building blocks of a React application. They can be class-based or functional and can be reused throughout the app.

7. What is the significance of keys in React?
Answer: Keys help React identify which items have changed, are added, or are removed. They should be unique among siblings to ensure efficient re-rendering.

8. How does React handle events?
Answer: React handles events using camelCase syntax and passes a synthetic event that normalizes the event properties across different browsers.

9. What are higher-order components (HOCs)?
Answer: HOCs are functions that take a component and return a new component with additional props or state.

10. What is the context API?
Answer: The context API is a way to pass data through the component tree without having to pass props down manually at every level.

Advanced react interview questions

11. 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`.

12. Explain the `useEffect` hook.
Answer: `useEffect` is used to perform side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM. It can replace lifecycle methods like `componentDidMount`, `componentDidUpdate`, and `componentWillUnmount`.

13. How do you optimize performance in a React application?
Answer: Performance optimization techniques include using React.memo, useCallback, and useMemo to prevent unnecessary re-renders, code-splitting, lazy loading, and optimizing the performance of lists with virtualization.

14. What are render props?
Answer: Render props is a technique for sharing code between React components using a prop whose value is a function.

15. What is the significance of error boundaries in React?
Answer: Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of crashing the whole component tree.

16. What is Redux and how does it relate to React?
Answer: Redux is a state management library often used with React to manage application state more predictably and efficiently. It uses a single store for the entire application state.

17. What are the core principles of Redux?
Answer: The core principles of Redux are a single source of truth, state is read-only, and changes are made with pure functions (reducers).

18. How does Redux differ from the Context API?
Answer: Redux is more suited for large-scale applications with complex state logic and provides middleware for side effects. The Context API is suitable for simpler applications or for passing data down the component tree without props drilling.

19. What is server-side rendering (SSR) in React?
Answer: SSR is the process of rendering React components on the server and sending the HTML to the client. This can improve performance and SEO.

20. What is Next.js and how does it enhance React applications?
Answer: Next.js is a React framework that enables server-side rendering, static site generation, and other features like file-based routing and API routes.

In-Depth React Interview Questions for Senior Developers

21. How do you handle side effects in React applications?
Answer: Side effects in React applications can be handled using hooks like `useEffect`, middleware in Redux (such as redux-thunk or redux-saga), or libraries like React Query for data fetching.

22. What are fragments and how are they used in React?
Answer: Fragments let you group multiple elements without adding extra nodes to the DOM. They can be used with `<React.Fragment>` or the shorthand `<>…</>`.

23. Explain the reconciliation process in React.
Answer: Reconciliation is the process through which React updates the DOM. It compares the new virtual DOM with the previous one and only makes changes to the actual DOM where differences are found.

24. How do you manage form state in React?
Answer: Form state in React can be managed using controlled components, where form inputs derive their values from the component’s state, or using libraries like Formik or React Hook Form for more complex forms.

25. What is the difference between `useCallback` and `useMemo`?
Answer: `useCallback` returns a memoized version of a callback function that only changes if one of its dependencies has changed, while `useMemo` returns a memoized value and recalculates it only when its dependencies change.

26. How do you test React components?
Answer: React components can be tested using frameworks like Jest and testing libraries like React Testing Library or Enzyme. Tests can include unit tests, integration tests, and end-to-end tests.

27. What is the purpose of the `useReducer` hook?
Answer: `useReducer` is an alternative to `useState` for managing state in complex components. It provides a way to handle state transitions using a reducer function.

28. How do you implement routing in a React application?
Answer: Routing in a React application can be implemented using libraries like React Router, which provides components like `BrowserRouter`, `Route`, `Link`, and `Switch` for defining routes and navigation.

29. What are controlled and uncontrolled components in React?
Answer: Controlled components have their state managed by React, while uncontrolled components manage their own state using the DOM.

30. How does React handle reconciliation for lists?
Answer: React uses keys to identify items in lists and manage their state. When the keys change, React updates the items accordingly, ensuring efficient re-rendering.

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React interview questions for a senior developer

31. What is the difference between class components and functional components?
Answer: Class components are ES6 classes that extend `React.Component` and have lifecycle methods, while functional components are simpler functions that can use hooks to manage state and lifecycle.

32. How do you manage global state in a React application?
Answer: Global state can be managed using state management libraries like Redux, MobX, or the Context API with custom hooks.

33. What is code splitting and why is it important?
Answer: Code splitting is the practice of breaking up a large bundle of code into smaller chunks that can be loaded on demand. It improves the performance and load time of applications.

34. How do you implement lazy loading in React?
Answer: Lazy loading can be implemented using `React.lazy` and `Suspense` to load components only when they are needed.

35. What is the purpose of `React.StrictMode`?
Answer: `React.StrictMode` is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants.

36. Explain the concept of prop drilling.
Answer: Prop drilling is the process of passing data through multiple nested components via props. It can be mitigated using the Context API or state management libraries.

37. What are custom hooks and how are they used?
Answer: Custom hooks are functions that encapsulate reusable logic using React hooks. They allow for cleaner and more maintainable code by abstracting complex logic.

38. How do you handle authentication in a React application?
Answer: Authentication can be handled using libraries like Firebase, Auth0, or custom solutions with JWT tokens and middleware for secure routing.

39. What is React Query and how does it help in data fetching?
Answer: React Query is a library for managing server-state in React applications. It simplifies data fetching, caching, synchronization, and more.

40. What are controlled and uncontrolled inputs?
Answer: Controlled inputs have their value controlled by React state, while uncontrolled inputs manage their own state via the DOM, usually accessed with refs.

Conclusion

We hope this article has provided you with a comprehensive understanding of the various ReactJS interview questions you might encounter and has better prepared you for your next interview. If you’re looking to further enhance your software development skills, we highly recommend exploring Shef Solutions LLC’s Data Science Course. This program is designed to equip you with the necessary skills to become job-ready in no time.
Preparing for a ReactJS interview involves not just understanding the complexities of this powerful JavaScript library but also ensuring that your user interface (UI) design skills are well-honed. ReactJS is a go-to for building dynamic and interactive UIs, making a solid grasp of UI design principles essential for creating engaging web applications.
To excel in ReactJS interviews, it’s crucial to have a strong foundation in both theoretical concepts and practical application.

Enrolling in a comprehensive course, such as the Data Science Course offered by Shef Solutions LLC, will not only prepare you with the necessary knowledge but also provide you with hands-on experience through project-based learning.
If you have any questions or queries, feel free to post them in the comments section below. Our team will get back to you promptly.