Top React.js Interview Questions and Answers: Your Ultimate Preparation Guide for 2024

Top React.js Interview Questions and Answers

1. What is React.js?

- Answer: React.js is an open-source JavaScript library developed by Facebook for building user interfaces, particularly for single-page applications. It allows developers to create reusable UI components.

2. Explain the concept of Virtual DOM in React.

- Answer: The Virtual DOM is a lightweight copy of the actual DOM that React uses to optimize updates. When the state of a component changes, React updates the Virtual DOM first and then compares it with the actual DOM to make the minimal changes required, improving performance.

3. What are React components?

- Answer: React components are the building blocks of a React application. They are independent, reusable pieces of UI that can be either class-based or function-based.

4. What is JSX?

- Answer: JSX (JavaScript XML) is a syntax extension for JavaScript that looks similar to HTML. It allows developers to write HTML elements in JavaScript and place them in the DOM.

5. Explain the difference between state and props.

- Answer: State is a local data storage that is specific to a component and can change over time, whereas props (short for properties) are read-only inputs passed from parent to child components.

6. What is a Higher-Order Component (HOC)?

- Answer: A Higher-Order Component is a pattern in React that allows a function to take a component as an argument and return a new component with additional props or functionalities.

7. Describe the lifecycle methods of a React component.

- Answer: React components go through a lifecycle of three phases: Mounting (componentDidMount), Updating (componentDidUpdate), and Unmounting (componentWillUnmount). These methods are used to execute code at specific points during a component’s existence.

8. How does React handle form events?

- Answer: React handles form events through synthetic events, which are wrappers around the browser’s native events, ensuring consistent behavior across different browsers.

9. What is the purpose of the useState hook?

- Answer: The useState hook is a React hook that allows functional components to have a local state. It returns an array containing the state variable and a function to update it.

10. How can you optimize a React application?

- Answer: Optimization techniques include code splitting, lazy loading, memoization, using PureComponent, and avoiding unnecessary re-renders by implementing shouldComponentUpdate or React.memo.

11. What is React Router?

- Answer: React Router is a library that enables dynamic routing in React applications. It allows the creation of navigational components, rendering components based on the current URL, and defining nested routes.

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

- Answer: Controlled components are input elements that are controlled by React through state, whereas uncontrolled components maintain their own internal state and rely on refs to access DOM elements directly.

13. What is Prop Drilling and how can it be avoided?

- Answer: Prop Drilling is the process of passing data from a parent component to deeply nested child components through intermediate components. It can be avoided using Context API or state management libraries like Redux.

14. What is Context API in React?

- Answer: Context API is a feature in React that allows sharing state across the entire component tree without passing props down manually at every level. It's useful for global state management.

15. How do you create a ref in a React component?

- Answer: Refs are created using the React.createRef() function and are assigned to an element via the ref attribute, allowing direct access to a DOM element or a React element.

16. Explain the useEffect hook.

- Answer: The useEffect hook in React allows you to perform side effects in function components, such as fetching data, setting up subscriptions, or manually changing the DOM. It replaces lifecycle methods like componentDidMount and componentDidUpdate.

17. What is the difference between useEffect and useLayoutEffect?

- Answer: useEffect runs asynchronously after the render, while useLayoutEffect runs synchronously after all DOM mutations but before the browser has a chance to paint, making it suitable for synchronizing with the DOM.

18. What is memoization in React?

- Answer: Memoization is an optimization technique used in React to avoid unnecessary re-rendering of components by caching the results of expensive calculations and reusing them when the same inputs occur.

19. How do you handle errors in React components?

- Answer: Errors in React components can be handled using Error Boundaries, which are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI.

20. What is React.memo?

- Answer: React.memo is a higher-order component that prevents a functional component from re-rendering if its props haven't changed, optimizing performance by memoizing the result.

21. Explain the useReducer hook.

- Answer: The useReducer hook is an alternative to useState for managing complex state logic in React components. It takes a reducer function and an initial state and returns the current state and a dispatch function.

22. What is React's Reconciliation process?

- Answer: Reconciliation is React's process of updating the DOM by comparing the new element tree with the previous one (the Virtual DOM), identifying changes, and making minimal updates to the actual DOM.

23. How does React handle rendering performance?

- Answer: React optimizes rendering performance through techniques like the Virtual DOM, memoization, code splitting, lazy loading, and avoiding unnecessary re-renders using tools like React.memo and PureComponent.

24. What are fragments in React?

- Answer: Fragments allow grouping multiple elements without adding an extra node to the DOM. They can be written as <React.Fragment> or simply <>.

25. How do you implement code splitting in React?

- Answer: Code splitting in React can be implemented using dynamic import() statements and the React.lazy() function, often combined with React.Suspense for loading components asynchronously.

26. What are portals in React?

- Answer: Portals in React allow rendering a child component into a different part of the DOM than its parent component, typically used for modals, tooltips, or pop-ups.

27. What is server-side rendering (SSR) in React?

- Answer: Server-side rendering (SSR) is the process of rendering React components on the server and sending fully rendered HTML to the client, improving performance and SEO.

28. How do you hydrate a React app?

- Answer: Hydration is the process of attaching event listeners and making a server-rendered React application fully interactive on the client-side using ReactDOM.hydrate().

29. What is the purpose of React.StrictMode?

- Answer: React.StrictMode is a tool that highlights potential problems in an application by activating additional checks and warnings in development mode, helping developers write better code.

30. How do you handle forms in React?

- Answer: Forms in React can be managed using controlled components, where form data is handled by the state, or uncontrolled components, where refs are used to access form values directly.

31. What is React.PropTypes?

- Answer: React.PropTypes was a type-checking feature for props in React components. It's now deprecated in favor of the prop-types library, which serves the same purpose.

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

- Answer: React is a library for building web applications, while React Native is a framework for building native mobile applications using React. They share a similar syntax and structure but differ in platform-specific components.

33. How do you optimize images in a React application?

- Answer: Image optimization in React can be done using techniques like lazy loading, responsive images with srcset, using image formats like WebP, and compressing images to reduce file sizes.

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

- Answer: TypeScript adds static typing to React, improving code quality, reducing bugs, enhancing IDE support, and making large-scale applications more maintainable by catching errors at compile time.

35. How do you manage state in a large React application?

- Answer: State management in large React applications can be handled using libraries like Redux, MobX, or the Context API, often combined with custom hooks to encapsulate complex logic.