React 20 Interview Question for 2024

Here’s a list of 20 intermediate-level React interview questions along with answers:

1. **What are the key features of React?**
— Answer: React is a JavaScript library for building user interfaces. Its key features include virtual DOM for efficient rendering, component-based architecture for reusability, JSX syntax for describing UI elements, and one-way data flow for predictable updates.

2. **Explain the concept of JSX in React.**
— Answer: JSX (JavaScript XML) is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. It makes React components more readable and intuitive by combining UI markup with JavaScript logic.

3. **What are props in React? How do you pass props from parent to child components?**
— Answer: Props (short for properties) are inputs that are passed into React components. They allow components to receive data from their parent components. Props are passed as attributes in JSX syntax, and they are accessed within the component via the `props` object.

4. **What is the purpose of the `key` attribute in React lists?**
— Answer: The `key` attribute is used to provide a unique identifier to elements in a React list. It helps React identify which items have changed, added, or removed from the list, improving performance and reducing unnecessary re-renders.

5. **Explain the difference between state and props in React.**
— Answer: Props are immutable and are passed from parent to child components, while state is mutable and managed internally within a component. Props are used to pass data from one component to another, while state is used for managing component-specific data that can change over time.

6. **What is the significance of the `useState` hook in React?**
— Answer: The `useState` hook is used in functional components to add state management capabilities. It allows functional components to have stateful behavior by declaring state variables and providing functions to update them. This helps simplify state management and makes functional components more powerful.

7. **Explain the purpose of the `useEffect` hook in React.**
— Answer: The `useEffect` hook is used to perform side effects in functional components. It allows you to run code in response to component lifecycle events, such as mounting, updating, and unmounting. Common use cases include fetching data, subscribing to events, and performing cleanup tasks.

8. **What is the role of the `useReducer` hook in React?**
— Answer: The `useReducer` hook is an alternative to `useState` for managing complex state logic in React. It takes a reducer function and an initial state as arguments and returns the current state and a dispatch function. It’s useful for managing state that involves complex updates or multiple sub-values.

9. **Explain how to handle forms in React.**
— Answer: Forms in React can be handled using controlled components or uncontrolled components. Controlled components are form elements whose values are controlled by React state, while uncontrolled components allow form data to be controlled by the DOM itself.

10. **What is the purpose of the `Context` API in React?**
— Answer: The `Context` API in React allows you to pass data through the component tree without having to pass props manually at every level. It provides a way to share values like themes, user authentication, or language preferences across the component tree.

11. **What are React fragments, and why are they useful?**
— Answer: React fragments are a lightweight syntax for grouping multiple elements in React without adding extra nodes to the DOM. They allow you to return multiple elements from a component’s render method without wrapping them in a div or other container element.

12. **Explain the concept of component lifecycle in React.**
— Answer: Component lifecycle refers to the series of events that occur during the lifecycle of a React component, including mounting, updating, and unmounting. Each lifecycle event has corresponding lifecycle methods that can be overridden to execute custom logic.

13. **What is the `useContext` hook in React, and how is it used?**
— Answer: The `useContext` hook is used to access the value of a React context within a functional component. It takes a context object as an argument and returns the current context value. It’s useful for consuming context values without nesting consumer components.

14. **How do you optimize performance in React applications?**
— Answer: Performance optimization techniques in React include minimizing re-renders, using memoization for expensive computations, implementing lazy loading and code splitting, optimizing component rendering with shouldComponentUpdate or React.memo, and optimizing network requests with debouncing or throttling.

15. **Explain the purpose of server-side rendering (SSR) in React.**
— Answer: Server-side rendering (SSR) in React involves rendering React components on the server and sending the HTML markup to the client, instead of relying on client-side rendering. SSR can improve perceived performance, SEO, and initial load times by delivering pre-rendered content to the browser.

16. **What are React Hooks, and why were they introduced?**
— Answer: React Hooks are functions that enable functional components to use state, lifecycle methods, and other React features without writing class components. They were introduced to simplify state management and lifecycle handling in functional components, making them more expressive and concise.

17. **Explain the concept of lazy loading in React.**
— Answer: Lazy loading is a technique used to improve performance by loading components, images, or other assets asynchronously, only when they are needed. In React, lazy loading can be achieved using React.lazy and Suspense to dynamically import components or modules when they are rendered for the first time.

18. **What are Higher Order Components (HOCs) in React, and how are they used?**
— Answer: Higher Order Components (HOCs) are functions that take a component as input and return a new component with enhanced functionality. They are used to share code, add cross-cutting concerns like authentication or logging, or compose components with reusable logic.

19. **Explain how you would implement client-side routing in a React application.**
— Answer: Client-side routing in React can be implemented using libraries like React Router. You define route components for different URLs and use the BrowserRouter or HashRouter component to wrap your application, providing routing functionality based on the URL path.

20. **What are the advantages of using TypeScript with React?**
— Answer: TypeScript is a statically typed superset of JavaScript that offers benefits like improved code quality, better tooling support, enhanced developer experience, and