Top 10 React Interview Questions and Answers

 As React continues to be a leading library for building user interfaces, it's crucial to understand its core concepts. Here are ten common React interview questions along with detailed answers to help you prepare.

1. What is React?

Answer: React is an open-source JavaScript library developed by Facebook for building user interfaces, particularly single-page applications. It allows developers to create reusable UI components, manage the application state efficiently, and optimize rendering performance using a virtual DOM.

2. What are components in React?

Answer: Components are the building blocks of a React application. They are JavaScript functions or classes that return a React element, which is a description of what the UI should look like. Components can be functional (stateless) or class-based (stateful), and they can accept inputs called "props" to render dynamic content.

3. What is the virtual DOM?

Answer: The virtual DOM is a lightweight representation of the actual DOM. React creates a virtual DOM to optimize rendering performance by minimizing direct manipulation of the actual DOM, which is slow. When the state of a component changes, React updates the virtual DOM, compares it with the previous version (a process called "reconciliation"), and then efficiently updates only the parts of the actual DOM that have changed.

4. What are props in React?

Answer: Props (short for "properties") are inputs to React components. They are used to pass data from one component to another, allowing for dynamic rendering and reusability. Props are read-only and cannot be modified by the child component receiving them.

5. What is state in React?

Answer: State is an object that represents the dynamic data of a component. Unlike props, which are passed down from parent to child, state is managed within the component itself. Changes to the state trigger re-renders, allowing the UI to update in response to user interactions or other events.

6. What is the purpose of the useEffect hook?

Answer: The useEffect hook allows you to perform side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM. It runs after the component renders and can be configured to run on specific state or prop changes, or when the component mounts and unmounts.

7. What are controlled and uncontrolled components?

Answer:

• Controlled Components: These are form elements whose value is controlled by React state. The component’s value is set by the state, and updates are handled via event handlers.
• Uncontrolled Components: These are form elements that maintain their own internal state. You can access their values using a ref but do not control them via React state.

8. What is React Router?

Answer: React Router is a library for managing navigation and routing in React applications. It enables you to define routes in your application, allowing for dynamic rendering of components based on the URL. It supports features like nested routes, route parameters, and programmatic navigation.

9. What is context in React?

Answer: Context is a React feature that allows you to share values (like global state) between components without passing props explicitly through every level of the component tree. It is useful for managing global data such as themes, user authentication, or language settings.

10. What is the difference between functional and class components?

Answer:

• Functional Components: These are simpler, stateless components defined as JavaScript functions. They can use hooks for state and lifecycle management.
• Class Components: These are ES6 classes that extend React.Component. They can have their own state and lifecycle methods. Class components are more verbose and were the standard way to handle state and lifecycle before hooks were introduced in React 16.8.