React.js interview questions with answers

 1. What is React.js?

• React.js is an open-source JavaScript library developed by Facebook for building user interfaces, specifically for single-page applications. It allows developers to create reusable UI components that can be composed together to build complex UIs efficiently.

2. What are the key features of React.js?

• Virtual DOM: React.js uses a virtual DOM to improve performance by minimizing the number of DOM manipulations.
• Component-based architecture: React.js follows a component-based architecture where UIs are composed of reusable and independent components.
• JSX: JSX is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript.
• Unidirectional data flow: React.js follows a unidirectional data flow, where data flows from parent components to child components via props.

3. What is JSX?

• JSX (JavaScript XML) is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. It makes it easier to write and understand React components.

4. What is a component in React.js?

• A component in React.js is a reusable and independent piece of UI that encapsulates a specific piece of functionality. Components can be composed together to build complex UIs.

5. What is a state in React.js?

• State in React.js is an object that represents the internal state of a component. It can be used to store and manage data that changes over time, such as user input or application state.

6. What is props in React.js?

• Props (short for properties) are a way of passing data from parent components to child components in React.js. They are read-only and are used to customize the behavior of a component.

7. What are the lifecycle methods in React.js?

• Lifecycle methods are special methods that are invoked at different stages of a component's lifecycle. Some common lifecycle methods include componentDidMount(), componentDidUpdate(), and componentWillUnmount().

8. What is the difference between state and props in React.js?

• State is an internal data of a component that can change over time, whereas props are external data passed into a component as parameters. State is mutable and controlled by the component itself, while props are immutable and controlled by the parent component.

9. What are controlled components in React.js? - Controlled components are React components whose state is controlled by React. They receive their current value through props and notify changes through callbacks like onChange. This allows React to control the input value and ensure that the UI reflects the state.

10. What are hooks in React.js? - Hooks are functions that enable functional components to use state and other React features without writing a class. They allow developers to reuse stateful logic across multiple components.

11. What is the significance of keys in React.js? - Keys are special attributes that provide a unique identity to elements in a list. They help React identify which items have changed, been added, or been removed, improving performance and efficiency in rendering lists.

12. What are higher-order components (HOCs) in React.js? - Higher-order components (HOCs) are functions that take a component as input and return a new component with enhanced functionality. They are commonly used for code reuse, logic abstraction, and cross-cutting concerns like authentication and logging.

13. How do you handle events in React.js? - Events in React.js are handled using camelCase event names like onClick and onChange. You can pass event handlers as props to components, and access event properties like event.target.value to handle user interactions.

14. What are the benefits of using React.js? - Some benefits of using React.js include improved performance with virtual DOM, component reusability, better code organization with component-based architecture, and a large and active community with extensive documentation and support.

15. How do you optimize performance in React.js? - Performance in React.js can be optimized by using techniques like memoization, code splitting, lazy loading, optimizing render methods, minimizing re-renders with shouldComponentUpdate or React.memo, and using production builds for smaller bundle sizes.