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ReactJS has become one of the most popular front-end libraries for building interactive and dynamic web applications. If you're preparing for a ReactJS interview Questions, it's essential to have a strong grasp of the fundamentals, advanced concepts, and best practices. Below is a list of top ReactJS interview questions along with their answers to help you prepare effectively.

1. What is ReactJS?

ReactJS is an open-source JavaScript library developed by Facebook for building user interfaces, especially for single-page applications where UI updates dynamically.

2. What are the key features of React?

• JSX (JavaScript XML): A syntax extension for JavaScript to write HTML-like code inside JavaScript files.

• Virtual DOM: Optimizes performance by updating only the necessary parts of the actual DOM.

• Component-Based Architecture: UI is divided into reusable components.

• Unidirectional Data Flow: Data flows in a single direction, improving control and debugging.

• Hooks: Functional components can now use state and lifecycle features.

3. What is JSX?

JSX is a syntax extension that looks similar to HTML but is used with JavaScript. It makes writing UI components easier and more readable.

const element = <h1>Hello, World!</h1>;

4. What is the Virtual DOM, and how does it work?

The Virtual DOM is an in-memory representation of the real DOM elements. React updates the Virtual DOM and then efficiently updates the real DOM only where changes have occurred, improving performance.

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

6. What are React Hooks?

Hooks allow functional components to use state and lifecycle methods without writing a class.

import { useState } from "react";

function Counter() {

  const [count, setCount] = useState(0);

  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;

}

7. What is useState and useEffect in React?

• useState is used to declare state variables in functional components.

• useEffect is used to handle side effects like fetching data, subscriptions, or updating the DOM.

8. What is a React Router?

React Router is a library for managing navigation in a React application, enabling single-page application behavior with multiple views.

import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

9. What is Redux?

Redux is a state management library that provides a single source of truth for application state and facilitates predictable state updates.

10. What is the difference between Context API and Redux?

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

A HOC is a function that takes a component and returns a new component with additional functionality.

const withLogger = (WrappedComponent) => {

  return (props) => {

    console.log("Rendered with props: ", props);

    return <WrappedComponent {...props} />;

  };

};

12. What is the significance of keys in lists?

Keys help React identify which items have changed, helping in efficient re-renders. Each child in a list should have a unique key.

{items.map(item => <li key={item.id}>{item.name}</li>)}

13. What are controlled and uncontrolled components?

• Controlled Components: Form inputs controlled by React state.

• Uncontrolled Components: Form inputs managed by the DOM.

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14. What are React Fragments?

Fragments allow grouping multiple elements without adding extra nodes to the DOM.

<>

  <h1>Hello</h1>

  <p>World</p>

</>

15. What is reconciliation in React?

Reconciliation is the process React uses to update the DOM efficiently by comparing the Virtual DOM with the previous version.

16. What is lazy loading in React?

Lazy loading allows components to be loaded dynamically, improving performance.

const LazyComponent = React.lazy(() => import('./LazyComponent'));

17. What is React.memo?

React.memo is used to optimize functional components by preventing unnecessary re-renders.

const MemoizedComponent = React.memo(MyComponent);

18. What are error boundaries?

Error boundaries are components that catch JavaScript errors in their child components.

class ErrorBoundary extends React.Component {

  componentDidCatch(error, info) {

    console.log("Error: ", error);

  }

  render() {

    return this.props.children;

  }

}

19. What is Prop Drilling, and how do you avoid it?

Prop drilling is passing props through multiple levels of components. It can be avoided using Context API or state management libraries like Redux.

20. How do you optimize performance in a React application?

• Use React.memo for memoization.

• Use React.lazy and Suspense for lazy loading.

• Optimize component re-renders with useCallback and useMemo.

• Implement code splitting with dynamic imports.

Conclusion

These ReactJS interview questions cover basic to advanced concepts and will help you prepare effectively for your next interview. Keep practicing and building projects to solidify your knowledge!

Want more in-depth interview preparation? Keep exploring and coding!