React JS Interview Question Advance Level

Mastering React JS: Key Concepts and Best Practices

React JS is a powerful JavaScript library for building user interfaces. Its features and capabilities make it a popular choice for developing efficient and scalable applications. In this guide, we’ll explore some of the key concepts in React JS, including the Virtual DOM, Hooks, Context, and more.

1. What is the Virtual DOM in React JS?

The Virtual DOM is an abstraction of the real DOM (Document Object Model) in React. It acts as a lightweight copy of the real DOM, allowing React to efficiently update the UI when a component's state or props change. Here's how it works:

• When a component's state or props change, React creates a new Virtual DOM representation.
• React compares the new Virtual DOM with the previous one using a diffing algorithm.
• Only the necessary changes are applied to the real DOM, minimizing performance overhead.

Benefits of the Virtual DOM:

• Improved performance by reducing direct DOM manipulations.
• Efficient updates with minimal re-rendering.
• Cross-platform compatibility, including React Native for mobile apps.

2. What are Higher-Order Components (HOC) in React?

Higher-Order Components (HOC) are a pattern in React that allows you to reuse component logic. An HOC is a function that takes a component and returns a new component with enhanced functionality. Here's an example:

const withLogging = (WrappedComponent) => {
    return class WithLogging extends React.Component {
        componentDidMount() {
            console.log("Component is mounted");
        }
        render() {
            return <WrappedComponent {...this.props} />;
        }
    };
};
    

In this example, withLogging is an HOC that adds logging functionality to any component it wraps.

3. What are React Hooks?

React Hooks, introduced in React 16.8, allow you to use state and other React features in functional components. They provide a simpler and more concise way to manage component logic compared to class components. Popular hooks include:

• useState: Manages state in functional components.
• useEffect: Handles side effects like data fetching or subscriptions.
• useContext: Accesses context values without prop drilling.

4. What is React Context?

React Context is a feature that allows you to share data across the component tree without passing props manually at every level. It consists of two main components:

• Context.Provider: Supplies the data to the component tree.
• useContext or Context.Consumer: Accesses the shared data.

React Context is useful for global state management, such as themes, user authentication, or language preferences.

5. Controlled vs. Uncontrolled Components

In React, form elements can be either controlled or uncontrolled:

• Controlled Components: The form data is managed by React state. Changes are handled by updating the state, providing more control over user input.
• Uncontrolled Components: The form data is managed by the DOM itself. This approach is useful when you don’t need immediate validation or manipulation.

6. What is React Router?

React Router is a library that enables routing in React applications. It allows you to define routes and render different components based on the URL. Key components include:

• <Router>: Wraps the application and enables routing.
• <Route>: Defines a route and its corresponding component.
• <Link>: Provides navigation between routes.

7. What is Reconciliation in React?

Reconciliation is the process by React compares the previous and current Virtual DOM to determine the minimal set of changes needed to update the real DOM. This process ensures efficient rendering and optimal performance.

8. What is Lazy Loading in React?

Lazy loading is a technique that loads components or resources only when they are needed. React provides the lazy function and Suspense component to implement lazy loading, reducing the initial load time and improving performance.

9. What is the Purpose of shouldComponentUpdate?

The shouldComponentUpdate lifecycle method allows you to control whether a component should re-render. By implementing this method, you can optimize performance by preventing unnecessary re-renders.

10. What are React DevTools?

React DevTools is a browser extension that helps developers inspect and debug React components. It provides insights into component hierarchies, props, state, and performance, making it an essential tool for React development.

11. What are React Fragments?

React Fragments allow you to group multiple elements without adding an extra DOM node. They are useful when you need to return multiple elements from a component’s render method. Example:

const MyComponent = () => {
    return (
        <>
            <h1>Heading 1</h1>
            <p>Paragraph 1</p>
            <p>Paragraph 2</p>
        </>
    );
};
    

By mastering these concepts, you can build efficient, scalable, and maintainable React applications. Whether you're a beginner or an experienced developer, understanding these fundamentals is key to leveraging the full power of React JS.

Author: Vishal Thakur