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React.js is a powerful JavaScript library for building dynamic and efficient user interfaces. Whether you’re preparing for an interview or looking to strengthen your React skills, understanding these key concepts is essential.
1️⃣ What is React and Why is it Used?
React is a JavaScript library developed by Facebook for building reusable UI components. It simplifies frontend development with its component-based architecture and Virtual DOM, ensuring faster rendering and better performance.
2️⃣ What are React Hooks?
Hooks allow functional components to manage state and side effects. Key hooks include:
– useState: Manages component state.
– useEffect: Handles side effects (API calls, subscriptions).
– useContext: Accesses context without prop drilling.
– useMemo: Memoizes expensive calculations.
– useCallback: Optimizes performance by memoizing functions.
Example:
import { useState, useEffect } from 'react';
function Counter() {
const [count, setCount] = useState(0);
useEffect(() => {
document.title = <code>Count: ${count}</code>;
}, [count]);
return (
<button onClick={() => setCount(count + 1)}>
Clicked {count} times
</button>
);
}
3️⃣ What is the Virtual DOM?
The Virtual DOM is a lightweight copy of the real DOM. React compares the previous and new Virtual DOM states (diffing algorithm) and updates only the changed parts (reconciliation), improving efficiency.
4️⃣ Functional vs. Class Components
- Functional Components: Simpler, use hooks for state.
- Class Components: Use `this.state` and lifecycle methods (
componentDidMount,componentDidUpdate).
5️⃣ Avoiding Prop Drilling
Use React Context API or state management libraries like Redux or Zustand to avoid passing props through multiple layers.
6️⃣ React Router for Navigation
import { BrowserRouter, Route, Routes, Link } from 'react-router-dom';
function App() {
return (
<BrowserRouter>
<Link to="/home">Home</Link>
<Routes>
<Route path="/home" element={<Home />} />
</Routes>
</BrowserRouter>
);
}
7️⃣ Handling Forms in React
- Controlled Components: Use `useState` to manage form inputs.
- Uncontrolled Components: Access inputs via
useRef.
8️⃣ Redux for State Management
Redux follows a unidirectional data flow:
1. Actions describe changes.
2. Reducers update state.
3. Store holds the global state.
9️⃣ SSR vs. CSR
- SSR (Next.js): Better SEO, faster initial load.
- CSR (Create React App): Smoother dynamic updates.
🔟 React Fragments
function App() {
return (
<>
<h1>Hello</h1>
World
</>
);
}
You Should Know: Essential React.js Commands & Tools
– Create a React App:
npx create-react-app my-app
– Start Development Server:
npm start
– Build for Production:
npm run build
– Install React Router:
npm install react-router-dom
– Add Redux:
npm install @reduxjs/toolkit react-redux
What Undercode Say
Mastering React.js requires hands-on practice with hooks, state management, and performance optimization. Use React DevTools for debugging and Next.js for SSR. Strengthen your skills by building real-world projects and contributing to open-source React repositories.
Expected Output:
A well-structured React.js interview guide with practical examples, commands, and best practices for frontend developers.
References:
Reported By: Ajay Sharma – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
