Introduction
Are you preparing for a React.js interview? You're in the right place! This comprehensive guide covers 15 crucial React concepts you need to know, complete with code examples and practical explanations.
Table of Contents
React.js Fundamentals
Q1: What is React.js and what are its key features? 🤔
React.js is an open-source JavaScript library developed by Facebook for building user interfaces. Here's what makes it special:
Key Features:
- Virtual DOM 🔄
// React handles DOM updates efficiently
const element = <h1>Hello, React!</h1>;
- Component-Based Architecture 🏗️
// Function Component
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
// Class Component
class Welcome extends React.Component {
render() {
return <h1>Hello, {this.props.name}</h1>;
}
}
State and Props
Q2: What's the difference between state and props? 🤔
Understanding the distinction is crucial for React development:
Props Example:
// Parent Component
function Parent() {
return <Child name="John" age={25} />;
}
// Child Component
function Child(props) {
return (
<div className="user-card">
<p>Name: {props.name}</p>
<p>Age: {props.age}</p>
</div>
);
}
React Hooks
Q3: What are React Hooks? Explain the most commonly used hooks. 🎣
Hooks revolutionized how we write React components. Here are the essentials:
// useState Example
function NameInput() {
const [name, setName] = useState('');
return (
<input
value={name}
onChange={(e) => setName(e.target.value)}
className="input-primary"
/>
);
}
// useEffect Example
function UserProfile({ userId }) {
const [user, setUser] = useState(null);
useEffect(() => {
fetchUser(userId).then(data => setUser(data));
}, [userId]);
return user ? <div>{user.name}</div> : <div>Loading...</div>;
}
Component Lifecycle
Q4: Explain the React Component Lifecycle methods. ⚙️
Modern React offers two approaches to lifecycle management:
Class Components:
class LifecycleComponent extends React.Component {
componentDidMount() {
console.log('Component mounted');
}
componentDidUpdate(prevProps) {
if (this.props.id !== prevProps.id) {
console.log('Props changed');
}
}
componentWillUnmount() {
console.log('Component will unmount');
}
}
Context API
Q5: What is the Context API and when should you use it? 🌍
Context provides a way to share values between components without prop drilling:
const ThemeContext = React.createContext('light');
function App() {
const [theme, setTheme] = useState('light');
return (
<ThemeContext.Provider value={theme}>
<ThemedButton />
</ThemeContext.Provider>
);
}
React Router
Q6: What is React Router and how does it work? 🛣️
import { BrowserRouter, Route, Switch, Link } from 'react-router-dom';
function App() {
return (
<BrowserRouter>
<nav>
<Link to="/">Home</Link>
<Link to="/about">About</Link>
</nav>
<Switch>
<Route exact path="/" component={Home} />
<Route path="/about" component={About} />
</Switch>
</BrowserRouter>
);
}
Virtual DOM
Q7: How does React's Virtual DOM improve performance? 🚀
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<h1>Count: {count}</h1>
<button onClick={() => setCount(count + 1)}>
Increment
</button>
</div>
);
}
React Fragments
Q8: What are React Fragments and why do we need them? 🧩
// Better way using Fragments
function ListItems() {
return (
<>
<li>Item 1</li>
<li>Item 2</li>
</>
);
}
Higher-Order Components
Q9: What are Higher-Order Components (HOC)? 🔄
function withAuth(WrappedComponent) {
return function WithAuthComponent(props) {
const [isAuthenticated] = useState(true);
if (!isAuthenticated) {
return <Navigate to="/login" />;
}
return <WrappedComponent {...props} />;
};
}
useCallback Hook
Q10: When and why should you use useCallback? 🎣
function ParentComponent() {
const [count, setCount] = useState(0);
const memoizedCallback = useCallback(() => {
console.log(count);
}, [count]);
return <ChildComponent onClick={memoizedCallback} />;
}
Error Boundaries
Q11: How do Error Boundaries work in React? 🛡️
class ErrorBoundary extends React.Component {
state = { hasError: false };
static getDerivedStateFromError(error) {
return { hasError: true };
}
render() {
if (this.state.hasError) {
return <h1>Something went wrong</h1>;
}
return this.props.children;
}
}
useReducer Hook
Q12: When would you use useReducer over useState? 📊
const initialState = { count: 0 };
function reducer(state, action) {
switch (action.type) {
case 'increment':
return { count: state.count + 1 };
default:
return state;
}
}
function Counter() {
const [state, dispatch] = useReducer(reducer, initialState);
return (
<button onClick={() => dispatch({ type: 'increment' })}>
Count: {state.count}
</button>
);
}
React Portals
Q13: What are React Portals and when would you use them? 🚪
function Modal({ children }) {
return ReactDOM.createPortal(
<div className="modal">
{children}
</div>,
document.getElementById('modal-root')
);
}
Strict Mode
Q14: What benefits does React's Strict Mode provide? 🔍
function App() {
return (
<React.StrictMode>
<div>
<Header />
<Main />
<Footer />
</div>
</React.StrictMode>
);
}
Code Splitting
Q15: How does Suspense enable better code splitting? 📦
const LazyComponent = lazy(() => import('./LazyComponent'));
function App() {
return (
<Suspense fallback={<Loading />}>
<LazyComponent />
</Suspense>
);
}
Wrap Up 🎉
Congratulations! You've covered the most important React concepts for your interview. Remember:
Focus on understanding the concepts deeply
Practice implementing these patterns
Think about real-world use cases
Be ready to explain your reasoning
Interview Tips 💡
Start with simple explanations, then dive deeper
Use practical examples from your experience
Discuss trade-offs and best practices
Show enthusiasm for learning and problem-solving
What's Next?
Stay tuned for next Part, where we'll cover:
Performance optimization techniques
Testing strategies
Advanced state management
Server-side rendering
And much more!
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Tags: #React #JavaScript #WebDevelopment #Programming #TechnicalInterview #ReactJS #FrontEnd #Coding #Interview #WebDev
Performance optimization
Testing strategies
State management patterns
And much more!
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