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Commonly asked React.js Low-Level Design (LLD) interview questions:
- How would you implement a toast notification system in React?
- How would you build a comment thread feature with nested replies?
- How would you design a responsive sidebar navigation component?
- How would you implement a tab component with animated switching?
- How would you build a filterable and sortable data table?
- How would you design a favorite/like button with optimistic updates?
- How would you implement a live chat feature in React?
- How would you build a rate limiter or throttling logic for a button click?
- How would you build a collapsible ACCORDION component?
- How would you implement theming (dark/light mode) in a React app?
You Should Know:
Practical implementation with code examples:
1. Toast Notification System:
[javascript]
// Using Context API
const ToastContext = createContext();
function ToastProvider({ children }) {
const [toasts, setToasts] = useState([]);
const addToast = (message, type = ‘info’) => {
const id = Date.now();
setToasts([…toasts, { id, message, type }]);
setTimeout(() => removeToast(id), 5000);
};
const removeToast = (id) => {
setToasts(toasts.filter(toast => toast.id !== id));
};
return (
<ToastContext.Provider value={{ addToast }}>
{children}
))}
</ToastContext.Provider>
);
}
[/javascript]
2. Optimistic Updates for Like Button:
[javascript]
function LikeButton({ postId }) {
const [isLiked, setIsLiked] = useState(false);
const [isLoading, setIsLoading] = useState(false);
const handleLike = async () => {
const previousState = isLiked;
setIsLiked(!previousState);
setIsLoading(true);
try {
await api.likePost(postId, !previousState);
} catch (error) {
setIsLiked(previousState); // Rollback on error
} finally {
setIsLoading(false);
}
};
return (
);
}
[/javascript]
3. Theme Implementation with CSS Variables:
[javascript]
// Theme context
const ThemeContext = createContext();
function ThemeProvider({ children }) {
const [theme, setTheme] = useState(‘light’);
const toggleTheme = () => {
const newTheme = theme === ‘light’ ? ‘dark’ : ‘light’;
setTheme(newTheme);
document.documentElement.setAttribute(‘data-theme’, newTheme);
};
return (
<ThemeContext.Provider value={{ theme, toggleTheme }}>
{children}
</ThemeContext.Provider>
);
}
// CSS
:root {
–bg-color: #ffffff;
–text-color: #333333;
}
[data-theme=”dark”] {
–bg-color: #1a1a1a;
–text-color: #f0f0f0;
}
[/javascript]
4. Throttling Button Clicks:
[javascript]
import { throttle } from ‘lodash’;
function SearchButton() {
const handleSearch = throttle(() => {
// API call here
console.log(‘Search executed’);
}, 1000);
return (
);
}
[/javascript]
5. WebSocket Chat Implementation:
[javascript]
function ChatComponent() {
const [messages, setMessages] = useState([]);
const [socket, setSocket] = useState(null);
useEffect(() => {
const ws = new WebSocket(‘wss://your-chat-server.com’);
setSocket(ws);
ws.onmessage = (event) => {
const newMessage = JSON.parse(event.data);
setMessages(prev => […prev, newMessage]);
};
return () => ws.close();
}, []);
const sendMessage = (text) => {
socket.send(JSON.stringify({ text }));
};
return (
);
}
[/javascript]
What Undercode Say:
For developers working with React.js, mastering these low-level design patterns is crucial for building scalable applications. Here are some additional Linux/IT commands that can help in your development workflow:
1. Process Management:
– `ps aux | grep node` – Find all Node.js processes
– `kill -9 [PID]` – Force kill a process
2. Network Debugging:
– `curl -v http://localhost:3000` – Test your React dev server
– `netstat -tulnp | grep 3000` – Check port usage
3. Performance Monitoring:
– `top` – Monitor system resources
– `htop` – Interactive process viewer
4. Development Tools:
– `npm run build` – Create production build
– `npm audit` – Check for vulnerabilities
5. Windows Equivalent Commands:
– `tasklist | findstr node` – Find Node processes
– `netstat -ano | findstr 3000` – Check port usage
6. Docker Commands for React Development:
– `docker build -t react-app .`
– `docker run -p 3000:3000 react-app`
7. Git Commands for Team Collaboration:
– `git rebase -i HEAD~3` – Interactive rebase
– `git cherry-pick [commit-hash]` – Selective commit application
8. CI/CD Helpers:
– `npm test — –coverage` – Run tests with coverage
– `npx lighthouse http://localhost:3000` – Audit your app
Remember to always test your components thoroughly and consider edge cases when implementing these patterns. The key to successful React development lies in understanding both the high-level architecture and low-level implementation details.
Expected Output:
A comprehensive guide to React.js low-level design patterns with practical code examples and related development commands.
References:
Reported By: Sakshi Gawande – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
