React Component Resets on Key Change

⚛️ Top 150 React Interview Questions – 43/150 📌 Topic: componentDidMount 🔹 WHAT is it? componentDidMount() is a lifecycle method in Class Components. It runs exactly once, immediately after the component is mounted (inserted into the DOM). In simple terms: 👉 The component is now visible on the screen 👉 The HTML is fully rendered in the browser 🔹 WHY is it designed this way? In React, the render() method must remain pure, meaning it should only return JSX and perform no side effects. componentDidMount() exists as a safe place for side effects. DOM Access: The component is already in the DOM, so you can safely access elements. Data Loading: Perfect place to fetch data from APIs so it runs only once when the component loads. Subscriptions: Ideal for starting timers, WebSockets, or event listeners when the component appears. 🔹 HOW do you use it? (Implementation) class UserList extends React.Component { componentDidMount() { fetch('https://lnkd.in/gEvy2BDJ') .then(res => res.json()) .then(data => this.setState({ users: data })); console.log("Component is now live!"); } render() { return <div>Users Loaded</div>; } } 🔹 WHERE are the best practices? When to Use: • Initial API calls • Starting timers or subscriptions • Initializing third-party libraries (Google Maps, Charts, D3) State Updates: Calling this.setState() is allowed here. It may cause one extra render, but users will not see any UI flicker. Modern Replacement: In Functional Components, this logic is replaced by: useEffect(() => { // componentDidMount logic }, []); 📝 Summary for your notes: componentDidMount is like a housewarming party 🏠 The house (DOM) is ready, people have arrived — now you bring the food (data) and turn on the music (timers). 👇 Comment “React” if this series is helping you 🔁 Share with someone preparing for React interviews #ReactJS #ReactInterview #FrontendDevelopment #JavaScript #LearningInPublic #Top150ReactQuestions

⚛️ Top 150 React Interview Questions – 21/150 📌 Topic: Why is State Immutable in React? This is one of those concepts that separates React users from React engineers. 🔑 What does Immutable mean? Immutable means unchangeable. In React, you should never modify state directly. Instead, you create a new copy with the updated data and pass it to React. ❌ Wrong (Mutation): user.name = "Rahul"; ✅ Right (Immutable update): setUser({ ...user, name: "Rahul" }); ⚡ Why is state immutable? (Performance reason) React stays fast because of the Virtual DOM. To decide whether the UI needs updating, React compares: Old State New State ❌ If you mutate state directly: The object’s memory reference stays the same, so React thinks nothing changed and skips the UI update. ✅ With immutability: Creating a new copy changes the reference. React detects this instantly using shallow comparison (OldReference !== NewReference) and triggers a re-render. Fast. Efficient. Predictable. 🧪 How does immutability help with debugging? Predictability: State never changes silently in the background. Time-travel debugging: Since old states aren’t overwritten, tools like Redux DevTools let you rewind and inspect previous app states. 🚫 Common beginner mistake Arrays and objects. ❌ Mutating an array: items.push(newItem); ✅ Always return a new array: setItems([...items, newItem]); or items.map(...) 📝 Final takeaway: Immutability doesn’t mean data can’t change. It means replace, don’t modify. This allows React to use a quick reference check instead of a slow deep comparison — which is a big reason React is fast. 👇 Comment “React” if this series is helping you 🔁 Share with someone preparing for React interviews #ReactJS #FrontendDevelopment #ReactInterview #JavaScript #WebDevelopment #LearningInPublic #Top150ReactQuestions

This React interview question sounds simple…but it often reveals how well someone understands hooks 👀 “How would you access the previous state or prop in React?” React doesn’t provide a built-in usePrevious hook. Solving this correctly comes down to understanding how values persist across renders. The core idea 👇 ➡️ useRef stores a value without triggering re-renders ➡️ useEffect updates it after each render ➡️ Together, they expose the last rendered value That’s exactly what this custom usePrevious hook does. Why this hook matters 🔥 ➡️ Common React interview pattern ➡️ Clean way to compare current vs previous values ➡️ Useful for counters, animations, forms, and UI transitions ➡️ Avoids unnecessary state and extra renders ➡️ Reusable and readable I’ve attached a clean code snippet image 👇 Hook + simple counter demo. Note: This is a foundational implementation...ideal for learning and interviews, and easy to extend. 💬 BTW Have you ever needed the previous value in a real project? How did you handle it? Don't forget to share your thoughts in the comments below ⬇️ Happy coding 💙 #React #JavaScript #CustomHooks #ReactHooks #Frontend #WebDevelopment #Interviews

⚛️ Top 150 React Interview Questions – 40/150 📌 Topic: React Router Basics 🔹 WHAT is it? React Router is the standard library used for routing in React applications. It allows you to navigate between different pages (views) without reloading the browser, while keeping the UI in sync with the URL. In short: URL changes, page doesn’t refresh — only components change. 🔹 WHY is it designed this way? React is used to build Single Page Applications (SPAs). No Page Refresh: Only the required component updates, giving a smooth app-like experience. Browser History Support: Users can use Back / Forward buttons naturally. Deep Linking: Bookmarking URLs like /profile/123 works perfectly and opens the same page later. 🔹 HOW do you do it? (Basic Setup) In React Router v6+, routing is defined using BrowserRouter, Routes, and Route. import { BrowserRouter, Routes, Route, Link } from 'react-router-dom'; function App() { return ( <BrowserRouter> <nav> <Link to="/">Home</Link> | <Link to="/about">About</Link> </nav> <Routes> <Route path="/" element={<Home />} /> <Route path="/about" element={<About />} /> </Routes> </BrowserRouter> ); } 🔹 WHERE are the best practices? When to Use: Any app with multiple pages, protected routes, or dynamic URLs. Use <Link> instead of <a>: <a> reloads the page and breaks SPA performance. Route Nesting: Structure complex UIs using nested routes like /dashboard/settings, /dashboard/stats. 📝 Summary for your notes: React Router is like a GPS for your App 🧭 You enter a destination (URL), and it shows the correct view (Component) without restarting the journey (page reload). 👇 Comment “React” if this series is helping you 🔁 Share with someone preparing for React interviews #ReactJS #ReactRouter #FrontendDevelopment #JavaScript #WebDevelopment #LearningInPublic #Top150ReactQuestions

⚛️ Top 150 React Interview Questions – 53/150 📌 Topic: Code Splitting 🔹 WHAT is it? Code Splitting is a technique that breaks one large JavaScript bundle into smaller chunks. Instead of loading the entire app at once, React loads only the code that is needed right now. 🔹 WHY use it? Loading everything upfront slows down the app. ✅ Faster Initial Load The Home page loads immediately without waiting for unused pages ✅ Bandwidth Efficient Users don’t download code for features they never visit ✅ Better Performance Browsers process smaller files much faster than one massive bundle 🔹 HOW do you implement it? Use React.lazy() to load a component on demand and Suspense to show a fallback UI while it downloads. const HeavyChart = React.lazy(() => import("./HeavyChart")); function App() { return ( <React.Suspense fallback={<p>Loading...</p>}> <HeavyChart /> </React.Suspense> ); } 👉 The component loads only when required. 🔹 WHERE / Best Practices ✔ Split by Routes (/dashboard, /profile, /admin) ✔ Split Heavy Features Large libraries (charts, PDF tools) used in limited areas ✔ Always use Suspense Without it, the app will break during lazy loading 📝 Summary (Easy to Remember) Code Splitting is like streaming a movie 🎬 You don’t wait for the full 2GB file to download — you start watching while the rest loads in the background. 👇 Comment “React” if this series is helping you 🔁 Share with someone preparing for React interviews #ReactJS #ReactInterview #FrontendDevelopment #JavaScript #WebPerformance #CodeSplitting #Top150ReactQuestions #LearningInPublic #Developers

⚛️ Top 150 React Interview Questions – 33/150 📌 Topic: Controlled vs Uncontrolled Components 🔹 WHAT is it? This topic describes how form data (input values) is handled in React. Controlled Component: The input value is driven by React State. React is the boss of the data. Uncontrolled Component: The input value is handled by the DOM itself. You read the value only when needed using a ref. 🔹 WHY is it designed this way? React provides these two patterns to balance control and simplicity. Controlled (Standard): Needed for instant validation, showing errors while typing, or disabling buttons until the form is valid. Uncontrolled (Edge Case): Useful when integrating with non-React libraries or when you only care about the value at form submission. 🔹 HOW do you do it? (Implementation) Controlled Component (using State): function ControlledForm() { const [name, setName] = useState(""); return ( <input value={name} onChange={(e) => setName(e.target.value)} /> ); } Uncontrolled Component (using Ref): function UncontrolledForm() { const inputRef = useRef(null); const handleSubmit = () => { alert("Name: " + inputRef.current.value); }; return ( <> <input ref={inputRef} type="text" /> <button onClick={handleSubmit}>Submit</button> </> ); } 🔹 WHERE are the best practices? Default to Controlled: Use Controlled Components in most cases. They are more predictable and easier to manage. Use for Validation: Real-time features like character count or password strength checks require Controlled components. defaultValue: In Uncontrolled components, use defaultValue instead of value so the DOM manages updates. 📝 Summary for your notes: A Controlled component is like a Puppet 🪀 — it moves only when React (State) pulls the strings. An Uncontrolled component is like a Wild Animal 🐾 — it does its own thing, and you check on it using a Ref. 👇 Comment “React” if this series is helping you 🔁 Share with someone preparing for React interviews #ReactJS #FrontendDevelopment #ReactInterview #JavaScript #WebDevelopment #LearningInPublic #Top150ReactQuestions

⚛️ Top 150 React Interview Questions – 41/150 📌 Topic: Lifecycle Methods in Class Components 🔹 WHAT is it? Lifecycle Methods are special functions in Class Components that run automatically at different stages of a component’s life. Every component goes through three main phases: Mounting (Birth) Updating (Growth) Unmounting (Death) These methods let you hook into each phase and run code at the right time. 🔹 WHY is it designed this way? React provides lifecycle methods so you can manage side effects properly. Resource Management: Start API calls, timers, or subscriptions exactly once when a component loads. Synchronization: Update the DOM or refetch data when props or state change. Cleanup: Prevent memory leaks by stopping timers or removing listeners before the component is destroyed. 🔹 HOW do you do it? (Core Methods) The three most important lifecycle methods are: 1️⃣ componentDidMount (After Birth) Runs once after the component is added to the DOM. Best for API calls and subscriptions. componentDidMount() { console.log("Component is ready!"); } 2️⃣ componentDidUpdate (After Change) Runs when props or state change. componentDidUpdate(prevProps) { if (prevProps.userId !== this.props.userId) { this.fetchData(this.props.userId); } } 3️⃣ componentWillUnmount (Before Death) Runs just before the component is removed. Used for cleanup. componentWillUnmount() { clearInterval(this.timer); } 🔹 WHERE are the best practices? When to Use: Mostly in legacy codebases or when working with existing Class Components. Avoid Side Effects in render(): render() should stay pure — no API calls or state changes. Always Cleanup: Remove event listeners and timers in componentWillUnmount to keep apps fast. 📝 Summary for your notes: Lifecycle Methods are like a Daily Routine 🕒 componentDidMount → Waking up componentDidUpdate → Adjusting during the day componentWillUnmount → Turning off lights before sleep 👇 Comment “React” if this series is helping you 🔁 Share with someone revising React fundamentals #ReactJS #ReactInterview #LifecycleMethods #FrontendDevelopment #JavaScript #LearningInPublic #Top150ReactQuestions

🚦 How does Next.js handle routing out of the box? One of the most common Next.js interview questions 👇 In Next.js, routing is handled using a file-based routing system—no need for React Router or manual route configs. ✨ Key highlights: Files & folders automatically become routes Supports dynamic & nested routing Built-in <Link /> for fast client-side navigation SEO-friendly URLs by default 📌 If you’re preparing for Next.js interviews, this is a core concept you must understand. 🧠 Discuss Next.js interview question 💬 Have you used Pages Router or App Router in production? Share your experience! #NextJS #ReactJS #FrontendInterview #WebDevelopment #JavaScript #NextJSInterview #Infographic #FullStackDeveloper

🚀 Top React.js Topics You Must Master for Frontend Interviews 👩🎓React.js continues to dominate the frontend ecosystem, and cracking React interviews today requires much more than memorizing definitions. 📚 You need clarity, depth, and real-world understanding of how React works under the hood. I recently explored a solid guide that covers the most essential React concepts every frontend developer should master — from fundamentals to advanced patterns used in real projects: ✅ Components & Props ✅ State & Component Lifecycle ✅ Hooks (useState, useEffect, useMemo, useCallback, etc.) ✅ Virtual DOM & Reconciliation ✅ Performance Optimization Techniques ✅ Context API for State Management ✅ Rendering Patterns in React ✅ Handling Forms, Events & API Calls ✅ React Router ✅ Creating & Reusing Custom Hooks ✅ Best Practices, Architecture & Clean Code Whether you're a beginner learning React, a mid-level developer preparing for interviews, or an experienced engineer revising core concepts, mastering these topics can significantly boost your confidence and performance in frontend interviews. Credit: Bosscoder 💡 Strong fundamentals + practical understanding = interview success. #ReactJS #FrontendDevelopment #JavaScript #WebDevelopment #FrontendInterviews #ReactHooks #CleanCode #DeveloperLife #Parmeshwarmetkar

⚛️ Top 150 React Interview Questions – 34/150 📌 Topic: Higher-Order Components (HOC) 🔹 WHAT is it? A Higher-Order Component (HOC) is an advanced pattern in React for reusing component logic. It is not part of the React API. It is a pattern where a function takes a component as an argument and returns a new, enhanced component. 🔹 WHY is it designed this way? HOCs follow the Don’t Repeat Yourself (DRY) principle. Reusability: Instead of writing the same logic (like authentication checks) in multiple components, you write it once in an HOC and reuse it. Separation of Concerns: UI components stay clean and focus only on rendering, while the HOC handles extra logic like data fetching, analytics, or permissions. 🔹 HOW do you do it? (The Implementation) An HOC is simply a function that wraps another component. Example: withLoading HOC // 1. The HOC (Wrapper) function withLoading(Component) { return function EnhancedComponent({ isLoading, ...props }) { if (isLoading) return <p>Loading...</p>; return <Component {...props} />; }; } // 2. Regular Component const UserProfile = ({ name }) => <h1>User: {name}</h1>; // 3. Enhanced Component const ProfileWithLoading = withLoading(UserProfile); // Usage: // <ProfileWithLoading isLoading={true} name="John" /> 🔹 WHERE are the best practices? When to Use: Authentication, Authorization, Logging, Analytics, or shared UI layouts. Don’t Mutate: Never modify the original component. Always return a new one. Pass Unrelated Props: Use ...props so the wrapped component works normally. Don’t Use Inside Render: Defining HOCs inside render causes unnecessary unmounting and remounting. 📝 Summary for your notes: An HOC is like a Phone Case 📱 The phone (Component) stays the same. The case (HOC) adds extra features without changing the phone itself. 👇 Comment “React” if this series is helping you 🔁 Share with someone preparing for React interviews #ReactJS #FrontendDevelopment #ReactInterview #JavaScript #WebDevelopment #LearningInPublic #Top150ReactQuestions