React Interview Questions: Virtual DOM, Rest Parameters & More

🚀 JavaScript Interview Quick Revision Q. What is the difference between localStorage and sessionStorage? Answer: Persistence duration — localStorage persists indefinitely; sessionStorage clears on tab close. Q. What is the difference between document.ready and window.onload? Answer: document.ready triggers when DOM is ready; window.onload when all assets load. Q. What are JavaScript timers? Answer: Functions like setTimeout and setInterval to schedule code execution. Q. What is debouncing? Answer: Technique to limit how often a function is called. Q. What is throttling? Answer: Technique to make sure a function is called at most once per interval. 📘 These are part of my 3000+ JavaScript Interview Questions & Answers book. If you're preparing for frontend interviews, structured revision matters. Comment “JS” if you want the book link.

🚀 JavaScript Interview Questions – Are You Really Prepared? Here are 5 questions many developers struggle to answer clearly: 1️⃣ What is the difference between parseInt() and Number()? Answer: parseInt() parses until invalid character; Number() converts entire string or returns NaN. 2️⃣ How do you check if a variable is an array? Answer: Using Array.isArray(). 3️⃣ What is the output of typeof null? Answer: "object" (a known JS quirk). 4️⃣ What is destructuring assignment? Answer: Syntax for unpacking values from arrays or properties from objects. 5️⃣ What is event bubbling? Answer: When an event propagates from child to parent elements. ⚠️ These are just 5 out of 3000+ JavaScript Interview Questions & Answers covered in my book. If you're preparing for: Frontend interviews React / Node roles Product-based companies Startup technical rounds This book is designed for clear, precise, interview-focused answers. 📘 3000+ Questions 📘 Beginner to Advanced 📘 Concise, interviewer-ready explanations If you're serious about cracking JavaScript interviews, this resource will save you months of preparation time.

🚀 JavaScript Interview Question That Confuses 80% Developers Think you truly understand JavaScript’s async behavior? Let’s test it 👇 console.log("Start"); setTimeout(() => console.log("Timeout"), 0); Promise.resolve().then(() => console.log("Promise")); console.log("End"); 👉 What will be the output? Most developers answer: Start Timeout Promise End ❌ That’s WRONG. ✅ Correct Output: Start End Promise Timeout 💡 Why Does This Happen? This happens because of how the Event Loop works in JavaScript. Promise.then() → goes to the Microtask Queue setTimeout() → goes to the Macrotask Queue After the Call Stack is empty → Microtasks run first Then Macrotasks execute Understanding this difference is crucial for writing predictable asynchronous code. 📌 If You’re Preparing for Frontend Interviews, Master These: ✔ Event Loop & Execution Context ✔ Closures ✔ Hoisting ✔ Debouncing vs Throttling ✔ Shallow Copy vs Deep Copy ✔ Async/Await vs Promises ✔ Call, Apply, Bind ✔ This keyword behavior These are frequently asked in React, Next.js and modern JavaScript interviews. Drop your answer in the comments before checking the solution 👇 And share one tricky JS question you’ve faced recently! #JavaScript #FrontendDeveloper #WebDevelopment #ReactJS #NextJS #InterviewPreparation #CodingInterview #SoftwareDeveloper #TechCareers #Programming #100DaysOfCode

Day 17 – JavaScript Interview Q&A Series 🚀 Continuing my JavaScript interview learnings – Day Series, focusing on how data is handled in real-world frontend applications. 🔹 Day 17 Topic: Mutability vs Immutability 1️⃣ What is Mutability? Mutability means changing the original object or array directly. 📌 Examples: • push(), pop() • Direct object property assignment 2️⃣ What is Immutability? Immutability means creating a new copy instead of modifying existing data. 📌 Examples: • Spread operator (...) • map, filter, concat 3️⃣ Why is immutability important? • Predictable state updates • Efficient change detection • Easier debugging and time-travel debugging 4️⃣ How does this affect React & Angular? • React relies on reference changes to trigger re-renders • Angular’s OnPush change detection benefits from immutability 5️⃣ Interview takeaway Immutability helps avoid side effects and unexpected UI bugs. 📌 This concept separates beginner vs experienced frontend developers. ➡️ Day 18 coming soon… (JavaScript Design Patterns – Module, Singleton) 🧠⚙️ #JavaScript #Immutability #FrontendDeveloper #InterviewPreparation #Angular #React #LearningInPublic

ReactJS Interview Prep – Scenario-Based Questions You Must Know Most companies now focus on real-world, scenario-based questions to test your problem-solving, component design, and state management skills. Here are some important scenarios every Front-End / React Developer should prepare: 🔹 Component Design Build a reusable button supporting multiple styles & sizes using props, conditional classes, CSS modules, or styled-components. 🔹 State Management Manage shopping cart logic (add, remove, update quantity) using useState, Context API, or Redux with actions & reducers. 🔹 Handling Side Effects Fetch API data with useEffect, while managing loading states, error handling, and cleanup. 🔹 Performance Optimization Improve large list rendering using React.memo, useMemo, useCallback, and list virtualization (react-window / react-virtualized). 🔹 Routing in React Implement dynamic & nested routing with React Router, useParams, and route configuration. 🔹 Form Handling & Validation Handle complex forms using controlled components, validation logic, Formik, or React Hook Form. 🔹 Component Communication Share data across deeply nested components using Context API, useContext, or Redux. Get ebook with (detailed 232 ques = 50+ Reactjs Frequent Ques & Answers, 45+ Reactjs scenario based questions, 90+ frequently asked interview questions and answers, 50+ Output based ques & ans, 25+ Coding Questions & ans) 𝐄𝐛𝐨𝐨𝐤 𝐋𝐢𝐧𝐤: https://lnkd.in/gJMmH-PF Interview Guidance: https://lnkd.in/gr9PCuxd Follow on Instagram : https://lnkd.in/g-iSTsRd #javascript #javascriptdeveloper #reactjs #reactnative #vuejsdeveloper #angular #angulardeveloper

🚀 Commonly Asked #JavaScript #Interview #Questions (1–45) Agar tum JavaScript interviews ki preparation kar rahe ho, to yeh list core + async + frontend concepts cover karti hai. 🔹 #Core_JavaScript Difference between var, let, and const What are closures and how do they work? Explain the this keyword in different contexts What is a Promise in JavaScript? What is the Event Loop? What is hoisting in JavaScript? JavaScript data types Difference between null and undefined What is a callback function? How do you handle errors in JavaScript? 🔹 #Asynchronous_JavaScript Difference between setTimeout() and setInterval() How do Promises work? What is then(), catch(), and finally()? What is async/await? Advantages of async/await over callbacks How to handle multiple promises (Promise.all) What is Promise.allSettled()? 🔹 #Modern_JavaScript (ES6+) What are higher-order functions? What is destructuring in JavaScript? What are template literals? How does the spread operator work? What is the rest parameter? Arrow functions vs normal functions 🔹 #Objects_Arrays Difference between object and array How to clone an object or array Object.keys(), Object.values(), Object.entries() How does map() work? Difference between map() and forEach() Difference between filter() and reduce() 🔹 #Advanced_JavaScript What is event delegation? What are JavaScript modules? What is the prototype chain? Difference between bind(), call(), and apply() Difference between == and === What is currying in JavaScript? 🔹 #Frontend_Browser_Concepts What is the DOM? How does JavaScript interact with the DOM? preventDefault() vs stopPropagation() What is an event object? What are custom events? How do you optimize JavaScript performance? What is debouncing? What is throttling? What is memory leak in #JavaScript? How to avoid memory leaks? 📌 Tip: Interviewers want clarity + real examples, not book definitions. 💡 Save this list for #revision. #JavaScript #JavaScriptInterview #FrontendDevelopment #WebDevelopment #ES6 #AsyncJavaScript #DOM #InterviewPreparation #Developers

JavaScript interview question that fails 80% of candidates: Implement Promise.all from scratch. Most answers: "Loop through promises and resolve when all done" That gets you 3/10. Here's the 9/10 answer: function promiseAll(promises) { return new Promise((resolve, reject) => { const results = []; let completed = 0; promises.forEach((promise, index) => { Promise.resolve(promise) .then(value => { results[index] = value; completed++; if (completed === promises.length) { resolve(results); } }) .catch(reject); }); }); } Key points interviewers want: ✓ Handle non-promise values (Promise.resolve wrapper) ✓ Maintain order (results[index]) ✓ Reject on first failure ✓ Edge case: empty array There are 40+ JavaScript problems like this on FE Master. All with detailed solutions. Link in the first comment #javascript #frontend

Day 22/50 – JavaScript Interview Question? Question: What is the this keyword and how is it determined? Simple Answer: this refers to the object that is executing the current function. Its value is determined by how the function is called: in a method it's the object, in a regular function it's the global object (or undefined in strict mode), in an arrow function it's lexically inherited from the enclosing scope. 🧠 Why it matters in real projects: Understanding this is critical for object-oriented programming, event handlers, and React class components. Incorrect this binding is one of the most common sources of bugs, especially when passing methods as callbacks. 💡 One common mistake: Losing the this context when passing object methods as callbacks (like event handlers), resulting in this being undefined or pointing to the wrong object. 📌 Bonus: const user = { name: 'Alice', greet: function() { console.log(`Hello, ${this.name}`); }, greetArrow: () => { console.log(`Hello, ${this.name}`); // Won't work! } }; user.greet(); // "Hello, Alice" ✓ const greetFunc = user.greet; greetFunc(); // "Hello, undefined" - lost context! // Solutions for callback context: // 1. Bind button.addEventListener('click', user.greet.bind(user)); // 2. Arrow function wrapper button.addEventListener('click', () => user.greet()); // 3. Arrow function in class (React) class Component { handleClick = () => { // Auto-bound! console.log(this); } } #JavaScript #WebDevelopment #Frontend #LearnInPublic #InterviewQuestions #Programming #TechInterviews #ThisKeyword #InterviewPrep #OOP

Day 26/50 – JavaScript Interview Question? Question: What is currying in JavaScript? Simple Answer: Currying transforms a function that takes multiple arguments into a sequence of functions, each taking a single argument. Instead of add(a, b), you get add(a)(b). 🧠 Why it matters in real projects: Currying enables partial application, function composition, and more reusable code. It's fundamental to functional programming libraries like Ramda and is used in Redux middleware, event handlers, and configuration functions. 💡 One common mistake: Over-currying simple functions where it adds complexity without benefit. Use currying when you need partial application or composition, not everywhere. 📌 Bonus: // Regular function function add(a, b, c) { return a + b + c; } add(1, 2, 3); // 6 // Curried version function curriedAdd(a) { return function(b) { return function(c) { return a + b + c; }; }; } curriedAdd(1)(2)(3); // 6 // Practical use: Partial application const add5 = curriedAdd(5); const add5and10 = add5(10); console.log(add5and10(3)); // 18 // Generic curry function function curry(fn) { return function curried(...args) { if (args.length >= fn.length) { return fn.apply(this, args); } return function(...nextArgs) { return curried.apply(this, [...args, ...nextArgs]); }; }; } // Usage const multiply = (a, b, c) => a * b * c; const curriedMultiply = curry(multiply); curriedMultiply(2)(3)(4); // 24 curriedMultiply(2, 3)(4); // 24 - flexible! #JavaScript #WebDevelopment #Frontend #LearnInPublic #InterviewQuestions #Programming #TechInterviews #FunctionalProgramming #Currying #WebDev #InterviewPrep

💡React Interview Question💡 Why do we always use useState, useRef, useReducer hook to store any information in the functional component instead of using normal variables? Answer: Whenever we create a React functional component, React behind the scenes creates a JavaScript function, and uses bind method to pass the props as arguments for each component like this: const User = User.bind(null, { userId: 10, username: 'Mike' }) Here, because of the bind method so we can re-use that component later multiple times with different props like this: <User userId={10} username="Mike" /> <User userId={12} username="Jerry" /> And because 𝗨𝘀𝗲𝗿 component is converted to a JavaScript function, each time you call that function, all the local variables, event handlers declared in the component will be re-created on every function call/re-render of the component. That's the reason we use hooks like 𝘂𝘀𝗲𝗦𝘁𝗮𝘁𝗲, 𝘂𝘀𝗲𝗥𝗲𝗱𝘂𝗰𝗲𝗿 or 𝘂𝘀𝗲𝗥𝗲𝗳 to store the values inside the component to retain the values across multiple re-renders. Even though we declare state or ref in a component, state or ref value is actually stored outside the component linked to that particular component, that's why we don't loose its value during re-render. If we declare a local variable 𝗰𝗼𝘂𝗻𝘁𝗲𝗿 along with the 𝘂𝘀𝗲𝗦𝘁𝗮𝘁𝗲 hook inside a 𝗨𝘀𝗲𝗿 component like this: 𝘧𝘶𝘯𝘤𝘵𝘪𝘰𝘯 𝘜𝘴𝘦𝘳() { 𝘤𝘰𝘯𝘴𝘵 [𝘶𝘴𝘦𝘳, 𝘴𝘦𝘵𝘜𝘴𝘦𝘳] = 𝘶𝘴𝘦𝘚𝘵𝘢𝘵𝘦(𝘯𝘶𝘭𝘭); 𝘤𝘰𝘯𝘴𝘵 𝘤𝘰𝘶𝘯𝘵𝘦𝘳 = 𝟣𝟢; 𝘤𝘰𝘯𝘴𝘵 𝘩𝘢𝘯𝘥𝘭𝘦𝘊𝘭𝘪𝘤𝘬 = (𝘦𝘷𝘦𝘯𝘵) => { // 𝘴𝘰𝘮𝘦 𝘤𝘰𝘥𝘦 } // 𝘴𝘰𝘮𝘦 𝘑𝘚𝘟 } then the 𝗰𝗼𝘂𝗻𝘁𝗲𝗿 variable and 𝗵𝗮𝗻𝗱𝗹𝗲𝗖𝗹𝗶𝗰𝗸 method will be re-created on every re-render of the 𝗨𝘀𝗲𝗿 component so 𝗰𝗼𝘂𝗻𝘁𝗲𝗿 variable value will be reset to 𝟭𝟬 on every re-render of the component, but the 𝘂𝘀𝗲𝗿 state will maintain its previous value across multiple re-render of the 𝗨𝘀𝗲𝗿 component and it 𝘄𝗶𝗹𝗹 𝗻𝗼𝘁 𝗯𝗲 re-initialized to null on re-render. Because even though the state is declared inside a component, React actually stores the state information outside the component so as to not lose its value during re-render. 𝗙𝗼𝗿 𝗺𝗼𝗿𝗲 𝘀𝘂𝗰𝗵 𝘂𝘀𝗲𝗳𝘂𝗹 𝗰𝗼𝗻𝘁𝗲𝗻𝘁, 𝗱𝗼𝗻'𝘁 𝗳𝗼𝗿𝗴𝗲𝘁 𝘁𝗼 𝗳𝗼𝗹𝗹𝗼𝘄 𝗺𝗲. #javascript #reactjs #nextjs #webdevelopment