Top 10 Common React.js Interview Questions

Recently, I interviewed for multiple Senior React.js & Tech Lead roles — and noticed a pattern. Most interviewers asked basic but frequently repeated questions that test your clarity of concepts + coding approach. Here are the Top 10 common questions I was asked 👇 1️⃣ Call, Apply, Bind → Difference + Polyfill implementation 2️⃣ Flatten an Array without Array.flat() 👉 Input: [1,2,3,[4,5,6,[7,8,[10,11]]],9] 👉 Output: [1,2,3,4,5,6,7,8,10,11,9] 3️⃣ Inline 5 divs in a row without flex/margin/padding (Hint: display: inline-block) 4️⃣ Find sum of numbers without a for loop (Hint: reduce() / recursion) 5️⃣ Deep Copy vs Shallow Copy — behavior & how to achieve it 6️⃣ Promise & Async/Await output puzzle 7️⃣ Find first repeating character (e.g., "success" → "c") 8️⃣ Stopwatch Implementation (Start, Stop, Reset + live timer) 9️⃣ Build a To-Do List (Vanilla JS/React) → optimize re-renders 🔟 Currying for Infinite Sum 👉 sum(10)(20)(30)() → 60 👉 sum(10)(20)(30)(40)(50)(60)() → 210 𝐠𝐞𝐭 𝐞𝐛𝐨𝐨𝐤 𝐰𝐢𝐭𝐡 (detailed 232 ques = 90+ frequently asked Javascript interview questions and answers, 70+ Reactjs Frequent Ques & Answers, 50+ Output based ques & ans, 23+ Coding Questions & ans, 2 Machine coding ques & ans) 𝐄𝐛𝐨𝐨𝐤 𝐋𝐢𝐧𝐤: https://lnkd.in/gJMmH-PF Follow on Instagram : https://lnkd.in/gXTrcaKP #javascript #javascriptdeveloper #reactjs #reactnative #vuejsdeveloper #angular #angulardeveloper

Recently, I interviewed for multiple Senior React.js & Tech Lead roles — and noticed a pattern. Most interviewers asked basic but frequently repeated questions that test your clarity of concepts + coding approach. Here are the Top 10 common questions I was asked 👇 1️⃣ Call, Apply, Bind → Difference + Polyfill implementation 2️⃣ Flatten an Array without Array.flat() 👉 Input: [1,2,3,[4,5,6,[7,8,[10,11]]],9] 👉 Output: [1,2,3,4,5,6,7,8,10,11,9] 3️⃣ Inline 5 divs in a row without flex/margin/padding (Hint: display: inline-block) 4️⃣ Find sum of numbers without a for loop (Hint: reduce() / recursion) 5️⃣ Deep Copy vs Shallow Copy — behavior & how to achieve it 6️⃣ Promise & Async/Await output puzzle 7️⃣ Find first repeating character (e.g., "success" → "c") 8️⃣ Stopwatch Implementation (Start, Stop, Reset + live timer) 9️⃣ Build a To-Do List (Vanilla JS/React) → optimize re-renders 🔟 Currying for Infinite Sum 👉 sum(10)(20)(30)() → 60 👉 sum(10)(20)(30)(40)(50)(60)() → 210 𝐠𝐞𝐭 𝐞𝐛𝐨𝐨𝐤 𝐰𝐢𝐭𝐡 (detailed 232 ques = 90+ frequently asked Javascript interview questions and answers, 70+ Reactjs Frequent Ques & Answers, 50+ Output based ques & ans, 23+ Coding Questions & ans, 2 Machine coding ques & ans) 𝐄𝐛𝐨𝐨𝐤 𝐋𝐢𝐧𝐤: https://lnkd.in/gJMmH-PF Follow on Instagram : https://lnkd.in/gXTrcaKP #javascript #javascriptdeveloper #reactjs #reactnative #vuejsdeveloper #angular #angulardeveloper

Recently, I interviewed for multiple Senior React.js & Tech Lead roles — and noticed a pattern. Most interviewers asked basic but frequently repeated questions that test your clarity of concepts + coding approach. Here are the Top 10 common questions I was asked 👇 1️⃣ Call, Apply, Bind → Difference + Polyfill implementation 2️⃣ Flatten an Array without Array.flat() 👉 Input: [1,2,3,[4,5,6,[7,8,[10,11]]],9] 👉 Output: [1,2,3,4,5,6,7,8,10,11,9] 3️⃣ Inline 5 divs in a row without flex/margin/padding (Hint: display: inline-block) 4️⃣ Find sum of numbers without a for loop (Hint: reduce() / recursion) 5️⃣ Deep Copy vs Shallow Copy — behavior & how to achieve it 6️⃣ Promise & Async/Await output puzzle 7️⃣ Find first repeating character (e.g., "success" → "c") 8️⃣ Stopwatch Implementation (Start, Stop, Reset + live timer) 9️⃣ Build a To-Do List (Vanilla JS/React) → optimize re-renders 🔟 Currying for Infinite Sum 👉 sum(10)(20)(30)() → 60 👉 sum(10)(20)(30)(40)(50)(60)() → 210 𝐠𝐞𝐭 𝐞𝐛𝐨𝐨𝐤 𝐰𝐢𝐭𝐡 (detailed 232 ques = 90+ frequently asked Javascript interview questions and answers, 70+ Reactjs Frequent Ques & Answers, 50+ Output based ques & ans, 23+ Coding Questions & ans, 2 Machine coding ques & ans) 𝐄𝐛𝐨𝐨𝐤 𝐋𝐢𝐧𝐤: https://lnkd.in/gJMmH-PF Follow on Instagram : https://lnkd.in/gXTrcaKP #javascript #javascriptdeveloper #reactjs #reactnative #vuejsdeveloper #angular #angulardeveloper

Today, I appeared for an interview for a React.js role.  Sharing one of the questions here—it might help others who are preparing  for similar frontend roles. Question 1: What is the Virtual DOM in React? The Virtual DOM is a lightweight JavaScript representation of the real DOM. React updates it first and efficiently applies only required changes to the real DOM. Question 2 : What is a Rest Parameter in JavaScript? A rest parameter (...args) allows a function to accept multiple arguments and collect them into a single array, making functions more flexible and cleaner. Question 3: If an object is created using const, does changing its properties cause an error? No. You can modify properties of a const object. An error occurs only when you try to reassign the object itself. Question 4 : Why is React JS used? React JS is used to build fast, scalable, and reusable user interfaces using a component-based architecture and an efficient Virtual DOM. Question 5 : What is “input search while typing,” and why are debouncing and throttling important? Input search while typing updates results instantly. Debouncing and throttling limit function calls, improving performance and reducing unnecessary API requests. Question 6 : What are Cookies, Session Storage, and Local Storage? Cookies store small data sent with server requests. Session storage stores data temporarily per tab. Local storage stores data persistently in the browser. Question 7: What are find() and apply() methods in JavaScript? find() returns the first array element that matches a condition. apply() calls a function with a specific this context and arguments passed as an array. 👉 Follow ARUN DUBEY↗ for more React & JavaScript interview prep 👉 Like • Comment • Share to help others #JavaScript #ReactJS #FrontendDevelopment #WebDevelopment #InterviewPreparation #CodingInterviews

𝐅𝐫𝐨𝐧𝐭𝐞𝐧𝐝 𝐃𝐞𝐯𝐞𝐥𝐨𝐩𝐞𝐫𝐬: 𝐒𝐭𝐨𝐩 𝐋𝐞𝐚𝐫𝐧𝐢𝐧𝐠 𝐑𝐚𝐧𝐝𝐨𝐦𝐥𝐲. 𝐒𝐭𝐚𝐫𝐭 𝐋𝐞𝐚𝐫𝐧𝐢𝐧𝐠 𝐒𝐭𝐫𝐚𝐭𝐞𝐠𝐢𝐜𝐚𝐥𝐥𝐲. I’ve created a complete Frontend Development Roadmap + Interview Checklist to help you: ✅ Master core fundamentals (HTML, CSS, JavaScript) ✅ Understand modern frameworks (React, Angular, Vue) ✅ Strengthen problem-solving & debugging skills ✅ Prepare confidently for frontend interviews ✅ Avoid tutorial-hopping & confusion This isn’t just a roadmap. It’s a structured action plan designed to take you from beginner → job-ready. If you're: Starting your frontend journey Switching to frontend Preparing for interviews Feeling stuck despite learning This guide will bring clarity and direction. 📄 Download the PDF. 📌 Follow Esha Tariq for practical tech insights. 🔁 Repost to help another developer grow. Let’s build scalable, beautiful, and performant web experiences together. #frontend #webdevelopment #javascript #react #angular #vuejs #frontenddeveloper #interviewprep #coding #techcareers #roadmap #career

Frontend developers — if you’re tired of learning randomly and want a clear, strategic path, this roadmap is worth checking out. ✅ Strengthening core fundamentals (HTML, CSS, JavaScript) ✅ Understanding modern frameworks (React, Angular, Vue) ✅ Improving problem-solving & debugging skills ✅ Preparing confidently for frontend interviews ✅ Avoiding tutorial hopping What I like most is the focus on moving from beginner → job-ready with a structured plan instead of scattered learning. Credit to Esha Tariq for putting together a practical and helpful guide for the community 👏 #frontend #webdevelopment #javascript #react #angular #vuejs #frontenddeveloper #interviewprep #coding #techcareers #roadmap

🚀 Day 11/100 – Understanding Prototypes in JavaScript Today I deep dived into one of the most fundamental concepts in JavaScript: Prototypes. 🧠 Problem: How does JavaScript enable inheritance between objects? ✅ Example: function Person(name) { this.name = name; } Person.prototype.greet = function () { console.log(`Hello, my name is ${this.name}`); }; const user1 = new Person("Bunty"); user1.greet(); // Output: Hello, my name is Bunty 💡 What’s Happening Here? Every JavaScript function has a prototype property. Objects created using new inherit from that prototype. Methods defined on the prototype are shared across all instances. This improves memory efficiency. 📌 Why Not Define Method Inside Constructor? If we write: function Person(name) { this.name = name; this.greet = function () { console.log(`Hello, my name is ${this.name}`); }; } Now every instance gets its own copy of greet() ❌ Which increases memory usage. Prototype keeps it optimized ✅ 🧠 Key Learnings: JavaScript uses prototypal inheritance (not classical inheritance). The __proto__ links objects to their prototype. Method lookup happens through the prototype chain. Core JS concepts like Array, Object, Function are built using prototypes. Understanding prototypes makes closures, inheritance, and even React internals easier to grasp. I’m currently open to Frontend Developer opportunities (React / Next.js) and available for immediate joining. 📩 Email: bantykumar13365@gmail.com 📱 Mobile: 7417401815 If you're hiring or know someone who is, feel free to connect. #OpenToWork #FrontendDeveloper #JavaScript #Prototypes #ReactJS #ImmediateJoiner #100DaysOfCode

⚛️ 𝗥𝗲𝗮𝗰𝘁 𝗝𝗦 𝗚𝘂𝗶𝗱𝗲 𝗳𝗼𝗿 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 — 𝗕𝘂𝗶𝗹𝗱 𝗠𝗼𝗱𝗲𝗿𝗻 & 𝗦𝗰𝗮𝗹𝗮𝗯𝗹𝗲 𝗪𝗲𝗯 𝗔𝗽𝗽𝘀! React JS is one of the most popular JavaScript libraries for building fast, interactive, and scalable user interfaces. Whether you're a beginner or an experienced developer, mastering React helps you create dynamic single-page applications with better performance and maintainable code. Learn core concepts like components, props, state, hooks, lifecycle methods, and virtual DOM. Explore advanced topics such as custom hooks, context API, performance optimization, routing, state management, and API integration. Build real-world projects, follow best practices, and write clean, reusable components to become a confident React developer. Start building amazing user experiences with React today! #ReactJS #FrontendDevelopment #WebDevelopment #JavaScript #Programming #Developers #SoftwareEngineering #Coding #SPA #TechLearning #CodeWithGandhi

Frontend Developer Reality: It’s Never “Almost Done” You start with confidence. HTML? “Almost done.” CSS? “Almost there.” Add Bootstrap? “Nice… this looks smooth.” Then someone says: 👉 Can we build it in React? Or Vue. Or Angular. And suddenly you’re climbing a whole new mountain. 🧱 The Truth About Frontend Frontend looks simple from the outside. But behind every clean UI, there’s: • Layout debugging at 2 AM • Cross-browser issues nobody warned you about • Responsive design headaches • State management confusion • Endless framework decisions And somehow… it’s still fun. 🚀 What This Really Means Learning HTML & CSS isn’t the finish line. It’s the foundation. Frameworks don’t replace fundamentals. They build on them. If you’re a student or early-stage developer, focus on: ✅ Strong HTML structure ✅ Deep CSS understanding (Flexbox, Grid) ✅ Solid JavaScript fundamentals ✅ THEN move to React / Vue / Angular Climb one mountain at a time. 💬 Frontend developers — be honest… Which part made you say “Oh No” the most? 😄 #WebDevelopment #FullStackDeveloper #FrontendDevelopment #BackendDevelopment #JavaScript #ReactJS #NodeJS #APIs #SoftwareEngineering #Programming #Developers #ReactJS #VueJS #ComputerScience