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

👋 Hi LinkedIn! I’m a Frontend Developer working with JavaScript, React, Angular and modern frontend technologies. Over the next few weeks, I’ll be sharing: • JavaScript concepts • Frontend interview questions • Performance optimization tips • Real-world frontend problems & solutions If you're preparing for frontend interviews or building scalable UI applications, please feel free to follow along. Let’s grow together 🚀 Comment down your favorite topics. #javascript #frontend #webdevelopment #js #UI #AI #angular #reactjs #webdevelopment #softwareengineering #programming

🚀 Day 20/100 – Implementing a curry() Function in JavaScript Today I explored an advanced JavaScript concept: Currying. Currying transforms a function with multiple arguments into a sequence of functions, each taking a single argument. 🧠 Problem: Convert a function so that it can be called like: sum(1)(2)(3) // 6 ✅ Solution: function curry(fn) { return function curried(...args) { if (args.length >= fn.length) { return fn(...args); } else { return function (...nextArgs) { return curried(...args, ...nextArgs); }; } }; } function sum(a, b, c) { return a + b + c; } const curriedSum = curry(sum); console.log(curriedSum(1)(2)(3)); console.log(curriedSum(1, 2)(3)); console.log(curriedSum(1)(2, 3)); ✅ Output: 6 6 6 💡 Key Learnings: • Currying allows partial application of functions • Helps create reusable and flexible functions • Improves function composition • Common in functional programming 📌 Real World Usage: • Used in utility libraries like Lodash • Helps in writing cleaner React code • Useful in event handling and reusable logic Understanding currying improves how you think about functions and composition in JavaScript. 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, I’d love to connect. #OpenToWork #FrontendDeveloper #JavaScript #ReactJS #NextJS #ImmediateJoiner #100DaysOfCode

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

If you can clearly explain these React concepts… you’re already ahead of most frontend interview candidates. ⚛️📚 I compiled the most commonly asked React interview concepts into a simple React Fundamentals Cheat Sheet – Part 1 to make revision faster and more practical. If you're preparing for a React developer role or strengthening your frontend development fundamentals, these core topics appear repeatedly in interviews and real-world projects: ✅ React Core Features ⚛️ – Understand JSX, components, Virtual DOM, and one-way data binding, the backbone of modern React applications. ✅ Element vs Component 🧩 – Learn how React elements define UI structure, while components create reusable and scalable interfaces. ✅ Component Creation 🛠️ – Build dynamic UI using function components, class components, props, and JSX syntax. ✅ Virtual DOM Explained ⚡ – See how React improves web application performance by updating only the changed parts of the DOM. ✅ Keys & Dynamic Lists 🔑 – Use keys with arrays and map() to manage dynamic rendering in React efficiently. 🚀 Level Up Your Skills For deep-dives into these concepts, I highly recommend checking out the latest documentation and tutorials from JavaScript Mastery and GeeksforGeeks. 💬 Comment Below: Which React topic should be covered in Part 2? #imperio_coders #Reactjs #Nextjs #Javascript #WebDevelopment #FullStack #Frontend #Developers #Community #Education #Careers

Want a Quick but Powerful Overview of JavaScript? Let’s Break It Down! Are you learning JavaScript or preparing for front-end developer interviews? Before diving into advanced frameworks like React, Next.js, or Node.js, every developer must master the core fundamentals of JavaScript. I created a quick JavaScript overview / cheat sheet that covers essential concepts every developer should know. 📌 In this overview you’ll explore: What JavaScript is and why it powers the modern web JavaScript data types and variables (var, let, const) Functions and reusable code patterns Arrays, strings, and objects Conditional statements and loops DOM manipulation and selecting elements Events and user interactions Error handling using try...catch Window methods like setTimeout, setInterval, alert, and more JavaScript is the foundation of modern web development, and understanding these concepts helps you: ✔ Build interactive web applications ✔ Prepare for developer interviews ✔ Write cleaner and more efficient code ✔ Move faster into frameworks like React and Node.js Whether you're a beginner developer, student, or preparing for frontend interviews, mastering these fundamentals will strengthen your development journey. 💡 Remember: Great developers don't skip the basics — they master them. 💬 Which JavaScript concept took you the longest to fully understand? #JavaScript #FrontendDevelopment #WebDevelopment #FullStackDevelopment #SoftwareEngineering #Programming #Coding #DeveloperCommunity #DeveloperLife #LearnToCode #CodingJourney #TechCareers #TechHiring #HiringDevelopers #InterviewPreparation #TechJobs #DeveloperSkills #ProgrammingTips #CareerGrowth #CodingEducation

🚀 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

Most React developers think they know React. But when it comes to writing scalable, production-level code… they struggle. I realized this after seeing the same mistakes again and again: ❌ Unnecessary re-renders ❌ Misusing hooks ❌ Poor component structure ❌ Slow and messy code So I compiled “Top 50 React Tips Every Developer Should Know.” This isn’t basic tutorial content. These are practical tips senior developers actually use to write cleaner, faster, and more scalable React applications. If you’re: • Learning React • Preparing for frontend interviews • Or trying to level up as a developer This guide will help you. Comment “REACT” and I’ll share the PDF. And follow me for more developer resources, coding tips, and interview prep. Having Doubts in technical journey? 🚀 Book 1:1 demo with me : https://thevinia.com 🚀 Subscribe and stay up to date: https://lnkd.in/g-Rf8EgT follow instragram page : https://lnkd.in/g5jfDRxy 🚀 Get Complete React JS Interview Q&A Here: https://lnkd.in/gCs_jvJf #react #reactjs #frontenddeveloper #javascript #webdevelopment #codingtips #softwareengineering