MERN Interview Question

[Shubhamkahar196]

MERN Interview Sheet (2026 Edition)

The only 100 questions you need to confidently crack most MERN Stack interviews in 2026. >

📌 JavaScript (20)

• What is the difference between var, let, and const?
• What is Hoisting?
• What is the Temporal Dead Zone (TDZ)?
• What are Closures?
• What is Lexical Scope?
• Explain the JavaScript Event Loop.
• What is the difference between the Call Stack, Callback Queue, and Microtask Queue?
• Explain the lifecycle of a Promise.
• How does async/await work internally?
• What is Callback Hell? How can it be avoided?
• What is Debouncing? Where would you use it?
• What is Throttling? Where would you use it?
• What is the difference between Shallow Copy and Deep Copy?
• What is the difference between == and ===?
• Explain the this keyword in JavaScript.
• What are Arrow Functions? How are they different from regular functions?
• What is the Prototype Chain?
• What is Event Delegation?
• What causes Memory Leaks in JavaScript?
• What is the difference between CommonJS and ES Modules?

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⚛️ React (25)

• What is React?
• Explain the Virtual DOM.
• What is React Reconciliation?
• Explain React's Rendering Process.
• Functional Components vs Class Components.
• Props vs State.
• Controlled vs Uncontrolled Components.
• Explain the React Component Lifecycle.
• How does useState work?
• How does useEffect work?
• What is useRef used for?
• When should you use useMemo?
• When should you use useCallback?
• When should you use useReducer?
• What is Context API?
• Why use Redux Toolkit?
• What is React.memo()?
• What are Higher Order Components (HOCs)?
• What are Custom Hooks?
• What is Lazy Loading?
• What is Code Splitting?
• Explain React Router and Protected Routes.
• SSR vs CSR.
• How do you optimize React performance?
• What is Optimistic UI?

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🟢 Node.js & Express (18)

• Explain the Node.js architecture.
• Explain the Node.js Event Loop.
• What are Worker Threads?
• What are Streams?
• What are Buffers?
• What is the purpose of package.json?
• What is Middleware in Express?
• What is the purpose of next()?
• How does Error Handling Middleware work?
• Difference between app.use() and app.get().
• How does Routing work in Express?
• Explain the Request-Response lifecycle.
• How should you structure an Express project?
• Why are Environment Variables important?
• How do you implement Logging?
• What are REST API best practices?
• How do you implement File Uploads?
• How should APIs be Versioned?

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🍃 MongoDB (15)

• SQL vs NoSQL.
• What is BSON?
• Collections vs Documents.
• How should you design MongoDB Schemas?
• What is Mongoose?
• Explain CRUD operations.
• What is Indexing?
• Explain the Aggregation Pipeline.
• What is populate()?
• What are Transactions?
• Explain Replication and Replica Sets.
• What is Sharding?
• Explain the CAP Theorem.
• How do you optimize MongoDB queries?
• When would you use Redis instead of MongoDB?

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🔐 Authentication & Security (10)

• How does JWT Authentication work?
• How do Refresh Tokens work?
• Cookies vs LocalStorage.
• Why use bcrypt?
• Authentication vs Authorization.
• What is Role-Based Access Control (RBAC)?
• What is CORS?
• What is XSS? How do you prevent it?
• What is CSRF? How do you prevent it?
• What is Rate Limiting?

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🐳 Deployment & DevOps (5)

• What is Docker?
• Explain a CI/CD pipeline.
• What is a Reverse Proxy (e.g., Nginx)?
• Horizontal Scaling vs Vertical Scaling.
• What is Redis? Why is it used for caching?

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🏗️ System Design (7)

• Design a URL Shortener.
• Design a Real-Time Chat Application.
• Design a JWT Authentication Flow.
• Design a File Upload Service.
• Design a MongoDB Schema for a Social Media App.
• How would you scale a MERN application?
• Explain your project's architecture end-to-end.

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📚 Project Questions (Bonus)

• Explain your latest project in detail.
• Why did you choose this architecture?
• Why did you choose MongoDB?
• Why did you choose React?
• Why did you choose Express?
• Why did you choose Node.js?
• Explain your authentication flow.
• Explain your database design.
• What was the hardest bug you solved?
• What would you improve if you rebuilt the project today?