Node.js Interview Guide tailored for backend developers, ranging from beginner to advanced levels

Node.js Interview Guide tailored for backend developers, ranging from beginner to advanced levels. This will help you prepare for interviews with clear structure, concepts, code snippets, and frequently asked questions.

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🔰 1. Node.js Basics

✅ Topics:

• What is Node.js?
• Node.js architecture (Event Loop)
• Blocking vs Non-blocking I/O
• Global objects (__dirname, __filename, require, etc.)
• npm vs npx

❓ Sample Questions:

• What is Node.js and why is it single-threaded?
• Explain the Event Loop with phases.
• Difference between require and import.

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📦 2. Modules and File System

✅ Topics:

• CommonJS and ES Modules
• Built-in modules (fs, path, http, etc.)
• Synchronous vs Asynchronous file operations
• Module caching

❓ Sample Questions:

• How does module caching work in Node.js?
• Explain the difference between fs.readFile and fs.readFileSync.

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📡 3. Asynchronous Programming

✅ Topics:

• Callbacks
• Promises
• async/await
• Callback hell and how to avoid it

❓ Sample Questions:

• How does Node handle asynchronous code under the hood?
• Convert a callback-based function to Promise-based.

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🌐 4. HTTP and Express.js

✅ Topics:

• Creating an HTTP server
• Express.js basics (middleware, routing, error handling)
• Request lifecycle
• REST API design patterns

❓ Sample Questions:

• How do you handle global error middleware in Express?
• Difference between middleware and route handler?

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🗃️ 5. Database Integration

✅ Topics:

• MongoDB with Mongoose
• SQL with Sequelize / TypeORM
• Connecting to DBs asynchronously
• CRUD operations with async/await

❓ Sample Questions:

• How do you handle database connection failures?
• Compare ODM and ORM in Node.js.

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🔐 6. Authentication & Authorization

✅ Topics:

• JWT tokens
• Passport.js
• Role-based access
• Session vs Token-based auth

❓ Sample Questions:

• How does JWT work and where is it stored?
• How would you protect admin routes in an Express app?

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🛠️ 7. Best Practices & Clean Code

✅ Topics:

• Folder structure
• Error handling
• DTOs and validation (Joi, Zod)
• Environment variables (dotenv)
• Logging (winston, morgan)

❓ Sample Questions:

• How do you structure a large-scale Node.js project?
• What tools do you use for logging and monitoring?

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🧪 8. Testing

✅ Topics:

• Unit testing with Jest / Mocha
• Supertest for API testing
• Mocks and spies
• Test structure and coverage

❓ Sample Questions:

• How do you test an Express middleware?
• What is the difference between unit and integration tests?

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🧵 9. Performance & Advanced Topics

✅ Topics:

• Clustering and Worker Threads
• Memory leaks & Garbage collection
• Streams
• Rate limiting, throttling
• Redis caching

❓ Sample Questions:

• How would you scale a Node.js app?
• What is a stream in Node.js and how is it useful?

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📦 10. DevOps & Production

✅ Topics:

• PM2 process manager
• Dockerizing a Node.js app
• CI/CD pipeline (basic overview)
• Environment config and secret handling

❓ Sample Questions:

• How do you handle Node app crashes in production?
• How would you use Docker for deploying a Node.js app?

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📁 Bonus: Sample Project for Practice

Build a REST API for a To-Do App with:

• CRUD endpoints using Express
• MongoDB + Mongoose
• JWT Authentication
• Redis Caching
• Jest tests
• Dockerfile & PM2 setup