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.

🔰 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.

📦 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.

📡 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.

🌐 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?

🗃️ 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.

🔐 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?

🛠️ 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?

🧪 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?

🧵 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?

📦 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?

📁 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