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Node.js & Express.js Interview Questions - Practice Set

This document contains practice interview questions organized by difficulty level. Use these to test your knowledge and prepare for technical interviews.

Difficulty Levels

  • Beginner: Fundamental concepts
  • Intermediate: Practical application
  • Advanced: Complex scenarios and optimization

Beginner Level Questions

1. What is Node.js and what problem does it solve?

2. Explain the difference between JavaScript in the browser and Node.js.

3. What is npm and what is it used for?

4. What is the difference between var, let, and const?

5. What is module.exports and how do you use it?

6. What is the difference between require() and import?

7. What is Express.js and why would you use it instead of plain Node.js?

8. What is middleware in Express.js?

9. How do you create a route in Express?

10. What is the difference between app.get() and app.use()?

11. How do you send JSON responses in Express?

12. What is req.params used for?

13. What is req.query used for?

14. What is req.body and how do you access it?

15. What is a callback function?

16. What is a Promise?

17. What is async/await?

18. What are HTTP status codes? Give examples.

19. What is REST and what makes an API RESTful?

20. What is the difference between GET and POST requests?

Intermediate Level Questions

21. Explain the Node.js event loop. How does it work?

22. What is the difference between blocking and non-blocking code?

23. How do you handle errors in Express applications?

24. What is the order of middleware execution in Express?

25. How do you structure a Node.js/Express project?

26. What are environment variables and why use them?

27. How do you implement authentication in an Express app?

28. What is JWT and how does it work?

29. Why should you never store passwords in plain text?

30. What is CORS and how do you handle it in Express?

31. What is the difference between authentication and authorization?

32. How do you validate input data in Express?

33. What is the difference between PUT and PATCH?

34. How do you organize routes in Express?

35. What is Express Router and when should you use it?

36. How do you handle file uploads in Express?

37. What is rate limiting and why is it important?

38. How do you implement pagination in an API?

39. What is the difference between req.params, req.query, and req.body?

40. How do you handle async errors in Express?

41. What is separation of concerns in backend development?

42. How do you implement search functionality in an API?

43. What is the purpose of process.env in Node.js?

44. How do you test Express APIs?

45. What is input validation and why is it important?

46. How do you implement error handling middleware?

47. What is the difference between throw and next(error)?

48. How do you create a CRUD API in Express?

49. What is the difference between SQL and NoSQL databases?

50. What security best practices should you follow in Express?

Advanced Level Questions

51. Explain the Node.js event loop phases in detail.

52. How do you implement database transactions in Node.js?

53. What is connection pooling and why is it important?

54. How do you optimize Node.js application performance?

55. What is the difference between process.nextTick() and setImmediate()?

56. How do you handle memory leaks in Node.js applications?

57. What is the cluster module in Node.js and when would you use it?

58. How do you implement caching in a Node.js application?

59. What is the difference between child_process and worker_threads?

60. How do you implement real-time features (WebSockets) in Express?

61. How do you handle database migrations in Node.js?

62. What is the difference between an ORM and ODM?

63. How do you implement microservices architecture with Node.js?

64. How do you handle distributed transactions?

65. What is the purpose of message queues and when would you use them?

66. How do you implement API versioning?

67. How do you handle large file uploads in Node.js?

68. What is the difference between horizontal and vertical scaling?

69. How do you implement observability (logging, monitoring, tracing) in Node.js?

70. How do you secure an Express API against common vulnerabilities?

71. How do you implement GraphQL in Node.js?

72. What is the difference between server-side rendering and API-based architecture?

73. How do you implement rate limiting with different strategies?

74. How do you handle graceful shutdown in Node.js applications?

75. What is the difference between event-driven and request-response architectures?

Scenario-Based Questions

76. How would you design an ecommerce API with products, users, and orders?

77. How would you implement a social media API with posts, comments, and likes?

78. How would you build a real-time chat application backend?

79. How would you design a file upload service with progress tracking?

80. How would you implement a notification system?

81. How would you build a search API that can handle millions of records?

82. How would you implement a payment processing system?

83. How would you design a multi-tenant SaaS application backend?

84. How would you implement a recommendation engine API?

85. How would you build a scalable image processing service?

Coding Challenges

Challenge 1: Create a RESTful API

Build a complete RESTful API for a blog with:

  • Posts (CRUD operations)
  • Comments (CRUD operations)
  • Users (registration, login)
  • Authentication middleware
  • Input validation
  • Error handling

Challenge 2: Implement Authentication

Create a complete authentication system with:

  • User registration
  • User login
  • JWT token generation
  • Protected routes
  • Password hashing
  • Token refresh

Challenge 3: Build a File Management API

Create an API for file management with:

  • File upload
  • File download
  • File listing
  • File deletion
  • File metadata
  • File type validation

Challenge 4: Create a Search API

Build a search API that can:

  • Search across multiple fields
  • Filter results
  • Sort results
  • Paginate results
  • Handle fuzzy search

Challenge 5: Implement Rate Limiting

Create a rate limiting system with:

  • Different limits for different routes
  • IP-based limiting
  • User-based limiting
  • Sliding window algorithm
  • Redis-based storage

Debugging Scenarios

Scenario 1: Memory Leak

Your Node.js application's memory keeps increasing. How do you identify and fix it?

Scenario 2: Slow API Response

Your API is responding slowly. What steps do you take to diagnose and fix it?

Scenario 3: Database Connection Issues

Your application is losing database connections. How do you troubleshoot?

Scenario 4: Authentication Not Working

Users can't log in. What do you check?

Scenario 5: CORS Errors

Your frontend can't call your API. How do you fix CORS issues?

Practice Tips

  1. Answer Out Loud: Practice explaining concepts verbally
  2. Write Code: Don't just explain, show code examples
  3. Think Aloud: Show your thought process during coding challenges
  4. Ask Questions: Clarify requirements before solving
  5. Consider Edge Cases: Think about error scenarios
  6. Optimize: Consider performance implications
  7. Review: Review your solutions and improve them

Solutions

For detailed answers to all these questions, see INTERVIEW_QUESTIONS_SOLUTIONS.md

This solutions file contains brief, comprehensive answers to all 85 questions plus coding challenges and debugging scenarios.