Node.js Interview Questions
🧠 Basic Level
- What is Node.js?
- Is Node.js a programming language? Explain.
- What is the V8 engine?
- What are the main features of Node.js?
- What is event-driven programming?
- What is non-blocking I/O?
- Difference between Node.js and JavaScript in browser?
- What is npm?
- What is package.json?
- How do you install a package using npm?
⚙️ Core Concepts
- What is the event loop in Node.js?
- Explain callbacks in Node.js.
- What is callback hell?
- What are Promises?
- Difference between callbacks and promises?
- What is async/await?
- How does Node.js handle concurrency?
- What is the difference between process.nextTick() and setImmediate()?
- What is buffer in Node.js?
- What are streams in Node.js?
🔌 Modules & Architecture
- What are modules in Node.js?
- Difference between CommonJS and ES Modules?
- What is require() function?
- What is module.exports?
- What are built-in modules in Node.js?
- Name some core modules (fs, http, path, etc.).
- What is the fs module used for?
- What is the path module?
- What is the http module?
- What is middleware in Node.js?
🌐 Express.js (Very Important)
- What is Express.js?
- Why is Express used with Node.js?
- What is routing in Express?
- What is middleware in Express?
- Difference between app.use() and app.get()?
- What is request and response object?
- How do you handle errors in Express?
- What is REST API?
- Difference between PUT and PATCH?
- What is CORS?
🔐 Advanced Level
- How does Node.js scale applications?
- What is clustering in Node.js?
- What is worker threads?
- Difference between single-threaded and multi-threaded models?
- How does Node.js handle memory management?
- What is garbage collection in Node.js?
- What is event emitter?
- How do you secure a Node.js application?
- What are common performance optimization techniques?
- What is logging and monitoring in Node.js apps?
🚀 Scenario-Based Questions
- How would you handle a slow API in Node.js?
- How do you manage large file uploads?
- How do you debug Node.js applications?
- How do you prevent callback hell in real projects?
- How do you structure a Node.js project?
Node.js Interview Answers
🧠 Basic Level
1. What is Node.js?
Node.js is a runtime environment that allows JavaScript to run on the server side.
2. Is Node.js a programming language?
No, Node.js is not a language. It is a runtime built on JavaScript.
3. What is V8 engine?
V8 is Google’s JavaScript engine that executes JS code in Node.js.
4. Features of Node.js
Fast, event-driven, non-blocking I/O, scalable, single-threaded.
5. Event-driven programming?
It means execution is based on events like requests, clicks, etc.
6. Non-blocking I/O?
Operations don’t wait; they run asynchronously.
7. Node.js vs Browser JS?
Browser JS runs in browser; Node.js runs on server.
8. What is npm?
Node Package Manager used to install libraries.
9. package.json?
File that contains project info and dependencies.
10. Install package?npm install package-name
⚙️ Core Concepts
11. Event loop?
It handles asynchronous operations in Node.js.
12. Callback?
A function passed as argument to another function.
13. Callback hell?
Nested callbacks making code unreadable.
14. Promise?
An object representing future completion of async task.
15. Callback vs Promise?
Promises are cleaner and easier to manage than callbacks.
16. async/await?
Syntax to handle promises in synchronous style.
17. Concurrency?
Handled using event loop, not multiple threads.
18. process.nextTick vs setImmediate?
nextTick runs before event loop phase; setImmediate runs after.
19. Buffer?
Used to handle binary data.
20. Streams?
Used to process data in chunks.
🔌 Modules & Architecture
21. Modules?
Reusable blocks of code.
22. CommonJS vs ES Modules?
CommonJS uses require(), ES uses import/export.
23. require()?
Used to include modules.
24. module.exports?
Used to export functions/objects.
25. Built-in modules?
fs, http, path, os.
26. Core modules?
fs, http, path, url, events.
27. fs module?
Used for file operations.
28. path module?
Handles file paths.
29. http module?
Creates web servers.
30. middleware?
Function that runs between request and response.
🌐 Express.js
31. Express.js?
Web framework for Node.js.
32. Why Express?
Simplifies routing and APIs.
33. Routing?
Defines API endpoints.
34. Middleware?
Functions executed during request lifecycle.
35. app.use vs app.get?
use = all methods, get = only GET requests.
36. req & res?
Request and response objects.
37. Error handling?
Using middleware with error parameters.
38. REST API?
API that uses HTTP methods.
39. PUT vs PATCH?
PUT replaces full resource, PATCH updates part.
40. CORS?
Allows cross-origin requests.
🔐 Advanced
41. Scaling?
Using clustering and load balancing.
42. Clustering?
Running multiple Node processes.
43. Worker threads?
Used for CPU-heavy tasks.
44. Single vs multi-threaded?
Node is single-threaded but handles async tasks.
45. Memory management?
Handled by V8 garbage collector.
46. Garbage collection?
Removes unused memory.
47. Event emitter?
Handles custom events.
48. Security?
Use authentication, validation, encryption.
49. Optimization?
Caching, clustering, async code.
50. Logging?
Tracking app behavior and errors.
🚀 Scenario Answers
51. Slow API?
Use caching, async calls, or optimization.
52. Large uploads?
Use streams.
53. Debugging?
Using console, debugger, or logging tools.
54. Callback hell solution?
Use promises or async/await.
55. Project structure?
Separate routes, controllers, models.