Level 1: Core JavaScript (Backend Foundation)
1. What is the event loop in JavaScript?
Expected answer:
JavaScript is single-threaded; the event loop manages execution of synchronous code and handles async callbacks (via call stack, callback queue, microtask queue).
2. Difference between setTimeout and Promise.then execution?
Expected answer:Promise.then runs in the microtask queue (higher priority), while setTimeout runs in the callback queue (lower priority), so promises execute first.
3. What is async/await and how does it work?
Expected answer:
Syntactic sugar over Promises. await pauses execution inside an async function until the Promise resolves/rejects.
4. Difference between == and ===?
Expected answer:== does type coercion, === checks both value and type (strict equality).
5. What happens if you don’t handle a rejected Promise?
Expected answer:
It results in an unhandled promise rejection, which can crash the Node.js process (or log warnings depending on config).
🟡 Level 2: HTTP & REST Fundamentals
6. What is REST?
Expected answer:
An architectural style using HTTP methods to perform CRUD operations on resources (stateless, client-server separation).
7. Difference between GET and POST?
Expected answer:
GET retrieves data (no side effects), POST sends data to create resources (can modify server state).
8. What are common HTTP status codes?
Expected answer:
200 (OK), 201 (Created), 400 (Bad Request), 401 (Unauthorized), 404 (Not Found), 500 (Server Error).
9. What is statelessness in REST?
Expected answer:
Each request contains all required information; server doesn’t store client session state.
10. What is the difference between PUT and PATCH?
Expected answer:
PUT replaces the entire resource; PATCH updates partial fields.
🟠 Level 3: Backend Thinking & Node.js Concepts
11. How does Node.js handle multiple requests if it is single-threaded?
Expected answer:
Uses non-blocking I/O and event loop; heavy tasks are delegated to system threads or worker pool.
12. What is middleware in backend systems?
Expected answer:
Functions that run before/after request handlers to process requests (e.g., logging, auth, validation).
13. What is JSON and why is it commonly used?
Expected answer:
Lightweight data format used for client-server communication; easy to parse in JavaScript.
14. What is CORS?
Expected answer:
Security mechanism that controls cross-origin requests from browsers.
15. What is input validation and why is it important?
Expected answer:
Ensures incoming data is correct/safe; prevents bugs, security issues (like injection attacks).
🔵 Level 4: Debugging & Developer Tools Awareness
16. How would you debug a failing API?
Expected answer:
Check logs, inspect request/response, use browser dev tools/Postman, verify payload and status codes.
17. What can you do with browser developer tools (Network tab)?
Expected answer:
Inspect API calls, headers, payload, response, timing, status codes.
18. What is a 500 error vs 400 error?
Expected answer:
400 = client mistake, 500 = server-side failure.
19. How would you test an API without frontend?
Expected answer:
Use tools like Postman/cURL to send requests and inspect responses.
20. What are logs and why are they important?
Expected answer:
Records of application activity used for debugging, monitoring, and tracing issues.
🔴 Level 5: Problem Solving & Applied Scenarios
21. If an API is slow, how would you investigate?
Expected answer:
Check DB queries, network latency, logs, response time, optimize heavy operations.
22. How would you design an API for “Create User”?
Expected answer:
POST /users, validate input, store data, return 201 + created resource.
23. What happens if two users update the same data simultaneously?
Expected answer:
Race condition; need handling (locking, versioning, last-write-wins, etc.).
24. How would you handle errors in an API?
Expected answer:
Use try-catch, return meaningful status codes and messages, log errors.
25. What is the difference between synchronous and asynchronous code?
Expected answer:
Sync blocks execution; async allows non-blocking execution and better performance.