Backend Fundamentals:

1. How the Server Works (Request–Response Cycle)
2. REST API Design Principles
3. HTTP Methods & Status Codes
4. Middleware & Request Flow
5. Authentication vs Authorization

Node.js & Backend Runtime:

6. Event Loop & Non-Blocking I/O
7. Async/Await & Promises
8. Error Handling & Global Error Middleware
9. Environment Variables & Config Management
10. Logging & Debugging

Databases:

11. SQL vs NoSQL (When to Use What)
12. Database Schema Design
13. Indexing & Query Optimization
14. Transactions & ACID Properties
15. Relationships (1–1, 1–Many, Many–Many)

Security & Scalability:

16. JWT, Sessions & Cookies
17. Password Hashing & Security Best Practices
18. Rate Limiting & Caching (Redis)
19. Scalability Concepts (Load Balancing, Horizontal Scaling)
20. API Versioning & Deployment Basics

If you master these, most backend interviews become predictable