Understanding Core Concepts in NestJS

Welcome to the core concepts of NestJS — the Marvel Cinematic Universe of backend frameworks. If you’ve been wondering what’s behind the curtain of all those @Decorators, or how Nest magically wires everything up, this is your backstage pass! 🎟️

NestJS isn't just another framework; it’s a full-blown, opinionated, TypeScript-powered backend circus 🎪 — where every act (and file) has a role and a funny hat to wear.

So here’s the deal:

🧱 Modules — The Office Floors 🏢

Imagine your app is a giant corporate building (with working AC). Each Module is a floor with a dedicated team. Users? They're visiting for specific reasons. Want to register a new user? Go to the “Auth” floor. Need to check product stock? Try the “Inventory” department.

Modules are the foundation — they bundle related logic (controllers, services, etc.) together so your app doesn’t turn into a spaghetti monster. 🍝

🚦 Controllers — The Route Traffic Cops

Controllers are those cool folks at the reception. 🧑‍✈️ They handle HTTP requests like GET, POST, PUT, DELETE, and route them to the appropriate handler (aka your service). Think of them as Google Maps for your API — they know where things should go.

When a request comes in:

Controller: “Ah, you need user data? One sec, I’ll talk to the UserService for you.”

👷 Services — The Overworked Backend Engineers

Services are where the actual business logic lives. They fetch data, apply rules, save things, calculate taxes (eww), and handle the "heavy lifting" while controllers pretend to be busy.

They are:

• Reusable 💡
• Injected into controllers 🔌
• Basically NestJS's unsung heroes 🦸

If your controller is Batman, the service is Alfred — doing everything behind the scenes.

🧃 Dependency Injection — The Magic Sauce

You don’t “new Up” your services here like it’s 2010. NestJS has a better idea. It says:

“You chill. I’ll give you what you need.”

That’s Dependency Injection (DI) — Nest’s way of handing over the right services, repositories, or anything else, directly to your classes.

You register it once in a module, and boom — it’s available anywhere you ask, like an app-wide Amazon Prime. 🛒🚚