post: Added provider and dependency injection one

content/posts/provider-and-de-in-nestjs.md

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+---
+title: "Understanding Providers and Dependency Injection in NestJS"
+date: 2026-09-28T18:50:00+05:45
+draft: false
+tags: ["nestjs", "backend"]
+description: "Concept of dependency injection in NestJS"
+---
+
+When I first started working with NestJS, registering providers (and controllers) inside a module felt like nothing more than a chore.
+
+It was one of those things I did because *“that’s just how Nest is designed.”*
+
+Even after I learned that modules exist to build an application graph, which it uses internally to resolve relationships and dependencies between modules and providers, the idea still felt vague and abstract. I understood **what** was happening, but not really **why** it worked the way it did.
+
+Recently, I decided to take a deeper dive into how providers are registered and how Nest actually injects them. It turned out to be far more interesting than I expected.
+
+Here’s what I learned.
+
+## First, let’s understand what Dependency Injection is.
+
+Wikipedia defines Dependency Injection(DI) as:
+
+> **Dependency Injection** is a programming technique in which an object or function receives other objects or functions that it requires, as opposed to creating them internally.
+
+Let’s look at a simple example.
+
+Imagine a restaurant application. When a user’s order is ready for pickup, a notification needs to be sent.
+
+```tsx
+class OrderService {
+  // code operating over orders
+
+  const notificationService = new NotificationService()
+  notificationService.notify(userId, message)
+
+  // ...
+}
+```
+
+Here, `OrderService` handles order-related logic, and `NotificationService` handles notifications.
+
+However, because `NotificationService` is instantiated directly inside `OrderService` using `new`, this **is not** dependency injection.
+
+Now compare that with this:
+
+```tsx
+class OrderService {
+  constructor(
+    private readonly notificationService: NotificationService,
+  ) {}
+
+  // code operating over orders
+  notificationService.notify(userId, message)
+
+  // ...
+}
+```
+
+This is dependency injection.
+
+`OrderService` no longer cares about **how** `NotificationService` is created. It simply declares that it needs one, and something else provides it. That instance can also be cached and reused across every class that needs it, if configuration is same.
+
+## Why Dependency Injection?
+
+Dependency Injection helps by:
+
+- Reducing coupling between classes
+- Making testing easier (you can inject mock dependencies)
+- Encouraging modular, maintainable code
+
+## How Dependency Injection Works in NestJS
+
+In NestJS, the basic flow looks something like this:
+
+1. Define a provider using `@Injectable()`
+2. Register that provider with Nest
+3. Ask Nest to inject it using constructor-based injection
+
+Registration usually happens in a module:
+
+```tsx
+@Module({
+  providers: [DemoService],
+  controllers: [DemoController],
+})
+export class DemoModule {}
+```
+
+At this point, Nest becomes responsible for creating and managing instances of `DemoService`.
+
+## Nest’s IoC Container (The Missing Piece)
+
+NestJS uses an **IoC (Inversion of Control) container** to manage dependencies.
+
+You can think of this container as a **key-value** **registry**:
+
+- **Keys** are called *tokens*
+- **Values** describe how to create or retrieve a dependency
+
+When we write:
+
+```tsx
+providers: [DemoService]
+```
+
+Nest internally expands it into a  standard provider definition:
+
+```tsx
+providers: [
+  {
+    provide: DemoService,
+    useClass: DemoService,
+  },
+]
+```
+
+In this case:
+
+- The **token** is `DemoService` (the class itself)
+- `useClass` tells Nest which class to instantiate when that token is requested.
+
+So when Nest encounters:
+
+```tsx
+constructor(private readonly demoService: DemoService) {}
+```
+
+1. Nest looks up the `DemoService` token in the IoC container
+2. It finds the matching provider record
+3. It resolves the provider (`useClass` in this case)
+4. The instance is created (or reused, since providers are singletons by default)
+5. The resolved instance is injected into the class
+