Summary  
This chapter explains how to mark a class with an `@Injectable()` decorator and register it in a module’s `providers` array so that Nest.js can instantiate and inject a single shared instance wherever it’s needed.

General domain of usage  
Server-side application development

Previously, you learned that Nest.js can inject dependencies automatically. But how does Nest.js know which classes it can create?

To make a class available for dependency injection, you mark it with the `@Injectable()` decorator.

```ts
@Injectable()
export class UsersService {}
```

Next, register the service inside a module using the `providers` array.

```ts
@Module({
  providers: [UsersService],
})
```

Here is what is happening:

* `@Injectable()` tells Nest.js that the class can be managed by the framework;
* `providers` registers the service with the module;
* Nest.js creates a single instance of the service;
* Whenever another class requests `UsersService`, Nest.js provides the same instance automatically.

For example, the controller can now receive the service through its constructor:

```ts
constructor(private usersService: UsersService) {}
```

This system keeps your application organized by allowing different parts of your code to work together without manually creating or managing objects.

Learn how to build structured backend applications with Nest.js. Organize your code using controllers, services, and modules, handle requests and data validation, and create a complete API using a scalable architecture.

Set up your first Nest.js application and understand how it is structured. Learn how the main files work together to handle requests and organize backend logic.

Learn how to handle requests in Nest.js using controllers and routes. Define endpoints, work with route and query parameters, and control how your backend responds to client requests.

Learn how to organize backend logic using services. Move business logic out of controllers, structure your code properly, and understand how different parts of your application work together.

Learn how Nest.js organizes applications using modules and dependency injection. Understand how different parts of your backend connect and how to structure your code into scalable, maintainable features.

Learn how to handle and validate data in Nest.js. Work with request body data, define structured DTOs, and ensure incoming data is correct using validation pipes.

Summarize what you have learned throughout the track and reinforce your understanding of backend development. Review key concepts and see how they come together into a complete, structured application.

How Providers Work in Nest.js