Summary  
This chapter explains how to configure Nest.js’s dependency injection by marking classes with the @Injectable() decorator and registering them in a module’s providers array so that the framework creates a singleton instance and injects it into constructors automatically.

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.

How Providers Work in Nest.js