Summary  
This chapter explains how to use dependency injection in Nest.js by registering providers in a module and having services automatically instantiated and injected into controllers.  

General domain of usage  
Backend application development (e.g., connecting to databases in web APIs)

Dependency injection allows Nest.js to create and connect different parts of your application automatically.

When you use a service inside a controller, you don’t create it manually. Instead, Nest.js provides it for you.

For example:

```ts id="di4"
constructor(private usersService: UsersService) {}
```

Here is what is happening:

* `UsersService`: is requested by the controller;
* Nest.js looks for it in the module;
* The service is created and passed into the controller.

For this to work, the service must be registered:

```ts id="di5"
@Module({
  providers: [UsersService],
})
```

The `providers` array tells Nest.js which classes can be injected.

Nest creates a single instance of the service and reuses it wherever needed. This avoids creating multiple copies and keeps your application consistent.

In simple terms, dependency injection connects parts of your application without requiring you to manage them manually.


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.

Connects to the Database