Understanding Dependency Injection

Dependency injection is a way to provide one part of your application with access to another.

Instead of creating objects manually, Nest.js automatically provides them when needed.

In previous chapters, you used a service inside a controller:

constructor(private usersService: UsersService) {}

Here is what is happening:

• UsersService: is a dependency;
• The controller does not create it manually;
• Nest.js injects it automatically.

Without dependency injection, you would have to create the service yourself:

const usersService = new UsersService();

This approach makes code harder to manage and reuse.

With dependency injection, Nest.js handles this for you.

To make a class injectable, use:

@Injectable()

Then register it in the module. providers: tells Nest which services can be injected.

This system allows different parts of your application to work together without being tightly connected.