Dependency injection
NestJS is built around the strong design pattern commonly known as Dependency injection.
If you are new to NestJS and/or the concept of Dependency Injection, the NestJS official docs recommends to read this article about dependency injection from the angular official documentation.
NestJS Official Documentation:
Thanks to TypeScript capabilities, it's extremely easy to manage dependencies because they are resolved just by type. In the example below, Nest will resolve the catsService by creating and returning an instance of CatsService (or, in the normal case of a singleton, returning the existing instance if it has already been requested elsewhere). This dependency is resolved and passed to your router's constructor (or assigned to the indicated property)
With this adapter, it is possible to use dependency injection with you routers, middlewares or context providers with ease.
The adapter will automatically resolve the dependency and will inject the instance when invoking the method.
To do that we use the @Inject() decorator provided by NestJS. You can read more about it in their official documentation.
constructor(@Inject(CatsService) private catsService: CatsService) {}