Dependency Injection (DI) is a design pattern used in NestJS to manage dependencies between classes, promoting decoupling and making it easier to test and maintain the code. In NestJS, the DI system is built around the concept of providers and tokens.

• Providers: A provider is a class, value, or factory function responsible for creating an object that can be injected into other classes. Providers are typically registered in the providers array of a module. Here's an example of a provider class:

    import { Injectable } from '@nestjs/common';

    @Injectable()
    export class CatsService {
      // Implementation of the service
    }

• Tokens: Tokens are unique identifiers used by NestJS to locate providers. By default, a class’s token is the class itself. When a class needs a dependency, it requests it from the DI system using a token.

• Injecting Dependencies: To inject a dependency, you can use the @Inject() decorator in the constructor of a class. The DI system will then resolve the dependency and provide an instance of the requested class. Here's an example:
  

    import { Injectable, Inject } from '@nestjs/common';
    import { CatsService } from './cats.service';

    @Injectable()
    export class AppService {
      constructor(private readonly catsService: CatsService) {}
    }

In this example, the AppService class requests an instance of CatsService in its constructor. The DI system locates the CatsService provider, creates an instance, and injects it into the AppService class.

Dependency Injection in NestJS provides the following benefits:

• Decoupling: DI promotes loose coupling between classes by allowing them to depend on abstractions rather than concrete implementations. This makes it easier to modify or replace parts of the application without affecting other components.

• Testing: DI makes it simpler to test classes by allowing you to replace dependencies with mock implementations during testing.

• Reusability: By managing dependencies centrally, DI encourages the reuse of providers across multiple parts of the application.

Now that you understand Dependency Injection in NestJS, let’s discuss the role of pipes and their use cases. How can pipes help with validation and data transformation in a NestJS application?

to be continued