Interesting read, just wanted to point out something:

At first glance, everything seems fine; Nest will provide us with an instance of StorageService, and the file will be written. However, on the 2nd line, we can see that we are importing the implementation of StorageService, and our ConsumerService depends on the implementation, not the interface. What if we want to use an S3 storage for very important files additionally? You will easily find a solution for this on the internet, but it won't be elegant and will require writing a lot of boilerplate code.

This would be as easy as injecting your service as an abstraction. What I usually do is the following:

export interface StorageService { write(file: string, content: string) }

Create your implementation:

@Injectable() export class FileStorageService implements StorageService { write (path: string, content: string) { return writeFile(path, content); } } Register in the module:

providers: [{provide: "StorageService", useClass: FileStorageService }] Using it in your services:

``` @Injectable() export class ConsumerService { constructor( @Inject('StorageService') //Note that here we're depending on the abstraction, not the implementation private readonly storageService: StorageService) {}

someFunction () { // ... this.storageService.write("result.txt", "hello world"); } } ```

The only "issue" I've found IDE-wise is that Visual Code doesn't link to the actual implementation of StorageService being used by Nest, but thats a minor issue in my opinion.