Extend Type to support abstract classes

[oleksii-nikishkin]

Hello,

I am building an app with Nest.js. I don't like the idea of tokens (@Inject(SOME_TOKEN)) and depending on final classes. I prefer having an interface or an abstract class of a service that might have multiple implementations. Here is an example:

abstract class Logger {
  // ...
}

@Injectable()
class PinoLogger extends Logger {
  // ...
}

@Injectable()
class ConsoleLogger extends Logger {
  // ...
}

@Injectable()
class SomeService {
  construct(private readonly logger: Logger) {}

  // ...
}

It works fine with the Nest's DI. It also works with their testing modules. However, I found it's not possible to create a test unit with mocked abstract dependencies using Suites. Here is an example:

let service: SomeService;
let logger: Mocked<Logger>;

describe('SomeService', () => {
  beforeAll(async () => {
    const { unit, unitRef } = await TestBed.solitary(SomeService).compile();

    service = unit;
    logger = unitRef.get(Logger); // Here is the type error
  });
});

get() method doesn't let me access any dependency typed in the service as an abstract class or an interface.

There is an interface Type (link) in your types package. I think it can be extended to support at least abstract classes by adding the abstract key before new, like this:

export interface Type<T = any> {
  abstract new (...args: any[]): T;
}

I tried to tweak my unit test to consider Logger as Type<Logger> (even without the abstract key) – it seems to be working fine, it successfully mocked the abstract class and injected it into the service. I was later able to spy on the logger.