How to assign type to a class?

Question

I have a class which is actually a Service in NestJS, so the problem I faced is that I cannot type it properly. I mean of course we can do an interface and say that class implements it, but what if I need to use it in generic. Let's take a look what I mean.

For example I want to do a Provider that can choose right class according to some data for example .env, for this purpose we should write a generic

type ServiceProviderType<T> = Record<string, T>;

Now the implementation of such a Provider

const Provider: ServiceProviderType<Interface> = {
default: DefaultService                      //--------------Problem occurs here
custom: CustomService
}

Error says that typeof DefaultService cannot be assigned to interface cause it doesn't have required methods, even though it has.

For Example this is Interface

export interface ServiceInterface {
  getResult(expression: string): string;
  exponentialToDecimal(exponential: string): string;
}

and this is Class

@Injectable()
export class DefaultService implements Interface {
  constructor(
    @Inject(EXPRESSION_COUNTER_SERVICE)
    private readonly expressionCounterService: ExpressionCounterService,
    @Inject(REGEXP_CREATOR_SERVICE_INTERFACE)
    private readonly regExCreatorService: RegExCreatorService,
  ) {}

  getResult(expression: string): string {
//some code return string
}
exponentialToDecimal(string): {
//some code return string
}
}

Answer 1

DefaultService is the constructor while that T would represent an instance of T, that's why you got that error. You could use the Type<T> utility from @nestjs/common or write your own.