How to export multiple Type Aliases?

Question

For better capsulation I want to create a new type that includes multiple other types. entityService should be typed as OakService or MapleService which should be included inside TreeService .

One possible solition you can see inside the comments. If I uncomment the imports inside garden.component.ts and replace TreeService with OakService | MapleService OakService | MapleService , it will work, but I want some kind of parent type, that includes them all.

tree.service.ts

import { OakService } from './oak.service';
import { MapleService } from './maple.service';

export type TreeService = typeof OakService | MapleService;

oak.service.ts

import { BaseService } from './base.service';

@Injectable()
export class OakService extends BaseService {
  constructor() {
    super();
  }
}

maple.service.ts

import { BaseService } from './base.service';

@Injectable()
export class MapleService extends BaseService {
  constructor() {
    super();
  }
}

garden.component.ts

// import { OakService } from './oak.service';
// import { MapleService } from './maple.service';
import { TreeService } from './parent.service';

@Component()
export class GardenComponent {
  constructor(
    protected entityService: TreeService // OakService | MapleService
  ) {}
}

balcony.component.ts

import { MapleService } from './maple.service';

export class BalconyComponent extends GardenComponent {   
  constructor(
    protected mapleService: MapleService
  ) {
    super(mapleService);   
  }
}

Info: The code above does not work. Your have to use the code inside the comments. The error I've got is not inside balcony.component.ts : ERROR in : Can't resolve all parameters for BaseListComponent in .../garden.component.ts: (?).

Answer 1

Parameter types in injectable class allow to skip Angular @Inject decorator in TypeScript and annotate a constructor for dependency injection. Types don't exist at runtime and cannot be injected.

GardenComponent cannot be workable component because of that. Since is used as abstract class, it shouldn't have @Component decorator:

export abstract class GardenComponent {
  constructor(
    protected entityService: TreeService // OakService | MapleService
  ) {}
}

While child classes should have @Component :

@Component(...)
export class BalconyComponent extends GardenComponent {   
  constructor(mapleService: MapleService) {
    super(mapleService);
  }
}

Constructors are mandatory in child classes because parent class doesn't contain proper annotation, and their parameters shouldn't have visibility modifiers because this.entityService is already assigned by parent constructor.