moduleA.module.ts

import { ModuleWithProviders, NgModule } from '@angular/core';
import { SingletonService } from './singleton.service';

import { ModuleAComponent } from './moduleA.component';

@NgModule({
  declarations: [
    ModuleAComponent
  ],
  exports: [ModuleAComponent],
})
export class ModuleAModule {
  static forRoot(): ModuleWithProviders<ModuleAModule> {
    return { ngModule: ModuleAModule, providers: [SingletonService] };
  }

  static forChild(): ModuleWithProviders<ModuleAModule> {
    return {
      ngModule: ModuleAModule, providers: [{ provide: SingletonService, useValue: {} }]
    };
  }
}