[Angular 8] Module.forRoot() configuration

app.module.ts

import { FooModule } from "@foo/modules"
import { config } from "@foo/config"

@NgModule( {
  imports: [
    BrowserModule,
    FooModule.forRoot(config)
  ]
  bootstrap: [ AppComponent ],
})
export class AppModule {}

foo.module.ts

import { InjectionToken, ModuleWithProviders, NgModule, Optional, SkipSelf } from '@angular/core'
import { FooService } from '@foo/services'

export type AppConfig = { apiKey: string; databaseURL: string }
export const APP_CONFIG = new InjectionToken<string>('APP_CONFIG')

// Fix the AoT` error "Consider changing the function expression into an exported function"
export function buildApp(config: AppConfig) {
  return new FooService(config)
}

@NgModule()
export class FooModule {
  static forRoot(config: AppConfig): ModuleWithProviders {
    return {
      ngModule: FooModule,
      providers: [
        { provide: APP_CONFIG, useValue: config },
        {
          provide: FooService,
          useFactory: buildApp,
          deps: [APP_CONFIG]
        }
      ]
    }
  }

  constructor(@Optional() @SkipSelf() parentModule: FooModule) {
    if (parentModule) {
      throw new Error('FooModule is already loaded. Import it in the AppModule only')
    }
  }
}

// References:
// https://angular.io/guide/singleton-services#the-forroot-pattern
// https://angular.io/guide/singleton-services#prevent-reimport-of-the-greetingmodule
// https://github.com/angular/angular/issues/33539
// https://www.bennadel.com/blog/3565-providing-module-configuration-using-forroot-and-ahead-of-time-compiling-in-angular-7-2-0.htm