Angular CoreModule

core.module.ts

@NgModule({
  imports: [
    ...
  ],
  providers: [
    ...
  ],
  exports: [
    ...
  ]
})
export class CoreModule {
  constructor(@Optional() @SkipSelf() parentModule: CoreModule) {
    if (parentModule) {
      throw new Error(
        'CoreModule is already loaded. Import it in the AppModule only');
    }
  }
}