Provide promises to the angular APP_INITIALYZER to put application initialization on hold

medium-ngrx-indexeddb-APP_INITIALYZER.ts

export function accountReducerBootstrapFactory(reducer: AccountReducer) {
  // bootstrap() returns a Promise
  return () => reducer.bootstrap()
}

@NgModule({
...
  providers: [
    {
      provide: APP_INITIALIZER,
      deps: [AccountReducer],
      multi: true,
      useFactory: accountReducerBootstrapFactory
    }
  ]
...
})
export class AccountModule {}