app.module.ts

// in a real app could be environment.useMocks ?
const useMocks = true;

@NgModule({
  imports: [BrowserModule, CommonModule],
  declarations: [AppComponent, CryptoSelectorComponent],
  bootstrap: [AppComponent],
  providers: [
    {
      provide: PriceApiService,
      useClass: useMocks ? MockPriceApiService : PriceApiService
    }
  ]
})
export class AppModule { }