Is there a way in Angular 7 to decide at compile time which service to use?

Question

I've created an Angular 7 project with Ionic 4. In this project I have multiple environments e.g 'local' or 'firebase'.

In my environment.ts is a parameter which corresponds to the strings above.

export const environment = {
  production: false,
  ...,
  userApi: 'firebase'
};

I've created a module to allow my services to be lazy-loaded as described in this article. It looks as follows:

@NgModule({
  declarations: [],
  imports: [
    CommonModule,
    AngularFireAuthModule,
    AngularFireModule.initializeApp(environment.firebase),
  ]
})
export class UserApiModule { }

Next I have an InjectionToken in which I declare that I provide the service in the Module mentioned above and use a factory to decide which Service to use.

export const USER_SERVICE = new InjectionToken<AbstractUserService>('USER_SERVICE',
    {
        providedIn: UserApiModule,
        factory: UserApiFactory
    }
);

function UserApiFactory(): AbstractUserService {
    switch (environment.userApi) {
        case 'firebase':
            return new FirebaseUserService(inject(AngularFireAuth));
        case 'mock':
            return new MockUserService();
        default:
            throw new Error('UserService implementation not found');
    }
}

This solutions in general works fine. When in 'firebase'-mode the firebase services is loaded and in 'local'-mode the local service.

But when compiling and serving in 'prod'-mode the package sizes in the ChromeDev-Tools are the same in either service mode. Which leads me to the idea that, also never reachable, the other service is still compiled.

So is there a way to implement this feature to ignore the effectively unreachable code? So if I set 'local' in my environment file, the firebase service aswell as the dependencies are never included in the build?

Thanks in advance!

Answer 1

1. You can make use of conditional imports based on your environment configuration. There are multiple ways to do it but a easy one can be:

@NgModule({
  imports: [ 
       BrowserModule,
       environment.userApi == 'firebase' ? AngularFireAuthModule : []
  ],
  declarations: [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule {}

2. You can also create an array of dependencies (imports) in your environment file and add them in imports like (see more here:

imports: [...environment.dependencies]

where

//environment.ts
dependencies = [
    AngularFireAuthModule,
    AngularFireModule.initializeApp(environment.firebase)
];