Angular Jest - test service with dependency on another service and so on

Question

I'm starting with Jest and Unit test. I need to create unit tests for an already existing big application on angular 11, and I start by testing services.

Here is my BaseService. Most of my other services expends from this one.

@Injectable({
  providedIn: 'root'
})
export class BaseService {

  protected apiHost: string;
  protected httpClient: HttpClient;
  protected httpClientWithInterceptor: HttpClient;
  protected configurationService: ConfigurationService;
  protected translate: TranslateService;

  constructor(
    injector: Injector,
  ) {
    this.configurationService = injector.get(ConfigurationService);
    this.translate = injector.get(TranslateService);
    this.apiHost = this.configurationService.configuration.apiHost;
    this.httpClient = new HttpClient(injector.get(HttpBackend));
    this.httpClientWithInterceptor = injector.get(HttpClient);
  }

  // ...

}

As you see, it inject some other services like HttpClient, TranslateService and ConfigurationService.

I solved (I hope) the dependency for HttpClient and TranslateService but ConfigurationService is still a problem. This is another internal service to fetch an appSettings.json file and it has its own dependencies.

export class ConfigurationService {

  public configuration: Configuration;

  constructor(private httpClient: HttpClient, httpBackend: HttpBackend) {
    this.httpClient = new HttpClient(httpBackend);
    this.getConfigurationFromUrl();
  }

  // ...
}

And now my test file for BaseService

describe('BaseService', () => {
  let service: BaseService;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [
        HttpClientTestingModule,
        TranslateModule.forRoot(),
      ],
      providers: [BaseService, ConfigurationService]
    });
    service = TestBed.inject(BaseService);
  });

  it('should be created', () => {
    expect(service).toBeTruthy();
  });
});

Form now it does nothing and npm test return error

describe('BaseService', () => {
  let service: BaseService;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [
        HttpClientTestingModule,
        TranslateModule.forRoot(),
      ],
      providers: [BaseService, ConfigurationService]
    });
    service = TestBed.inject(BaseService);
  });

  it('should be created', () => {
    expect(service).toBeTruthy();
  });
});

the returned error

FAIL  projects/web-components/src/app/shared/services/base.service.spec.ts
 BaseService › should be created

    TypeError: Cannot read property 'apiHost' of null

      25 |     // this.configurationService = injector.get(ConfigurationService);
      26 |     // this.translate = injector.get(TranslateService);
    > 27 |     this.apiHost = this.configurationService.configuration.apiHost;
      at new BaseService (projects/web-components/src/app/shared/services/base.service.ts:27:60)
      at Object.BaseService_Factory [as factory] (ng:\BaseService\ɵfac.js:5:10)
      at R3Injector.Object.<anonymous>.R3Injector.hydrate (../packages/core/src/di/r3_injector.ts:412:29)
      at R3Injector.Object.<anonymous>.R3Injector.get (../packages/core/src/di/r3_injector.ts:209:23)
      at NgModuleRef$1.Object.<anonymous>.NgModuleRef$1.get (../packages/core/src/render3/ng_module_ref.ts:76:29)
      at TestBedRender3.Object.<anonymous>.TestBedRender3.inject (../packages/core/testing/src/r3_test_bed.ts:278:48)
      at Function.Object.<anonymous>.TestBedRender3.inject (../packages/core/testing/src/r3_test_bed.ts:167:33)
      at projects/web-components/src/app/shared/services/base.service.spec.ts:19:23
      at ZoneDelegate.Object.<anonymous>.ZoneDelegate.invoke (node_modules/zone.js/dist/zone.js:386:30)
      at ProxyZoneSpec.Object.<anonymous>.ProxyZoneSpec.onInvoke (node_modules/zone.js/dist/proxy.js:117:43)
      at ZoneDelegate.Object.<anonymous>.ZoneDelegate.invoke (node_modules/zone.js/dist/zone.js:385:36)
      at Zone.Object.<anonymous>.Zone.run (node_modules/zone.js/dist/zone.js:143:47)

Of course, a lot of other services extends from BaseService and they all have there own dependencies.

So here are my questions :

• Did I chose the right way to implement unit tests ?
• Can you help me with this dependencies injections error please ?