constructor(
  public router: Router,
  private profilService: ProfileService,
  private dragula: DragulaService,
  private alerter: AlertService
) {
  dragula.drag.subscribe((value) => {
    this.onDrag(value);
  });
  dragula.dragend.subscribe((value) => {
    this.onDragend(value);
  });
}

it('should call initializer function in constructor', () => {
  TestBed.createComponent(HomeComponent); // this is the trigger of constructor method
 expect(sideNavService.initialize).toHaveBeenCalled(); // sample jasmine spy based test case
});