javascript - Angular Unit Test:メソッドでプロパティをモックする方法は?

Question

テストしようとしているサービスは次のとおりです。

@Injectable()
export class BomRevisiosnsService {
    constructor(
        private baseService: BaseService,
        private appConstants: AppConstants,
        private dmConstants: DMConstants
    ) { }

    public getRevisionsData(): any {
        var itemId = this.appConstants.userPreferences.modelData['basicDetails']['itemId'];
        let url = this.dmConstants.URLs.GETBOMREVISIONS + itemId + "/GetRevisionsAsync";
        let headers = {
            "Content-Type": "application/json",
            UserExecutionContext: JSON.stringify(this.appConstants.userPreferences.UserBasicDetails),
        }
        if (itemId != null || itemId != undefined) {
            return this.baseService.getData(url, headers).map(response => {
                return response;
            });
        }
    }
}

仕様ファイル

describe('bom-revisions.service ',()=>{
    let bomRevisiosnsService:BomRevisiosnsService;
    let  baseService: BaseService;
    let  appConstants: AppConstants;
    let  dmConstants: DMConstants;
    beforeEach(()=>{
        baseService=new BaseService(null,null);
        appConstants=null;
        dmConstants=null;
        bomRevisiosnsService=new BomRevisiosnsService(baseService,appConstants,dmConstants);
    });
it('getRevisionsData() call base service getData()',()=>{
    let spy = spyOn(baseService, 'getData').and.returnValue(Observable.of())  
    bomRevisiosnsService.getRevisionsData();
    expect(baseService.getData).toHaveBeenCalled();
});

})

エラー: TypeError: null のプロパティ 'userPreferences' を読み取れません

私はいくつかのモック値を提供する必要があると信じていthis.appConstants.userPreferences.modelData['basicDetails']['itemId']; ますthis.dmConstants.URLs.GETBOMREVISIONS + itemId + "/GetRevisionsAsync";