Angular 2 Observable Service Karma Jasmine单元测试不起作用

我是Angular 2和Karma Jasmine单元测试的新手 . 我无法弄清楚我为了使这个单元测试使用模拟响应而产生的语义错误 . 在控制台中，当“expect（items [0] .itemId）.toBe（2）;”时运行时，它表示项目[0] .itemId未定义 .

有人能够帮助我或指出我正确的方向吗？如果您需要任何其他信息，请与我们联系 . 谢谢！

item.ts

export class Item {
     itemId: number;
     itemName: string;
     itemDescription: string;
}

item.service.ts

import { Injectable, Inject } from '@angular/core';
    import { Headers, Http } from '@angular/http';
    import { Observable } from 'rxjs/Rx';
    import { Item } from './item';
@Injectable()
export class ItemService {
    private headers = new Headers({'Content-Type': 'application/json'});

    constructor(
    private http: Http)
    {

    }

    getItems(listOptions: Object): Observable<Item[]> {
    return this.http.post('/listItems', listOptions, {headers:this.headers})
       .map(response => response.json() as Item[])
  }
}

item.service.spec.ts

import { TestBed, fakeAsync, inject, tick } from '@angular/core/testing';
import { MockBackend } from '@angular/http/testing';
import { Http, BaseRequestOptions, Response, ResponseOptions } from '@angular/http';
import { Observable } from 'rxjs/Rx';
import { ItemService } from './item.service';
import { Item } from './item';


describe('ItemService', () => {
  let mockResponse, matchingItem, connection;

  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [
        ItemService,
        MockBackend,
        BaseRequestOptions,
        {
          provide: Http,
          useFactory: (backend, defaultOptions) => new Http(backend, defaultOptions),
          deps: [MockBackend, BaseRequestOptions]
        },
        // { provide: XHRBackend, useClass: MockBackend }        
      ]
    });

    const items = [
      {
        "itemId":2,
        "itemName":"test item1",
        "itemDescription":"hello hello"        
      },
      {
        "itemId":1,
        "itemName":"name2124111121",
        "itemDescription":"description212412112"        
      }
    ];
    mockResponse = new Response(new ResponseOptions({body: {data: items}, status: 200}));
  });

  describe('getItems', () => {

    //Subscribing to the connection and storing it for later
    it('should return all the items',inject([ItemService, MockBackend], (service: ItemService, backend: MockBackend) => {

        backend.connections.subscribe(connection => {
          connection.mockRespond(mockResponse);
        });
        service.getItems({isActive: true, sortColumn: "lastModifiedDateUtc", sortOrder: "desc"})
        .subscribe((items: Item[]) => {
          expect(items.length).toBe(2);
        });
      }));
    });
  });

Plunkr：https://plnkr.co/edit/m7In2eVh6oXu8VNYFf9l?p=preview（Plunkr有一些错误，我也需要帮助，但主要文件在那里）