Angular 4单元测试模拟后端创建未定义的答案

import { TestBed, inject } from '@angular/core/testing';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';

import { DataService } from './data.service';

describe('DataService', () => {
  let service: DataService;
  let httpMock: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [DataService]
    });

    service = TestBed.inject(DataService);
    httpMock = TestBed.inject(HttpTestingController);
  });

  afterEach(() => {
    httpMock.verify();
  });

  it('should return undefined answer', () => {
    const mockResponse = undefined;

    service.getAnswer().subscribe(response => {
      expect(response).toBeUndefined();
    });

    const req = httpMock.expectOne('api/answer');
    expect(req.request.method).toBe('GET');
    req.flush(mockResponse);
  });
});