Angular 单元测试中的错误“在分配之前使用变量”，尽管它就像在文章中一样

Question

我是 Angular 单元测试的新手，尝试测试 API 访问。

这是我的代码：

import { ComponentFixture, TestBed } from '@angular/core/testing'
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing'
import { JwtHelperService, JWT_OPTIONS } from '@auth0/angular-jwt'

import { AuthService } from '../../services/auth.service'
import { UserService } from '../../services/user.service'
import { WorkService } from '../../services/work.service'
import { ErrorToLogService } from '../../services/errorToLog.service'

import { FooterComponent } from './footer.component'

describe('FooterComponent', () => {
  let component: FooterComponent
  let fixture: ComponentFixture<FooterComponent>
  let httpController: HttpTestingController

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [
        HttpClientTestingModule
      ],
      providers: [
        AuthService,
        UserService,
        WorkService,
        ErrorToLogService,
        JwtHelperService,
        { provide: JWT_OPTIONS, useValue: JWT_OPTIONS },
      ],
      declarations: [ FooterComponent ]
    })
    .compileComponents()
  })

  beforeEach(() => {
    fixture = TestBed.createComponent(FooterComponent)
    component = fixture.componentInstance
    fixture.detectChanges()
    httpController = TestBed.inject(HttpTestingController)
  })

  afterEach(() => {
    TestBed.resetTestingModule()
  })

  it('should create', () => {
    expect(component).toBeTruthy()
  })

  it('Check API returns the webmaster',  () =>  {
    let res: string
    component.getUserService().getWebmasterEmail().subscribe(
      data => res = data,
    )

    const req = httpController.expectOne({
      method: 'GET',
      url: `/api/getWebmasterEmail`,
    })

    req.flush("toto")
    expect(res).toEqual("toto")
  })
})

测试

Check API returns the webmaster

没有编译因为

Variable 'res' is used before being assigned

.

我有点理解，但我不明白为什么它会在这篇文章（测试“应该返回数据”）的“Angular Service Unit Testing Example with HttpClient”段落中工作得更好。

有人能阐明这个问题吗？