Error "Variable is used before being assigned" in an Angular unit test although it's like in an article

I'm new at Angular unit tests and try to test an API access.

Here's my code:

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")
  })
})

The test Check API returns the webmaster doesn't compile because Variable 'res' is used before being assigned.

I kind of understand but I can't see why it would work better in the paragraph "Angular Service Unit Testing Example with HttpClient" of this article (test "should return data").

Anyone to shed light on this issue?

req.flush() just means that you are resolving the promise. you'll still need to move your expect to within the promise.

it('Check API returns the webmaster',  () =>  {
    let res: string
    component.getUserService().getWebmasterEmail().subscribe(
      data => {
      res = data,
      expect(res).toEqual("toto");
    }
    )

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

    req.flush("toto")
  })