How to mock service in Angular unit test code example

Example: angular testing mock service

import { TestBed } from '@angular/core/testing';import {HttpClientTestingModule, HttpTestingController}        from '@angular/common/http/testing';import {Dog, DogService} from './dog.service';export const mockDog: Dog = {    message:     'https://images.dog.ceo/breeds/hound-basset/n02088238_9815.jpg',    status: 'success'};describe('DogService', () => {  let httpTestingController: HttpTestingController;  let service: DogService;  beforeEach(() => {    TestBed.configureTestingModule({      providers: [DogService],      imports: [HttpClientTestingModule]    });    httpTestingController = TestBed.get(HttpTestingController);    service = TestBed.get(DogService);  });  afterEach(() => {    httpTestingController.verify();  });  it('should be created', () => {    expect(service).toBeTruthy();  });  it('random should should provide data', () => {    service.randomDog().subscribe((dog: Dog) => {      expect(dog).not.toBe(null);      expect(JSON.stringify(dog)).toEqual(JSON.stringify(mockDog));    });    const req = httpTestingController              .expectOne(`https://dog.ceo/api/breeds/image/random`);    req.flush(mockDog);  });});