Jest Mock Service argument is not assignable

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I've a simple controller in my NestJs application.

@Post('/')
async create(@Body() createUserRequest: CreateUserRequest): Promise<User> {
  return await this.userService.create(createUserRequest);
}

My goal is to test this function using jest. As you can see the controller injects an instance of UserService. So I try to mock this service in my unit test. The test case looks like this.

describe('User Controller', () => {
  let userService: UserService;
  let userController: UserController;

  beforeEach(async () => {
    const module = await Test.createTestingModule({
      controllers: [UserController],
      providers: [UserService],
    }).compile();

    userService = module.get<UserService>(UserService);
    userController = module.get<UserController>(UserController);
  });

  describe('create', () => {
    it('should return a user', async () => {
      const result = new User();

      jest.spyOn(userService, 'create').mockImplementation(() => result);

      expect(await userController.create(new CreateUserRequest())).toBe(result);
    });
  });
});

My problem is that jest.spyOn where the mocking happens produces an error.

No overload matches this call.
Overload 1 of 4, '(object: UserService, method: never): SpyInstance<never, never>', gave the following error.
Argument of type '"create"' is not assignable to parameter of type 'never'.
Overload 2 of 4, '(object: UserService, method: never): SpyInstance<never, never>', gave the following error.
Argument of type '"create"' is not assignable to parameter of type 'never'.

Has anyone an idea what is wrong with my mock? I took this approach from the documentation.

nestjs jestjs typescript unit-testing
1 Answers

This is probably not the response you expected, however, you could achieve the same result implementing as below:

// ...
describe('create', () => {
  it('should return a user', async () => {
    const result = new User();

    ((userService as unknown) as any).create = jest.fn().mockResolvedValue(result);
    // Or to ensure that the value is only called once, 
    // go ahead and use 'mockResolvedValueOnce'

    expect(await userController.create(new CreateUserRequest())).toBe(result);
  });
});
// ...
user_60543580
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