mockResolvedValue returns undefined - node.js

Question

I want to test the function below using the tests below. However, I fail in mocking the userService.login function, which always returns undefined instead of desired value. I have tried these approaches https://jestjs.io/docs/mock-functions#mock-return-values (please see the tests), but nothing seems to work.
FUNCTION
static async login(req: Request, res: Response, next: NextFunction): Promise<void> {
const userDTO = req.body;
try {
const accessToken = await userService.login(userDTO);
// accessToken is undefined here :(
res.setHeader('authorisation', `Bearer ${accessToken}`);
res.send('Logged in');
} catch (e) {
// eslint-disable-next-line callback-return
next(e);
}
}
TESTS:
import UserService from '#services/users.service';
import UserController from '#routers/controller/users.controller';
import { NextFunction, Request, Response } from 'express';
import UserRepository from '#data-access/user.repository';
jest.mock('#services/users.service');
let userService: UserService;
const fakeToken = 's$sdfDgf45d';
beforeAll(() => {
userService = new UserService(UserRepository);
// userService.login = jest.fn().mockResolvedValue('s$sdfDgf45d').mockRejectedValue('error');
});
describe('UserController: ', () => {
const mockReq = {
body: {
login: 'Artyom',
password: 'qwerty123'
}
};
const mockRes = {
setHeader: jest.fn(),
send: jest.fn()
};
const nextFunction = jest.fn();
test('the class constructor should have been called', () => {
expect(UserService).toHaveBeenCalled();
});
test('should send auth token in case of success scenario', async () => {
// #ts-ignore
userService.login.mockResolvedValue('s$sdfDgf45d').mockRejectedValue('error');
// jest.spyOn(userService, 'login').mockResolvedValue('s$sdfDgf45d' as any).mockRejectedValue('error');
await UserController.login(mockReq as Request, mockRes as any as Response, nextFunction as NextFunction);
// that's the test that fails
expect(mockRes.setHeader.mock.calls[0]).toEqual(['authorisation', `Bearer ${fakeToken}`]);
expect(mockRes.send.mock.calls[0][0]).toEqual('Logged in');
});
test('should throw an error', async () => {
return expect((userService.login as jest.Mock)()).rejects.toMatch('error');
});
});
ERROR:
Array [
"authorisation",
- "Bearer s$sdfDgf45d",
+ "Bearer undefined",
CODE:
https://github.com/lion9/node-express