How to mock functions deeper in the code with jest - unit-testing

Question

Im trying to mock this mail function so I dont send mails everytime I test my code. But the mocking is not working. This code gives me the error: mockImplementation is not a function.
It's the add function that calls sendUserInvitationMail(). The mailer module export looks like this:
module.exports = {
sendUserInvitationMail,
};
this is the test code:
require('dotenv').config();
const { startWithCleanDb } = require('../../../utils/test.helpers');
const { add } = require('../invitation.service');
const { ADMIN_LEVELS, TABLES } = require('../../../constants');
const { AuthorizationError } = require('../../../errors');
const knex = require('../../../../db/connection');
const mailer = require('../../../mailer/index');
jest.mock('../../../mailer/index');
beforeEach(() => startWithCleanDb());
mailer.sendUserInvitationMail.mockImplementation(() => console.log('Mocked mail function called'));
mailer.sendUserInvitationMail();
describe('invitation.service', () => {
describe('add', () => {
it('adds an invitation to the db', async () => {
expect.assertions(2);
const result = await add(
{
email: 'tester#test.be',
badgeNumber: '344d33843',
},
{ currentZoneId: 1 },
ADMIN_LEVELS.ADMINISTRATOR,
);
const invitation = (await knex.select('*').from(TABLES.INVITATIONS))[0];
expect(invitation.id).toEqual(result.id);
expect(invitation.email).toEqual(result.email);
});
});
});

Answer 1

In mailer, sendUserInvitationMail is undefined, so it has no property mockImplementation.
Try:
mailer.sendUserInvitationMail = jest.fn().mockImplementation(() => console.log('Mocked mail function called'));
or
mailer.sendUserInvitationMail = jest.fn(() => console.log('Mocked mail function called'));