mocked dooms-day

__mocks__ "slash" doom-scheduler.js

export const theDay = Promise.resolve();

__mocks__ "slash" rocket-silo.js

export const Launch = jest.fn(); //this is "replacement" code

dooms-day.test.js

import {Launch} from './rocket-silo'; // magic! magic!
import {theDay} from 'doom-scheduler'; // might be not a great idea
import 'rocket-silo';

expect(Launch).toHaveBeenCalled();