app.test.js

'use strict'

const app = require('./app')
const math = require('./math')

test('calls math.add', () => {
	const addMock = jest.spyOn(math, 'add')

	// override the implementation
	addMock.mockImplementation(() => 'mock')
	expect(app.doAdd(1, 2)).toEqual('mock')
	expect(addMock).toHaveBeenCalledWith(1, 2)

	// restore the original implementation
	addMock.mockRestore()
	expect(app.doAdd(1, 2)).toEqual(3)
	console.log(addMock.mock.calls)
})