Jest Mocking examples

jeststuff.js

// To mock a function
// utils has a function called getWinner that takes two strings
// as arguments and return one of them as the winner. It 
// is used internally by thumbWar. Problem here is that the original
// function gets clobbered by our mock, so we have to manually 
// reassign the original back after the test has run.
// This is called monkey patching and only works with commonjs modules
const utils = require('./utils');
const thumbWar = require('./thumb-war');

test('returns winner', () => {
  const originalGetWinner = utils.getWinner;
  utils.getWinner = jest.fn((p1, p2) => p1);
  
  const winner = thumbWar('Ken', 'Colin');
 
  expect(utils.getWinner.mock.calls).toEqual([
    ['Ken', 'Colin']  
  ]);
 
  
  // Clean up
  utils.getWinner = originalGetWinner;
});


////////////////////////////////////////////////////////
// To mock a function a better way is to use jest.spyOn
// which keeps track of the original implementation.
// This is called monkey patching and only works with commonjs modules
const utils = require('./utils');
const thumbWar = require('./thumb-war');

test('returns winner', () => {
  jest.spyOn(utils, 'getWinner')
  utils.getWinner.mockImplementation((p1, p2) => p1)
 
  const winner = thumbWar('Ken', 'Colin');
 
  expect(utils.getWinner.mock.calls).toEqual([
    ['Ken', 'Colin']  
  ]);
  
  // cleanup
  utils.getWinner.mockRestore();
});

///////////////////////////////////////////////////////////
// But we can mock out an entire module with jest.mock
const thumbWar = require('../thumb-war')

jest.mock('../utils', () => {
  return {
    getWinner: jest.fn((p1, p2) => p1)
  }
})

test('returns winner', () => {
  const winner = thumbWar('Kent C. Dodds', 'Ken Wheeler')
  expect(winner).toBe('Kent C. Dodds')
  expect(utilsMock.getWinner.mock.calls).toEqual([
    ['Kent C. Dodds', 'Ken Wheeler'],
    ['Kent C. Dodds', 'Ken Wheeler']
  ])

  // cleanup
  utilsMock.getWinner.mockReset()
})

///////////////////////////////////////////////////
// If you want to mock part of a module but have rest of
// exports not set
//////////////////////////////

const OPERATIONS = {
  SEND_EMAIL: 'SEND_EMAIL',
  SEND_PUSH_NOTIFICATION: 'SEND_PUSH_NOTIFICATION'
};

function createEmailNotification(to, subject, content) {
}

function createPushNotification(to, title, content) {
}

async function sendNotification(action) {
  // send something to an API
}

module.exports = {
  OPERATIONS,
  createEmailNotification,
  createPushNotification,
  sendNotification
};
// Here we are only mocking sendNotification and leaving the rest of
// the exports in the module un touched.
jest.mock('./notifications', () => ({
  ...jest.requireActual('./notifications'),
  sendNotification: jest.fn(async () => {}),
));