Mocking moment() and moment().format using jest

Question

I'm unable to mock moment() or moment().format functions. I have states where, currentDateMoment and currentDateFormatted are getting set as below.

currentDateMoment: moment() //2019-04-23T17:45:26.339Z
currentDateFormatted: moment().format('MM-DD-YYYY').valueOf() //"04-23-2019"

Trying to mock both moment() and moment().format in my snapshot tests to return a particular date, but was unable to. Tried below.

jest.mock('moment', () => () => '2018–01–30T12:34:56+00:00');

jest.mock('moment', () => ({
  constructor: () => '2018–01–30T12:34:56+00:00'
})); 

jest.mock('moment', () => () => ({ format: () => '01–30-2018' }));

Answer 1

The easiest way to mock moment() and any function that it uses (i.e. .day(), .format()) is to change the Date that moment() uses under the hood

Add the snippet below inside your test file

Date.now = jest.fn(() => new Date("2020-05-13T12:33:37.000Z"));

This makes it so anytime moment() is called in your tests, moment() thinks that today is Wednesday, May 13th 2020

Answer 2

You can mock Moment to return a specific date, then format don't have to be mocked.

jest.mock('moment', () => {
  return () => jest.requireActual('moment')('2020-01-01T00:00:00.000Z');
});

By doing so, any call to Moment() will always return a moment object with date set to 2020-01-01 00:00:00

Here is an example with a function that return the date of tomorrow and the test for this function.

const moment = require('moment');
const tomorrow = () => {
  const now = moment();
  return now.add(1, 'days');
};

describe('tomorrow', () => {
  it('should return the next day in a specific format', () => {
    const date = tomorrow().format('YYYY-MM-DD');
    expect(date).toEqual('2020-01-02');
  });
});

Answer 3

Here is the solution:

index.ts:

import moment from 'moment';

export function main() {
  return {
    currentDateMoment: moment().format(),
    currentDateFormatted: moment()
      .format('MM-DD-YYYY')
      .valueOf()
  };
}

index.spec.ts:

import { main } from './';
import moment from 'moment';

jest.mock('moment', () => {
  const mMoment = {
    format: jest.fn().mockReturnThis(),
    valueOf: jest.fn()
  };
  return jest.fn(() => mMoment);
});

describe('main', () => {
  test('should mock moment() and moment().format() correctly ', () => {
    (moment().format as jest.MockedFunction<any>)
      .mockReturnValueOnce('2018–01–30T12:34:56+00:00')
      .mockReturnValueOnce('01–30-2018');
    expect(jest.isMockFunction(moment)).toBeTruthy();
    expect(jest.isMockFunction(moment().format)).toBeTruthy();
    const actualValue = main();
    expect(actualValue).toEqual({ currentDateMoment: '2018–01–30T12:34:56+00:00', currentDateFormatted: '01–30-2018' });
  });
});

Unit test result with 100% coverage:

 PASS  src/stackoverflow/55838798/index.spec.ts
  main
    ✓ should mock moment() and moment().format() correctly  (7ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        3.795s, estimated 8s

Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/55838798