I spent too many hours trying to mock correctly the JavaScript's Date object. I tried a few things I've found on Google... without success. I finally ended up with a kinda handcrafted solution.

Every time we mock, we diverge from the real world scenario.

For this reason, I tend not to mock... but sometimes, there are not many choices. Fortunately, Date is one good exception!

Everybody on the same timezone!

To prevent problems related to timezones (e.g. date formating), you can set node timezone in jest config file. Now you are sure all tests are executed on the same timezone no matter where your colleagues or your CI server are.

// jest.config.js
process.env.TZ = 'GMT';

module.exports = {
  // ...
}

See also: the full list of timezones (column TZ database name)

Mock Date.now

Let's say I want to test a dashboard component which tells me "hello" with the date of the day. The lazy way is to only test the Hello part (without the date). Because it changes every day. But you won't test the date formatting part.

If you want to, you gonna have to mock Date.now() and put a default one.

// your-test.spec.js

const RealDate = Date.now

beforeAll(() => {
  global.Date.now = jest.fn(() => new Date('2019-04-07T10:20:30Z').getTime())
})

afterAll(() => {
  global.Date.now = RealDate
})

Now in the same file, you can add something like the following:

it('should show a formatted date of today', async () => {
  const dashboard = await Mount(<Dashboard />)
  expect(dashboard).toHaveText('Hi Max, today is 7 April 2019')
})

💡RealDate store "real" Date instance, so we can put it back afterwards.

Using Moment.js?

You are probably using the very popular moment.js library. If so, mocking Date.now, will probably not be enough.
A workaround is to mock the entire node module.

// <root>/__mocks__/moment.js
const moment = jest.requireActual('moment');

Date.now = () => new Date('2019-04-07T10:20:30Z').getTime();

module.exports = moment;

With this solution, you don't need beforeAll()/afterAll() listener. This mock will be effective for all tests.
And, every time moment() is called, return date will be the same 🎉

The very first thing I recommend you to do is to set a default Timezone.

// jest.config.js
process.env.TZ = 'GMT';

module.exports = {
  // ...
}

Mock Date object

// your-test.spec.js

const RealDate = Date;

beforeEach(() => {
  global.Date.now = jest.fn(() => new Date('2019-04-22T10:20:30Z').getTime());
});

afterEach(() => {
  global.Date = RealDate;
});

Using Moment?

If you're using the popular moment.js library, mocking Date.now, will probably not be enough. If you check the moment's code source, you will see that
you have to mock JavaScript Date object through a different way.

// <root>/__mocks__/moment.js
const moment = jest.requireActual('moment');

Date.now = () => new Date('2019-04-22T10:20:30Z').getTime();

module.exports = moment;

Thank you for taking the time to read this post. I hope you found it useful! If you liked it, please give it a ❤️ or a 🦄! Also, feel free to comment or ask questions in the section below or on Twitter @_maxpou :)

Originally published on maxpou.fr.