Mocking JavaScript's current Date in Jest tests

There are situations where Date.now is used in application code. That code needs to be tested, and it’s always a struggle to remember how to mock Date.now. Here is the magic snippet:

const literallyJustDateNow = () => Date.now();

test('It should call and return Date.now()', () => {
  const realDateNow = Date.now.bind(global.Date);
  const dateNowStub = jest.fn(() => 1530518207007);
  global.Date.now = dateNowStub;

  expect(literallyJustDateNow()).toBe(1530518207007);
  expect(dateNowStub).toHaveBeenCalled();

  global.Date.now = realDateNow;
});

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This isn’t really a Jest-specific trick, we’re just accessing Node global object and replace Date.now with a stub.
We’re also being good unit-testing citizens and putting the original global.Date.now implementation back 😇.

Cover photo by Bryce Barker on Unsplash.

Comments

[JavaScript Joel]

You might run the risk of tests after this one failing if there is an exception in this test that prevents Date.now being set back.

Using beforeEach and after each could help with Code reuse and this possible scenario.

[Patrick Roza]

Wrap in try, finally. Alternatively create a utility function to create now date, and mock that

[Michał T.]

Also great for mocking Math.random I imagine.

[Borys Kupar]

How can you mock date constructor?
So imagine date is created with "const now = new Date()".
I would like to have "now" to have specific mocked date.

[Hugo Di Francesco]

global.Date = myMockConstructor