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.