🔍 How to Mock the `moment` Library Using Jest

Keywords: Jest mock moment, mock date with Jest, Jest moment.js mock, Jest unit testing moment, mocking npm packages Jest

🧠 TL;DR

If you're using moment in your JavaScript/TypeScript code and want to mock dates in your Jest tests, you can easily mock moment() using jest.mock. Here's the quick version:

jest.mock('moment', () => () => ({
  format: () => '2025-03-24T10:00:00Z',
}));

But let’s go beyond the basics. As a senior developer, you want your mocks clean, flexible, and reusable.

🧪 Why Mock `moment` in Jest?

When writing unit tests, time-based logic can get flaky. Your code might:

Generate timestamps
Format dates/times
Calculate durations
Compare time windows

You don’t want your tests to depend on actual time, because that makes them:

Non-deterministic
Difficult to reproduce bugs
Fragile across timezones or daylight saving

👉 So instead of relying on real-time, we mock it.

✅ Basic Example: Mock `moment()` to Return a Fixed Date

Let’s say your function looks like this:

import moment from 'moment';

export function getCurrentISODate() {
  return moment().format();
}

You want getCurrentISODate() to always return the same thing in tests.

🧪 Jest Test With Mock:

import { getCurrentISODate } from './date-utils';

jest.mock('moment', () => () => ({
  format: () => '2025-03-24T10:00:00Z',
}));

describe('getCurrentISODate', () => {
  it('returns mocked ISO date', () => {
    expect(getCurrentISODate()).toBe('2025-03-24T10:00:00Z');
  });
});

Boom. 🎯 Predictable, testable, and no time travel needed.

🧰 Advanced Mocking: Customize the Return Value

Sometimes you want control over what moment() returns in each test.

Here’s how to mock with a variable:

import moment from 'moment';
import { getCurrentISODate } from './date-utils';

let mockedDate = '2025-03-24T12:00:00Z';

jest.mock('moment', () => () => ({
  format: () => mockedDate,
}));

test('uses custom mock date', () => {
  mockedDate = '2025-03-24T18:30:00Z';
  expect(getCurrentISODate()).toBe('2025-03-24T18:30:00Z');
});

Now you’re mocking like a pro. 🧑‍💻

🧙‍♂️ Want to Mock `moment()` to Return a Specific Moment Instance?

Sometimes your app relies on things like moment().isAfter(...) or moment().add(...). In that case, just return a real moment object seeded with your fake date:

import realMoment from 'moment';

jest.mock('moment', () => {
  const mockedMoment = realMoment('2025-03-24T00:00:00Z');
  return () => mockedMoment;
});

This way, you can still use add(), isAfter(), etc., just frozen in time.

⌛ Alternative: Use `jest.useFakeTimers` + `Date`

If you’re using moment() just to get the current time, consider mocking the global Date object instead:

jest.useFakeTimers().setSystemTime(new Date('2025-03-24T00:00:00Z'));

Then moment() will pick up the frozen time too, because it reads from Date.now() under the hood.

🔚 Conclusion

Mocking moment in Jest is simple, but doing it right makes your tests stable and clean.

Quick Recap:

Use jest.mock('moment', () => ...) to fake moment()
Return real or partial moment objects depending on your use case
Consider jest.useFakeTimers for global date mocking

🔍 How to Mock the moment Library Using Jest