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Vitest Usefaketimers Works Differently From Jest

7 min read Oct 12, 2024
Vitest Usefaketimers Works Differently From Jest

Vitest: A Refreshing Take on JavaScript Testing, But What About useFakeTimers?

Vitest, the new kid on the block in the JavaScript testing world, has quickly gained popularity for its speed and ease of use. It's built on top of the tried and true Jest, but brings several improvements to the table. But, as with any new framework, there are bound to be differences, and one that has caught the attention of developers is the way Vitest handles useFakeTimers.

While Jest and Vitest share a common ancestor, their implementation of useFakeTimers diverges. This can lead to unexpected behavior if you migrate your tests from Jest to Vitest without understanding the differences.

Let's dive into these differences and see how to approach useFakeTimers effectively within the Vitest environment.

The Heart of the Matter: How Jest and Vitest Handle Time

At their core, both Jest and Vitest aim to control the flow of time within your tests. This is essential for scenarios where you need to test functions that rely on asynchronous operations, timers, or intervals.

Jest, through jest.useFakeTimers, effectively "pauses" time. It intercepts the native timer functions (like setTimeout, setInterval, and Date.now) and creates a controlled environment where you can manually advance time, providing fine-grained control over when your asynchronous code executes.

Vitest, on the other hand, takes a slightly different approach. It utilizes fakeTimers from the fake-timers library, and it's here where the divergence becomes apparent. Vitest allows you to manipulate time by using a "fake-timers" API that's distinct from the traditional jest.useFakeTimers approach. This means you'll need to adjust your test code accordingly.

The Key Differences: Navigating the useFakeTimers Landscape

Here's a breakdown of the key differences that developers need to be aware of:

1. API Changes:

  • Jest: jest.useFakeTimers(), jest.runTimersToTime()
  • Vitest: vi.useFakeTimers(), vi.runAllTimers()

2. Timer Handling:

  • Jest: Manages timers using a dedicated clock mechanism.
  • Vitest: Uses fake-timers for timer management.

3. Mocking Date Objects:

  • Jest: Offers jest.fn().mockReturnValue(new Date(timestamp)) for mocking Date.now().
  • Vitest: vi.setSystemTime(new Date(timestamp)) provides a more direct approach to managing time.

Moving from Jest to Vitest: Making the Transition Smooth

If you're migrating from Jest to Vitest, here are some tips for a seamless transition:

  1. Identify useFakeTimers Calls: Start by reviewing your tests that utilize jest.useFakeTimers. Make sure you've identified all instances where time manipulation is employed.

  2. Adjust API Usage: Replace any calls to jest.useFakeTimers() and related methods with their Vitest counterparts (e.g., vi.useFakeTimers() and vi.runAllTimers()).

  3. Adapt to Different Mocking Approaches: If you relied on jest.fn().mockReturnValue(new Date(timestamp)) for mocking Date.now(), switch to vi.setSystemTime(new Date(timestamp)).

  4. Test Thoroughly: Once you've made the necessary adjustments, run your tests to ensure everything functions as expected.

Code Examples: Illustrating the Differences

Let's take a look at how these differences manifest in code:

Example: Testing an Interval

Jest:

import { jest } from '@jest/globals';
import myFunction from './myFunction';

test('should execute the function every 5 seconds', () => {
  jest.useFakeTimers();

  const mockCallback = jest.fn();
  setInterval(mockCallback, 5000);

  jest.advanceTimersByTime(5000);
  expect(mockCallback).toHaveBeenCalledTimes(1);

  jest.advanceTimersByTime(5000);
  expect(mockCallback).toHaveBeenCalledTimes(2);

  jest.runAllTimers();
});

Vitest:

import { vi } from 'vitest';
import myFunction from './myFunction';

test('should execute the function every 5 seconds', () => {
  vi.useFakeTimers();

  const mockCallback = vi.fn();
  setInterval(mockCallback, 5000);

  vi.advanceTimersByTime(5000);
  expect(mockCallback).toHaveBeenCalledTimes(1);

  vi.advanceTimersByTime(5000);
  expect(mockCallback).toHaveBeenCalledTimes(2);

  vi.runAllTimers();
});

Example: Mocking Date.now()

Jest:

import { jest } from '@jest/globals';
import myFunction from './myFunction';

test('should return the correct timestamp', () => {
  jest.useFakeTimers();
  const mockDate = new Date(1679804800000); // Example timestamp
  jest.spyOn(Date, 'now').mockReturnValue(mockDate.getTime());

  expect(myFunction()).toBe(1679804800000); 
});

Vitest:

import { vi } from 'vitest';
import myFunction from './myFunction';

test('should return the correct timestamp', () => {
  vi.useFakeTimers();
  vi.setSystemTime(new Date(1679804800000)); // Example timestamp

  expect(myFunction()).toBe(1679804800000);
});

Conclusion

Vitest offers a valuable alternative to Jest, delivering faster execution times and a more intuitive user experience. However, its approach to useFakeTimers differs from Jest, requiring adjustments to your tests. By understanding these differences and adapting your code accordingly, you can make a smooth transition and leverage the power of Vitest for your JavaScript testing needs.

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