You should be able to, if you have at the top: vi.mock('node:fs', () => { return { existsSync: vi.fn(), }; });

All vi.mock calls are placed at the top of the file, and it's the first thing that's getting called. To change implementation for different tests, you can do: vi.spyOn(fs, 'existsSync').mockImplementation(() => { // new implementation })

Vitest makes writing tests directly within your source code easy, eliminating the need for separate test files. This approach, known as in-source testing, is useful when you want to quickly test individual functions without the overhead of creating and managing separate test files.