Mocking fs module for a 3rd party library

[PaulRosset]

Hello,

I'm currently trying to migrate from Jest to Vitest.
However, I'm encountering an issue when I want to mock the fs module with memfs for an external library that I'm using.

With Jest, the following was working fine:
jest.mock('node:fs', () => memFs)
jest.mock('fs', () => memFs)

But using vitest, it makes the external library that is performing a call to fs not mocked, however, calls that are performed outside of this lib are working fine.
But for some reason, I cant mock any call to fs that have been called inside this external library.

| my-app <-- My app is performing some `fs` calls that are mocked right with `memfs`
| node_modules
 | external-lib <-- This external lib is performing some `fs` calls that I want to be mocked, but for some reason, they are not...

I also did a reproduction repo here:

• https://stackblitz.com/edit/stackblitz-starters-dewcrr (Vitest - KO)
• https://stackblitz.com/edit/stackblitz-starters-ps1fn9 (Jest - OK)

Does anyone know what I could be missing here?

[hi-ogawa]

vi.mock only intercepts import inside the files which are processed by Vite. In your reproduction, the external dependency to-vfile's import will be kep as is, so it acceess non-mocked fs modules.

One way to extend this to external dependency is to use server.deps.inline: ["to-vfile"] option, which will make Vite process to-vfile just like your source code. Here is an updated reproduction:
https://stackblitz.com/edit/stackblitz-starters-rwafjx?file=vitest.config.ts

[PaulRosset]

Superb! Thank you a lot for your help @hi-ogawa :)

Do you think it could make sense to add it to the documentation on the "Migrating from jest" part ?

I can do it if you think it could make sense.

[hi-ogawa]

I'm not the only one to decide (and also I'm not so familiar with what jest does in comparison), but yeah, it sounds nice to mention something in the doc. Feel free to try!