Mock fs

[antfu]

No description provided.

[sheremet-va]

What do you mean by that?

[antfu]

https://peeky.netlify.app/

Just an idea from Peeky, not sure if we want it

[Maxim-Mazurok]

This seems like the most appropriate issue to share my struggle with mocking fs.
TL;DR I can mock it with inconsistent module name, but can't keep track of calls.

engines.ts

import { readFileSync } from "fs";

export const getEngines = () => {
  return readFileSync("engines.ts");
};

mock.vispec.ts

import { expect, test, vi } from "vitest";
import { getEngines } from "../engines.js";

test("works", async () => {
  // Arrange
  vi.mock("node:fs", async () => {
    return {
      ...(await vi.importActual<typeof import("node:fs")>("node:fs")),
      readFileSync: vi.fn().mockReturnValue(`mocked value`),
    };
  });
  const { readFileSync } = await import("node:fs");
  console.log(readFileSync);

  // Act
  const result = getEngines();

  // Assert
  expect(result).toBe("mocked value"); // works
  expect(readFileSync).toHaveBeenCalled(); // doesn't
});

As soon as I changed import { readFileSync } from "fs"; to import { readFileSync } from "node:fs"; in engines.ts - it started working.

But still, quite an interesting artifact that mocking was actually kinda working even with an inconsistent module name.

[mrmckeb]

This is something I think a lot of teams could benefit from too @antfu and @sheremet-va. I can see it being really useful for CLIs, scripts, and server code.

Mocking fs is a fine option, but isn't as powerful/intuitive as a mocked file system.

[sheremet-va]

The team thinks it should not be part of the core because it adds more maintenance burden that we don't want to take. Additionally, the use case is not popular enough to ship this to everyone.

Instead, we are adding a section "Mocking File System" to our documentation in #6021 recommending the usage of memfs.