Mocking Math.random and not restoring makes vitest hang up when coverage is enabled

[sapphi-red]

Describe the bug

When you

• enabled coverage
• mock Math.random and not restoring the original function after test

vitest hangs up.
If you run the same code with jest, it finishes.

const t = typeof vi !== 'undefined' ? vi : jest

test('Math.random mock', () => {
  // const original = Math.random
  const mocked = t.fn(() => 0.1);
  Math.random = mocked
  // Math.random = original
});

When you restore Math.random it will work. (uncomment line 4 & 7)

I think it does not need to end the test successfully, but at least fail with timeout.
Because it is hard to find which test is hanging up.

Reproduction

https://gist.github.com/sapphi-red/d3454a362c3b66e3654918516c808d7e

1. Download zip and extract
2. npm ci
3. npm run jest runs successfully
4. npm run vitest never ends

(stackblitz did not work)

System Info

System:
    OS: Windows 10 10.0.19044
    CPU: (8) x64 Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
    Memory: 12.61 GB / 31.93 GB
  Binaries:
    Node: 16.14.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.17 - C:\Program Files\nodejs\yarn.CMD
    npm: 8.1.3 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 98.0.4758.102
    Edge: Spartan (44.19041.1266.0), Chromium (99.0.1150.30)
    Internet Explorer: 11.0.19041.1202
  npmPackages:
    vitest: ^0.5.9 => 0.5.9

Used Package Manager

npm

Validations

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• Read the Contributing Guidelines.
• Read the docs.
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• The provided reproduction is a minimal reproducible example of the bug.

[sheremet-va]

We cannot guarantee that no one uses global APIs when running tests. Just restore it to original after each test:

afterEach(() => {
  Math.random = originalRandom
})

[sapphi-red]

I thought the environment which is running tests is isolated from the environment which is collecting coverage.
So even if a test is breaking the environment, it would not effect the main environment.

But actually the test did not fail with timeout. Is this intended? If it is intended, I am ok with closing this issue.

[sheremet-va]

They are run in the same env (not isolated, like jest does with vm module). Vitest provides isolation through workers, unlike jest, who uses it only to run tests in parallel.

Currently we are not planning on changing the way tests are run, because vm has a poor ESM support (that is why jest is so bad at ESM), but we are ESM-first and it is simply not good enough, so we are waiting for the Node team to make a better support.

[sapphi-red]

Thank you for the explanation! I understand.