Vitest spyOn has different behaviour than jest's

Describe the bug

Hey! It seems the vitest spyOn doesn’t have the same behaviour as the jest’s (see reproduction)

Reproduction

vitest project reproduction: https://stackblitz.com/edit/vitest-dev-vitest-ne8pti?file=src%2Fexample.ts,test%2Fbasic.test.ts,package.json&initialPath=__vitest__

jest project reproduction: https://github.com/galkatz373/jest-example

System Info

macOS 12.0.1, node 16.6.0

Used Package Manager

npm

Validations

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• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn’t already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

[sheremet-va]

If you store methods on an object or class, you can spy on them without problem:

// funcs.ts
class Helper {
  func() {}
  
  combine() {
    this.func()
  }
}

export const helper = new Helper()

// or

export const helper = {
  func: () => {},
  combine: () => {
    helper.func()
  }
}

// test.ts
import { helper } from './funcs'

vi.spyOn(helper, 'func').mockReturnOnce(100)

[sheremet-va]

@sheremet-va works as well without instances but class level? For example, Helper class is injected by DI inside a Component and I want to test the component by mocking the class methods. Does Vitest support something like vi.spyOn(Helper.prototype, "func").mockReturnOnce(100)? Thanks.

Yes, it works.