Describe the bug

TypeScript allows accessing private and protected methods using bracket notation. This is useful for unit testing, if you want to check a private/protected method works as expected.

Vitest does not allow using spyOn on a private/protected method (e.g. you want to test a public method that uses a private method to load data) unless you cast to any. So vi.spyOn(myObject, 'privateMethod') does not work while vi.spyOn(myObject as any, 'privateMethod') works as expected. But this comes with the caveat that Vitest does not know about the types this function returns.

See the linked StackBlitz for an example.

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-ucls7umq

System Info

System:
    OS: macOS 26.3.1
    CPU: (8) arm64 Apple M2
    Memory: 257.30 MB / 24.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.6.0 - /Users/jakob/.nvm/versions/node/v24.6.0/bin/node
    npm: 11.12.1 - /Users/jakob/.nvm/versions/node/v24.6.0/bin/npm
    bun: 1.2.18 - /Users/jakob/.bun/bin/bun
  Browsers:
    Chrome: 147.0.7727.56
    Edge: 147.0.3912.72
    Firefox: 149.0.2
    Firefox Nightly: 149.0a1
    Safari: 26.3.1
  npmPackages:
    @vitest/browser-playwright: ^4.1.2 => 4.1.3 
    @vitest/coverage-v8: ^4.1.2 => 4.1.3 
    playwright: ^1.58.0 => 1.59.1 
    vitest: ^4.1.2 => 4.1.3

Used Package Manager

npm

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