Window event listener fires but spy does not count it

[nick-roth-sas]

Describe the bug

Trying to migrate a test from jest to vitest. I have a use case where we are listening for message events on window, and I want to verify calls to the listener with a spy.

The provided reproduction shows two scenarios. One where the I call vi.spyOn before the event listener is added and the second where event listener is added first. The listener is called and runs in both cases but the spy only counts if the spy was created before adding to window.

This behavior seems unintuitive at best and like an error in the worst case.

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-t28wegld?file=test%2Fbasic.test.ts

System Info

System:
    OS: macOS 15.1.1
    CPU: (8) arm64 Apple M3
    Memory: 133.05 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.20.4 - ~/.nvm/versions/node/v18.20.4/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v18.20.4/bin/yarn
    npm: 10.7.0 - ~/.nvm/versions/node/v18.20.4/bin/npm
  Browsers:
    Chrome: 131.0.6778.206
    Safari: 18.1.1
  npmPackages:
    vitest: ^2.1.8 => 2.1.8

Used Package Manager

yarn

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• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

Can you check if this really works on Jest?

I don't think this would work because listenerObj.listener is simply accessed before spied. You would need to delay it like this to make it work:
https://stackblitz.com/edit/vitest-dev-vitest-ril9f8z1?file=src%2Fbasic.ts

export const register = () => {
  // window.addEventListener('message', listenerObj.listener);
  window.addEventListener('message', (e) => listenerObj.listener(e));
};