`toEqual` reports `undefined` when other keys do not match

Describe the bug

Consider the following case:

const obj1 = {
  foo: 'bar',
  qux: 'qux'
}

const obj2 = {
  foo: 'bar',
  extra: undefined,
  qux: 'qux2'
}

If you are using toEqual, since it should not take care about undefined, you should expect the following comparison to fail:

expect(obj1).toEqual(obj2)

Because of the qux key, but the error also takes in consideration the undefined value, according to the following error:

FAIL  test/toEqual.test.ts > Object equality > obj2 should equal obj1
AssertionError: expected { foo: 'bar', extra: undefined, …(1) } to deeply equal { foo: 'bar', qux: 'qux' }

- Expected
+ Received

  Object {
+   "extra": undefined,
    "foo": "bar",
-   "qux": "qux",
+   "qux": "qux2",
  }

Which seems to be inconsistent, because if you remove the qux key, toEqual does not complain about the undefined.

Reproduction

Just use the following test:

import { describe, expect, test } from 'vitest'

const obj1 = {
  foo: 'bar',
  qux: 'qux'
}

const obj2 = {
  foo: 'bar',
  extra: undefined,
  qux: 'qux2'
}

describe('Object equality', () => {
  test('obj1 should equal obj2', () => {
    expect(obj1).toEqual(obj2)
  })
})

System Info

System:
  OS: macOS 15.2
  CPU: (8) arm64 Apple M1
  Memory: 184.66 MB / 16.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 20.15.0 - ~/.nvm/versions/node/v20.15.0/bin/node
  npm: 10.7.0 - ~/.nvm/versions/node/v20.15.0/bin/npm
  pnpm: 9.4.0 - ~/.nvm/versions/node/v20.15.0/bin/pnpm
Browsers:
  Brave Browser: 131.1.73.101
  Chrome: 102.0.5005.61
  Safari: 18.2
npmPackages:
  vitest: ^2.1.8 => 2.1.8

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• 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.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the reproduction in test/toEqual.test.ts and run the provided toEqual case. Trace the object-equality reporting used by that test; done means the mismatch still reports the differing qux value without including the extra undefined key, while preserving the existing behavior when no other key differs.