Testing `expect(null).toMatchObject(...)` against a Map or Set should fail

[eemeli]

Describe the bug

The following tests should fail, but do not:

import { expect, test } from 'vitest'

test.fails('null does not match map', () => {
  expect(null).toMatchObject(new Map([[1, 2]]));
})

test.fails('null does not match set', () => {
  expect(null).toMatchObject(new Set([[1]]));
})

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-y9khowyu?file=test%2Fbasic.test.ts

System Info

System:
    OS: macOS 15.7.3
    CPU: (14) arm64 Apple M4 Pro
    Memory: 6.79 GB / 48.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 25.2.1 - /opt/homebrew/bin/node
    npm: 11.7.0 - /opt/homebrew/bin/npm
  Browsers:
    Firefox: 146.0
    Firefox Nightly: 149.0a1
    Safari: 18.6
  npmPackages:
    @vitejs/plugin-react: ^5.1.1 => 5.1.1
    @vitest/coverage-v8: ^4.0.14 => 4.0.14
    vitest: ^4.0.9 => 4.0.14

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.

[hi-ogawa]

In terms of "object shape", Map and Set is equivalent to empty object {}. Then if empty object on the right, toMatchObject({}) is essentially no-op, so that's what's happening:

https://stackblitz.com/edit/vitest-dev-vitest-3jseghq6?file=test%2Fbasic.test.ts

import { expect, test } from 'vitest';

test('passes', () => {
  expect(null).toMatchObject({});
  expect(undefined).toMatchObject({});
  expect(123).toMatchObject({});
  expect('hello').toMatchObject({});
  expect({ a: 1 }).toMatchObject({});

  expect(null).toMatchObject(new Map([[1, 2]]));
  expect(undefined).toMatchObject(new Map([[1, 2]]));
  expect(123).toMatchObject(new Map([[1, 2]]));
  expect('hello').toMatchObject(new Map([[1, 2]]));
  expect({ a: 1 }).toMatchObject(new Map([[1, 2]]));
});

@eemeli Can you explain actual test case and what you intend to assert?

(I'd feel all the primitives on the left being no error is slightly over permissive, but not sure the consequence of changing it now.)

[eemeli]

The specific test where this showed up for us is here, where we have a Map as a value we're expecting to see as an object property value. The intent here is to effectively do what the docs say .toMatchObject() does, i.e. assert that "an object matches a subset of the properties of an object."

With Jest, .toMatchObject() with Map and Set values effectively does the same as .toEqual(), so our expectation was that the same would apply to Vitest as well.

[hi-ogawa]

Good to know the mismatch from Jest behavior. Though technically "an object matches a subset of the properties of an object." means it's doing a job since Map/Set elements are not "properties" in javascript object sense, we can check Jest's behavior closely and discuss whether alignment is worth it.

[hi-ogawa]

Indeed Jest seems more strict, which I think is a better behavior from a quick look. I created an example here https://stackblitz.com/edit/github-hn1e8nse?file=repro.test.js

(still not sure how systematically Map/Set like equality is supposed to work though)

[hi-ogawa]

It's not just null/undefined/number/string primitives, but Set/Map comparison prototype seems ignored.

test('repro3', () => {
  // pass on vitest
  // fail on jest
  expect({}).toMatchObject(new Set());

  // both pass
  expect(new Set()).toMatchObject({});
});

This feels unexpected/bug since Vitest does support deep shape comparison inside Set/Map:

// These cases have the same behaviors between Vitest and Jest

test('repro-set', () => {
  // pass
  expect(new Set([{ x: "y" }])).toMatchObject(new Set([{}]));

  // fail
  // expect(new Set([{ x: "y" }])).toMatchObject(new Set([]));
  // expect(new Set([{}])).toMatchObject(new Set([{ x: "y" }]));
  // expect(new Set([])).toMatchObject(new Set([{ x: "y" }]));
  // expect(new Set([{}])).toEqual(new Set([{ x: "y" }]));
  // expect(new Set([{ x: "y" }])).toEqual(new Set([{}]));
});