No enumerable properties on PlainDate breaks testing

[andrewbihl]

Using Temporal.PlainDate fails in tests like vitest/jest, where you'll see code like

expect(Temporal.PlainDate.from('2000-01-01')).toEqual(Temporal.PlainDate.from('2010-03-05')

pass tests because there are not enumerable properties on PlainDate. As a consequence, the deep equality implementation interprets both objects simply as {}.

This is a concerning footgun.

[justingrant]

Do you know how jest or vitest handles other JS language built-ins like Date that also don't have any own properties?

expect(new Date('2000-01-01')).toEqual(new Date('2010-03-05'))

Once Temporal's native implementation is completed in V8 (it's actively being developed now) then jest and vitest can use the native implementation, not a polyfill.

[12wrigja]

Just chiming in here to say that we ran into this exact problem within Google, and we "solved" it by:

• locally patching Jasmine to never consider two Temporal objects equal (determined by looking at the Symbol.toStringTag value to see if it contained 'Temporal.'),
• creating a set of Jasmine custom matchers that use either equals or compare to do the comparison.

I agree generally with Justin though that this happens with other types in JS, and that Temporal is not unique in this regard.

I'd expect that once Temporal lands fully that test frameworks would update to include similar logic as for Date.

[fabon-f]

Note that now Vitest has built-in support for Temporal classes in toEqual. It works no matter where Temporal implementation comes from (polyfill or native implementation).

vitest-dev/vitest#8007

[dwjohnston]

Just to be clear, so you don't waste the several hours that I just did. If you're upgrading vitest to 3.2.0 or later, and your code base contains tests like this:

  it("this test was passing", () => {
    const today = Temporal.Now.plainDateISO();
    const tomorrow = today.add({ days: 1 });
    expect(today).toEqual(tomorrow);
  });

These tests will now fail. They should have never been passing in the first place.

Quick reproduction here: https://github.com/dwjohnston/js-temporal-polyfill-false-postive