1. run npm install
2. run the test with vitest npm run vitest
3. run the test with jest npm run jest

Observe that the test fails with vitest but succeeds with jest.

Luxon DateTime instances are not instanceof DateTime when using vitest. Same test ran with Jest succeeds.

describe('LuxonUtils', () => {
    it('DateTime', () => {
        const utils = new LuxonUtils()
        const datetime = DateTime.now()
        expect(utils.date(datetime).isValid).toBeTruthy()
    });
})

I have located the problem to a small diffence between what TypeScript source of the date function in DateUtils is vs what their compiled source is. If I copy their source to my own project and let vite process that the test succeeds.

Their TypeScript:

if (value instanceof DateTime) {
    return value
}

Their JavaScript:

if (value instanceof luxon.DateTime) {
    return value
 }

When run through vitest, value is not a instance of luxon.DateTime and the code takes a wrong branch resulting in an invalid DateTime.

Is this problem a result of my vite/Typescript config, your bug, or a @date-io bug?

@date-io/luxon is an adapter library that standardizes multiple date libraries to the same API. It’s used by e.g. MUI. The bug doesn’t occur when using DateUtils in a React application in the browser.