Date mocking does not work in V9

[melkras]

Hello
I am unable to mock dates with MockDate after migrating to V9.
This is necessary for testing purposes. I understand that this functionality can be replaced using the defaultMonth prop, but I wanted to inform you just in case.

[gpbl]

@melkras some help reproducing your issue with MockDate? Why are you unable to use it?

[melkras]

When rendering <DayPicker /> in the testing environment and setting a mocked date with MockDate.set('2024-07-01'), the mocked date wont affect DayPicker's currently visible month as it used to in V8.
When mocking it should render the currently visible month of July 2024, instead it renders September 2024.
So I'm assuming that for some reason the DayPicker's internal Date.now() cannot be overriden like this anymore.

[leonaves]

Hey, I also ran into this problem after updating and I think I've found the cause. Because of how the new custom dateLib functionality works, what's happening is that RDP defaults to using the system Date function here:

react-day-picker/src/lib/dateLib.ts

Line 54 in b5e554a

Date: Date as GenericDateConstructor<Date>,

And then that object reference is captured within this closure:

https://github.com/gpbl/react-day-picker/blob/9c45c9f698eea439cc722a10413f95bf41d43dfd/src/helpers/getDateLib.ts

Meaning that later when trying to work out "today", because RDP uses new datelib.Date() here:

react-day-picker/src/helpers/getInitialMonth.ts

Line 24 in b5e554a

today = props.timeZone ? TZDate.tz(props.timeZone) : new dateLib.Date(),

It uses that original Date reference. This will break any mocking library that replaces the global Date object after everything has been imported, which is pretty much how every library that does date mocking works. A workaround is to supply a dateLib property of:

<ReactDayPicker dateLib={{ Date }} />

When rendering.

[gpbl]

Good find @leonaves! A straight solution there could be to update the line above:

'dateLib' in props ? new dateLib.Date() : new Date(), 

Anyway, I could mock dates in Jest: https://github.com/gpbl/react-day-picker/blob/main/examples/Start.test.tsx#L14-L17

[gpbl]

Mocking date should be fixed in v9.1.2 - please write back if you are still encountering issues mocking the dates, thanks!