fp-like isPast, isToday, etc implementation

[willisplummer]

I noticed that the fp library doesn't expose any functions that use Date.now() internally.. What do you think about adding versions of these fn's that allows for injection of now (in general, this makes testing so much easier)

e.g.

export const isPast = (date: Date, now: Date = new Date()): boolean => (date.getTime() - now.getTime) < 0

consider the fn foo:

import { isPast } from 'date-fns';

export const foo = (date: Date): string => isPast(date) ? 'past' : 'future';

currently, we have to mock out isPast in order to test foo (or use arbitrarily futuristic and ancient dates)

if we have the ability to inject now into the fn for testing, we can simply update foo to also take an optional now:

export const foo = (date: Date, now: Date = new Date()): string => isPast(date, now) ? 'past' : 'future';

and then hardcode the current time in our test

expect(foo(new Date(100), newDate(0))).toBe('future')
expect(foo(new Date(100), newDate(200))).toBe('past')

i find this is a really great pattern for working around the concept of now with date functions and would love to see it supported by the lib, but i get that it represents a bit of a paradigm shift.. any way figured i'd suggest it. eager to hear your thoughts!

[kossnocorp]

Hey! We have such a function. It's isSameDay 😉 I think using isPast(now, date) in FP code is just confusing. Unless you have better reasons, I would like to keep it as is and recommend use isSameDay (and other FP-friendly alternatives).