Implement iterable/valueOf for decoupling from libraries

[andytson]

A dependent library could use this knowing only that expression: Iterable<{ valueOf(): number }> (where multiple date libraries support valueOf() already (including JS Date and luxon DateTime).

Fixes #82

[andytson]

An example I'm thinking of is a fork of node-scheduler that just implements one-off or iterable dates, so it doesn't need to bundle any specific cron-parser (and therefore date library), instead the user chooses this cron-parser and uses it like:

import { CronExpression } from 'cron-parser'
import { scheduleJob } from 'scheduler-fork'

scheduleJob(CronExpression.parse('*/20 * * * *'), () => ...)

i.e. I definitely want to avoid the moment-timezone bundle size/EOL but although my preference for change is Luxon, if instead DayJs is used, I can maintain a Luxon fork to avoid needing to bundle two date libraries in my project (and avoid DayJs timezone data).

[andytson]

tbh, not having this isn't really a blocker to my idea. I'd just need an adapter to map the expression to the format my hypothetical node-scheduler fork would support without making it a hard dependency. The valueOf functionality would still be useful though as is standard in JS Date/some other date libraries

[NGPixel]

@harrisiirak Any update on merging this PR? Not being able to use for...of with the iterator flag is a bit annoying right now.

[harrisiirak]

Prefer single quotes over double quotes.

[andytson]

yep fair enough. I'm just used to eslint formatting on save :)

[harrisiirak]

Hi @NGPixel!

Thanks for reminding me about this! @andytson are you able to sync this PR branch with master branch to resolve conflict in the typings file? If not, I can create separate PR from your branch and merge it from there.

[andytson]

master has gone quite in a bit of a different direction since, at least with the CronExpression type, so may be tricky to sort out

[andytson]

i.e I don't know how to integrate:

implements Iterable<CronDate> 

with

export type CronExpression<IsIterable extends boolean = false> = ICronExpression<CronFields, IsIterable>

so if you knew and could sort it out, that's fine.

[harrisiirak]

@andytson can't we just do:

export interface ICronExpression<CronFields, IsIterable extends boolean> extends Iterable<CronDate> {

Tested it locally and it seems to be working fine in this way.