Proposal: IndexesOf and ItemsOf

[lucasteles]

This is a way to have typed indexes of a tuple, useful when you have types deriving of a const value

definition:

type ItemOf<T extends readonly any[]> = T[number]
type IndexesOf<T extends readonly any[]> = Exclude<Partial<T>['length'], T['length']>

use case:

const WEEK_DAYS = [
  'Monday', 'Tuesday', 'Wednesday',
  'Thursday', 'Friday', 'Saturday',
  'Sunday' ] as const

type WeekDayName = ItemsOf<typeof WEEK_DAYS>
type WeekDay = IndexesOf<typeof WEEK_DAYS>

const getDayName = (day: WeekDay): WeekDayName => WEEK_DAYS[day]

[sindresorhus]

Related: #658

I'm not sure which names are best ItemsOf or ArrayValue.

[lucasteles]

@sindresorhus ArrayValue and ArrayIndex looks good