8640 - Number Range

[Kakeru-Miyazaki]

// your answers
type UnionFromNumber<L extends number, Ans extends unknown[] = []> =  Ans["length"] extends L ? Ans[number] : UnionFromNumber<L, [...Ans, Ans["length"]]>

type NumberRange<L extends number, H extends number> = Exclude<UnionFromNumber<H>, UnionFromNumber<L>> | H

UnionFromNumber<L> generates a union type of 0 | 1 | ... | L-1. Therefore, using Exclude<UnionFromNumber<H>, UnionFromNumber<L>> yields a type of H | H + 1 | ... | L-2 | L-1. To include L in the type, we simply append L at the end.