Typescript Numeric Range Type

So you want a numeric range type, for example, from 30–40

the usual way is

type Thirty_to_Forty = 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37| 38 | 39 |40

it is doable but 0 fun in it (for obvious reasons)

what if you want a meta solution, from any range to any range, is it even possible?

It is, here is how you do it

type CreateArrayWithLengthX<
    LENGTH extends number,
    ACC extends unknown[] = [],
> = ACC['length'] extends LENGTH
    ? ACC
    : CreateArrayWithLengthX<LENGTH, [...ACC,1]>

type NumericRange<
   START_ARR extends number[], 
   END extends number, 
   ACC extends number=never>
=START_ARR['length'] extends END 
   ? ACC | END
   : NumericRange<[...START_ARR,1], END, ACC | START_ARR['length']>

type TWENTY_TO_FORTY = NumericRange<CreateArrayWithLengthX<20>,40>

Playground

keep in mind that there is a limit to how deep Typescript can recurse, and that number is 1000

Typescript will complain if your starting number is 1000 or your range size is 1000.