LessThan.ts

/*interface BarData {
  day1: number;
  day2: number;
  ...
  day30: number;
}*/


type LessThan<N extends number, A extends number[] = []> =
    N extends A['length'] ? A[number] : LessThan<N, [...A, A['length']]>;

type OneToThirty = Exclude<LessThan<31>, 0>;

interface BarData extends Record<`day${OneToThirty}`, number> { }

function foo(bar: BarData) {
    bar.day14 = 3;
    bar.day100 = 1
}

test

test