static prime checking using typescript types

check-prime.ts

// Prime checking regex, which this code is based off of
// https://regex101.com/r/RIJkGF/1

type StringDivisible<n extends string, div extends string> = n extends `` ? true : n extends `${div}${infer r}` ? StringDivisible<r, div> : false;

type Substrings<n extends string, d extends string = ""> = n extends `${d}${infer r}` ? readonly [` ${d}`, ...Substrings<r, ` ${d}`>] : readonly [];
type StringFactors<n extends string> = Substrings<n> extends readonly [unknown, ...infer R extends readonly string[], unknown] ? R : never;
type StringIsPrime<n extends string, Factors extends readonly string[] = StringFactors<n>> = Factors extends readonly [infer s extends string, ...infer R extends readonly string[]] ? StringDivisible<n, s> extends true ? false : StringIsPrime<n, R> : true;

type RepeatStringByStringDigit<str extends string, d extends string> = 
  d extends `0` ? ""
  : d extends `1` ? `${str}`
  : d extends `2` ? `${str}${str}`
  : d extends `3` ? `${str}${str}${str}`
  : d extends `4` ? `${str}${str}${str}${str}`
  : d extends `5` ? `${str}${str}${str}${str}${str}`
  : d extends `6` ? `${str}${str}${str}${str}${str}${str}`
  : d extends `7` ? `${str}${str}${str}${str}${str}${str}${str}`
  : d extends `8` ? `${str}${str}${str}${str}${str}${str}${str}${str}`
  : d extends `9` ? `${str}${str}${str}${str}${str}${str}${str}${str}${str}`
  : string;

type _mul10<str extends string> = `${str}${str}${str}${str}${str}${str}${str}${str}${str}${str}`;

type digits = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9";

type pow10fromstr<s extends string> = s extends `${infer _}${infer r}` ? _mul10<pow10fromstr<r>> : ' '

type _NumberStrConversion<n extends string> = n extends `${infer i}${infer trail}` ? `${RepeatStringByStringDigit<pow10fromstr<trail>, i>}${_NumberStrConversion<trail>}` : ``

type NumberToStringOfLength<n extends number> = _NumberStrConversion<`${n}`>;

type NumberIsPrime<n extends number> = StringIsPrime<NumberToStringOfLength<n>>;

type answer = NumberIsPrime<42>; // false
//   ^?
type answer2 = NumberIsPrime<607>; // true
//   ^?

nice, but is there any practical use?

no