TS type-system hacks

Palindrome.ts

type str = "racecar"

// will require a-z
type Alphabet = 'r' | 'a' | 'c' | 'e'

type ReverseStr<Str extends string> = 
  Str extends '' 
    ? Str 
    : Str extends `${infer Chr}${infer Str}` 
      ? Chr extends Alphabet 
        ? `${ReverseStr<Str>}${Chr}`
        : never
      : never

type Palindrome<Str extends string> = Str extends ReverseStr<Str> ? string : never

type x = Palindrome<str>

Range.ts

type PrependNextNum<A extends Array<unknown>> =
  A['length'] extends infer T
    ? ((t: T, ...a: A) => void) extends ((...x: infer X) => void) ? X : never
    : never;

type EnumerateInternal<A extends Array<unknown>, N extends number> =
  N extends A['length'] ? A : EnumerateInternal<PrependNextNum<A>, N>;

export type Enumerate<N extends number> = EnumerateInternal<[], N> extends (infer E)[] ? E : never;

export type Range<FROM extends number, TO extends number> = Exclude<Enumerate<TO>, Enumerate<FROM>>;

type MyNum = Range<20, 30>;

const x: MyNum = 20;