00531-medium-string-to-union.ts

// ============= Test Cases =============
import type { Equal, Expect } from './test-utils'

type cases = [
  Expect<Equal<StringToUnion<''>, never>>,
  Expect<Equal<StringToUnion<'t'>, 't'>>,
  Expect<Equal<StringToUnion<'hello'>, 'h' | 'e' | 'l' | 'l' | 'o'>>,
  Expect<Equal<StringToUnion<'coronavirus'>, 'c' | 'o' | 'r' | 'o' | 'n' | 'a' | 'v' | 'i' | 'r' | 'u' | 's'>>,
]


// ============= Your Code Here =============
type StringToUnion<T extends string> = T extends ''
? never
: T extends `${infer Char}${infer Rest}` 
  ? Char | StringToUnion<Rest> 
  : T