531-string-to-union.ts

/**
 * 531 - String to Union
 *
 * Implement the String to Union type. Type take string argument. The output should be a union of input letters
 *
 * For example
 *
 * ```ts
 * type Test = "123"
 * type Result = StringToUnion<Test> // expected to be "1" | "2" | "3"
 * ```
 */

/* _____________ Your Code Here _____________ */

type Split<S extends string> = S extends `${infer A}${infer B}`
  ? [A, ...Split<B>]
  : []

type StringToUnion<T extends string> = Split<T>[number];

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/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'>>,
]