String to Union

In this medium-level challenge, you'll implement the StringToUnion<T> type, that turns a string into a union of its characters.

type Test = "123"
type Result = StringToUnion<Test> // expected to be "1" | "2" | "3"

// Video Solution: https://youtube.com/watch?v=E7fY3VIfvGU

/* _____________ Your Code Here _____________ */

type StringToUnion<T extends string> = any

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '../helpers'

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'
    >
  >,
]

Detailed Explanation

type StringToUnion<T extends string> = T extends `${infer First}${infer Rest}`
  ? First | StringToUnion<Rest>
  : never

How it works:

• First we constrain the type T to be a string.
• Then we use a template literal to check if the type is a string and use infer to capture the first letter and the rest of the string.
• If the type is a string, we return a union of the first letter and the rest of the string recursively passed into StringToUnion<Rest>, turning each letter into a member of the union.
• If the type is not a string, we return never.

This challenge helps you understand union type manipulation and how to apply this concept in real-world scenarios.