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.

Medium

Implement the String to Union type. Type take string argument. The output should be a union of input letters

For example

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

Change the following code to make the test cases pass (no type check errors).

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Video Walkthrough

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.