CheckRepeatedChars

In this medium-level challenge, you'll implement CheckRepeatedChars<S> which checks whether a string type contains any duplicated characters, returning true or false.

type CheckRepeatedChars<'abc'>   // false
type CheckRepeatedChars<'aba'>   // true

type cases = [
  Expect<Equal<CheckRepeatedChars<'abc'>, false>>,
  Expect<Equal<CheckRepeatedChars<'abb'>, true>>,
  Expect<Equal<CheckRepeatedChars<'cbc'>, true>>,
  Expect<Equal<CheckRepeatedChars<''>, false>>,
]

Detailed Explanation

type CheckRepeatedChars<T extends string> =
  T extends `${infer First}${infer Rest}`
    ? Rest extends `${string}${First}${string}`
      ? true
      : CheckRepeatedChars<Rest>
    : false

How it works:

• T extends `${infer First}${infer Rest}` splits the string into its first character First and the remaining string Rest
• Rest extends `${string}${First}${string}` checks whether First appears anywhere in the remaining string Rest by matching it with arbitrary strings on both sides
• If the character is found in the rest of the string, the type returns true since a duplicate exists
• If the character is not found, the type recurses with CheckRepeatedChars<Rest> to check subsequent characters
• The base case (empty string) returns false, meaning no duplicates were found

This challenge helps you understand template literal type pattern matching and recursive string analysis, and how to apply these concepts in real-world scenarios.