Kebab case

The solution

type KebabCase<S extends string> =
  S extends `${infer H}${infer R}`
    ? R extends Uncapitalize<R>
      ? `${Lowercase<H>}${KebabCase<R>}`
      : `${Lowercase<H>}-${KebabCase<R>}`
    : S

The common wrong answer

type KebabCase<S extends string> = Lowercase<S>

`Lowercase` changes letters but forgets where the word boundaries were. Once every capital is gone there is no information left from which to insert the hyphens.

Line by line

1. R extends Uncapitalize<R>

   The suffix begins lowercase when uncapitalizing it changes nothing. If it changes, its first character marks a new word.

Takeaway

Case conversion must detect boundaries before normalising case; transformation destroys the evidence you need for parsing.