Parse url path into Typescript types

GetColonKeysFromUrlPath.ts

/**
 * Extracts url wildcards formatted as a string beginning with colon.
 * ```
 * 'user/:userId/profile' -> 'userId'
 * ```
 *
 * Multiple instances yields a union type:
 * ```
 * 'user/:userId/contacts/:listId' -> 'userId' | 'listId'
 * ```
 */
type GetColonKeysFromUrlPath<T> = T extends `${string}:${infer InitialKey}`
  ? InitialKey extends `${infer Key}/${infer Rest}`
    ? Key | GetColonKeysFromUrlPath<Rest>
    : InitialKey
  : never;