toUpperCase.ts

/**
 * Enhanced `toUpperCase` function with improved type inference.
 *
 * Traditional methods return a general `string` type. 
 * This version gives a specific uppercase type based on the input.
 * 
 * E.g., for an input type 'hello', this returns type 'HELLO'.
 */

type ToUpperCase<S extends string> = S extends `${infer First}${infer Rest}`
  ? `${Uppercase<First>}${ToUpperCase<Rest>}`
  : S;

export function toUpperCase<S extends string>(str: S): ToUpperCase<S> {
  return str.toUpperCase() as ToUpperCase<S>;
}