Greatest English Letter in Upper and Lower Case

export function greatestLetter(s: string): string {
  const set = new Set(s);
 
  const [start, end] = ['A'.charCodeAt(0), 'Z'.charCodeAt(0)];
  for (let charCode = end; start <= charCode; charCode--) {
    const char = String.fromCharCode(charCode);
    if (set.has(char) && set.has(char.toLowerCase())) {
      return char;
    }
  }
  return '';
}