Maximum Number of Vowels in a Substring of Given Length

export function maxVowels(s: string, k: number): number {
  const isVowel = (char: string) => /^[aeiou]$/.test(char);
 
  let answer = 0;
  let vowelCount = 0;
  for (let i = 0; i < s.length; i++) {
    if (k <= i && isVowel(s[i - k])) {
      vowelCount -= 1;
    }
    if (isVowel(s[i])) {
      vowelCount += 1;
    }
    answer = Math.max(answer, vowelCount);
  }
 
  return answer;
}