// Calculate the number of vowels in a string
const countVowels = string => {
  const vowels = ['a', 'e', 'i', 'o', 'u'];
  let count = 0;

  string
    .toLowerCase()
    .split('')
    .forEach(char => {
      vowels.forEach(vowel => {
        if (char === vowel) count++;
      });
    });

  return count;
};

console.log(countVowels('queue')); // 4