Дарья

const vowelCount = str => str.replace(/[^aeiouy]/gi, '').length;

const vowelCount = str => ~-str.split(/[aeiouy]/i).length;

const vowelCount = str =>
  Array.prototype.reduce.call(
    str.toLowerCase(),
    (acc, n) => acc + 'aeiouy'.includes(n),
    0
  );