form.addEventListener('input', function (event) {
  const form = event.currentTarget;
  const artist = form.elements.artist;
  const song = form.elements.song;

  if (artist.value.length === 0 || song.value.length === 0) {
    addButton.setAttribute('disabled', true);
    addButton.classList.add('input__btn_disabled');
  } else {
    addButton.removeAttribute('disabled');
    addButton.classList.remove('input__btn_disabled');
  }
});