Messy code with comments

birthday.js

// Returns number of remaining days to birthday
function birthday(inputDate) {
  const today = new Date();
  const birthday = new Date(inputDate);

  const upcomingBday = new Date(today.getFullYear(), birthday.getMonth(), birthday.getDate());

  // if birthday has already passed, set next year as birthday
  if (today > upcomingBday) {
    upcomingBday.setFullYear(today.getFullYear() + 1);
  }

  // getTime() returns in milliseconds so we divide the result in milliseconds to get number of days
  const result = Math.ceil((upcomingBday.getTime() - today.getTime()) / (24 * 60 * 60 * 1000));

  return result;
}