window.addEventListener('DOMContentLoaded', (event) => {
  //任意の生年月日フィールドと年齢フィールドのname値を指定
  //サンプルでは生年月日＝birthday、年齢＝ageとしております。
  var birthdayYearName = 'birthday:year';
  var birthdayMonthName = 'birthday:month';
  var birthdayDayName = 'birthday:day';
  var ageName = 'age'
  
  var yearInput = document.getElementsByName(birthdayYearName)[0];
  var monthInput = document.getElementsByName(birthdayMonthName)[0];
  var dayInput = document.getElementsByName(birthdayDayName)[0];
  var ageInput = document.getElementsByName(ageName)[0];
  
  yearInput.addEventListener('change', setAge);
  monthInput.addEventListener('change', setAge);
  dayInput.addEventListener('change', setAge);
  
  function setAge() {
    var year = yearInput.value;
    var month = monthInput.value;
    var day = dayInput.value;
    if(!year || !month || !day) {
      return;
    }
    var birthday = new Date(year, month - 1, day);
    var today = new Date();
    var age = today.getFullYear() - birthday.getFullYear();
    var m = today.getMonth() - birthday.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthday.getDate())) {
        age--;
    }
    
    ageInput.value = age;
  }
}); 
            

window.addEventListener('DOMContentLoaded', (event) => {
  // 任意の生年月日フィールドと年齢フィールドのname値を指定
  // サンプルでは生年月日＝birthday、年齢＝ageとしております。
  var birthdayInputName = 'birthday';
  var ageName = 'age'
  
  var birthdayInput = document.getElementsByName(birthdayInputName)[0];
  var ageInput = document.getElementsByName(ageName)[0];
  
  birthdayInput.addEventListener('change', setAge);
  
  function setAge() {
    var birthdayString = birthdayInput.value;
    if (!birthdayString) {
      return;
    }
    
    // ハイフンまたはスラッシュで分割
    var delimiter = birthdayString.includes('-') ? '-' : '/';
    var birthdayParts = birthdayString.split(delimiter);
    var year = parseInt(birthdayParts[0], 10);
    var month = parseInt(birthdayParts[1], 10);
    var day = parseInt(birthdayParts[2], 10);
    
    var birthday = new Date(year, month - 1, day);
    var today = new Date();
    var age = today.getFullYear() - birthday.getFullYear();
    var m = today.getMonth() - birthday.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthday.getDate())) {
        age--;
    }
    
    ageInput.value = age;
  }
});