誕生日から、ある日付までの年齢計算 JavaScript

■ユーザーインターフェイス

■ソースコード

js main.js

    var birthYear = parseInt(document.getElementById('dead_birth_year').value);
    var birthMonth = parseInt(document.getElementById('dead_birth_month').value);
    var birthDay = parseInt(document.getElementById('dead_birth_day').value);

   
    var birthYearType = parseInt(document.getElementById('dead_birth_koyomi_type').value);
  
    if (birthYearType === -1) {
       
    } else if (birthYearType === 4) {
        birthYear += 2018;
    } else if (birthYearType === 3) { 
        birthYear += 1988;
    } else if (birthYearType === 2) {
        birthYear += 1925;
    } else if (birthYearType === 1) { 
        birthYear += 1911;
    } else if (birthYearType === 0) { 
        birthYear += 1868;
    }

    console.log("birthYear:::" + birthYear);

    
    var deadYear = parseInt(document.getElementById('dead_year').value);
    var deadMonth = parseInt(document.getElementById('dead_month').value);
    var deadDay = parseInt(document.getElementById('dead_day').value);

    
    var deadYearType = parseInt(document.getElementById('dead_koyomi_type').value);
    
    if (deadYearType === -1) { 
        
    } else if (deadYearType === 4) { 
        deadYear += 2018;
    } else if (deadYearType === 3) { 
        deadYear += 1988;
    } else if (deadYearType === 2) { 
        deadYear += 1925;
    } else if (deadYearType === 1) {
        deadYear += 1911;
    } else if (deadYearType === 0) {
        deadYear += 1868;
    }

    console.log("deadYear:::" + deadYear);
            
    
    var ageAtDeath = deadYear - birthYear;
    if ((deadMonth < birthMonth) || (deadMonth === birthMonth && deadDay < birthDay)) {
        ageAtDeath -= 1;
    }

    var resultTextBox = document.getElementById('Age_at_Death');

    if (!isNaN(ageAtDeath)) {
        var numericValue = Number(ageAtDeath);

        if (numericValue >= 0) {
            resultTextBox.value = numericValue;   
        }   
    } else {
   
    }
}
  
        document.getElementById('dead_birth_koyomi_type').addEventListener('change', calculateAge);
        document.getElementById('dead_birth_year').addEventListener('change', calculateAge);
        document.getElementById('dead_birth_month').addEventListener('change', calculateAge);
        document.getElementById('dead_birth_day').addEventListener('change', calculateAge);

        document.getElementById('dead_koyomi_type').addEventListener('change', calculateAge);
        document.getElementById('dead_year').addEventListener('change', calculateAge);
        document.getElementById('dead_month').addEventListener('change', calculateAge);
        document.getElementById('dead_day').addEventListener('change', calculateAge);