JS/javascript 만 나이계산

function calcAge(birth) {                 
    var date = new Date();
    var year = date.getFullYear();
    var month = (date.getMonth() + 1);
    var day = date.getDate();       
    if (month < 10){
    	month = '0' + month;
    }
    if (day < 10){
    	day = '0' + day;
    }
    var monthDay = month + day;
    birth = birth.replace('-', '').replace('-', '');
    var birthdayy = birth.substr(0, 4);
    var birthdaymd = birth.substr(4, 4);
    var age = monthDay < birthdaymd ? year - birthdayy - 1 : year - birthdayy;

    return age;

}