JS to obtain age through ID card

function getAge(identityCard) {
    
    var len = (identityCard + "").length;
    if (len == 0) {
    
        return '';
    } else {
    
        if ((len != 15) && (len != 18)) // The ID card number can only be 15 Bit or 18 Others are illegal 
        {
    
            return '';
        }
    }
    var strBirthday = "";
    if (len == 18) // Handle 18 The ID number of bits gets the birthday and gender code from the number 
    {
    
        strBirthday = identityCard.substr(6, 4) + "/" + identityCard.substr(10, 2) + "/" + identityCard.substr(12, 2);
    }
    if (len == 15) {
    
        strBirthday = "19" + identityCard.substr(6, 2) + "/" + identityCard.substr(8, 2) + "/" + identityCard.substr(10, 2);
    }
    // In the time string , Must be “/”
    var birthDate = new Date(strBirthday);
    var nowDateTime = new Date();
    var age = nowDateTime.getFullYear() - birthDate.getFullYear();
    // Think about the month 、 The element of heaven ;.getMonth() What we get is from 0 At the beginning , Here's a comparison , No need to add 1
    if (nowDateTime.getMonth() < birthDate.getMonth() || (nowDateTime.getMonth() == birthDate.getMonth() && nowDateTime.getDate() < birthDate.getDate())) {
    
        age--;
    }
    return age;
}