Javascript Date getAge()

Date.prototype.getAge = function() {
    var today = new Date();
    var birthDate = this;
    var age = today.getFullYear() - birthDate.getFullYear();
    var m = today.getMonth() - birthDate.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
        age--;//from   ww w . j av a  2  s.  c o m
    }
    return age;
}