check为什么会出错（IE）

check.prototype={
pattern_user_name:/[a-z]+/,
checkUserName:function(){
var user_name=document.getElementById("username");
if(this.pattern_user_name.test(user_name.value)){
window.alert("user name does not match\n");
}
                else{
                alert("pp");
                }
}, //这个逗号有啥用啊，去掉试试
}