asp.net中button控制先执行js再执行后台程序的方法

if (!IsPostBack)
{
 //给button1添加客户端事件
 btnSave.Attributes.Add("OnClick", "return UserAddVerify()");
}

function UserAddVerify() {
    var userName = document.getElementById("TxtUserName").value;
    var password = document.getElementById("TxtUserPassword").value;
    var repassword = document.getElementById("TxtUserPasswordConfirm").value;
    var identity = document.getElementById("TxtUserIdentity").value;
    var mobile = document.getElementById("TxtUserMobile").value;
    var realName = document.getElementById("TxtUserRealName").value;
    var btnSave = document.getElementById("btnSave");
    var identityReg = /(^\d{15}$)|(^\d{18}$)|(^\d{17}(\d|X|x)$)/;
    var mobileReg = /1[3-8]+\d{9}/;
    if (userName == "" || userName == null) {
        alert("用户名不能为空");
        return false;
    }
    else if (password == "" || password == null) {
        alert("密码不能为空");
        return false;
    }
    else if (repassword == "" || repassword == null || repassword != password) {
        alert("对不起，两次输入密码不一样");
        return false;
    }
    else if (identity == "" || identity == null || identityReg.test(identity) === false) {
        alert("请输入合法的身份证号码");
        return false;
    }
    else if (mobile == "" || mobile == null || mobileReg.test(mobile) == false) {
        alert("请输入合法的手机号码");
        return false;
    }
    else if (realName == "" || realName == null) {
        alert("姓名不能为空");
        return false;
    }
    return true;
}