JS在onclientclick里如何控制onclick的执行

<asp:Button ID="btn_Save" runat="server" Text="保存" CssClass="button_bak" OnClientClick="return whetherEmpty();" OnClick="btn_Save_Click" />
//JavaScript实现如下：
<script language="javascript" type="text/javascript">
function whetherEmpty()
{
//alert("KPI信息不能全为空！");
emptflag = false ;
var kpiName = document.getElementById("<%=txt_KPIName.ClientID%>").value;
var jobgoal = document.getElementById("<%=txt_JobGoal.ClientID%>").value;
var weight = document.getElementById("<%=txt_Weight.ClientID%>").value;
var standard = document.getElementById("<%=txt_Standard.ClientID%>").value;
if (kpiName == "" && jobgoal == "" && weight =="" && standard =="")
{
emptflag = true ;
}
if (emptflag)
{
alert("KPI信息不能全为空！");
return false;
}
}
</script>

<script language="javascript" type="text/javascript">
function buttonClick() {
alert("我是客户端点击事件");
return false; 
}
</script> 

<form id="form1" runat="server">
<div>
<asp:Button ID="Button1" runat="server" Text="些Button是服务端控件，默认使用OnClientClick事件，请禁用浏览器的Js功能体验"
OnClick="Button1_Click" OnClientClick="return buttonClick();" />
<asp:Label ID="Label1" runat="server" Text=""></asp:Label>
</div>
</form> 

protected void Button1_Click(object sender, EventArgs e)
{
this.Label1.Text = "我是服务端点击事件";
}