js驗證asp.net控制項

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="WebUserControl.ascx.cs" Inherits="WebUserControl" %>

  <script   language="javascript"   type="text/javascript">  
  function   check()  
  {  
          var   id   =   '<%=this.txt123.ClientID%>';  
          var   obj   =   document.getElementById(id);  
          if(obj.value=="")
          {
            alert('不可為空')
          } 
          else
          {
            alert(obj.value)
          }
  }  
  </script>
  <asp:TextBox ID="txt123" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="return  check();"/>