js给texbox补0

<script language="javascript">
function test(txt)
{
 if(txt.value.length==0)
 txt.value = "00";
 if(txt.value.length == 1)
 txt.value = "0" + txt.value;
 
 var num = new Number(txt.value);
 
 if(isNaN(num))
 {
 alert("请输入数字");
 txt.value="";
 }
}
 </script>

===================
private void Page_Load(object sender, System.EventArgs e)
{
 this.TextBox1.Attributes.Add("onblur","test(this);");
}