Try this..please search for your question first..
function CalculateSum()
{
var Form = document.frmsum;
var txtbx1= Form.txt1.value;
var txtbx2 = Form.txt2.value;
var allsum = Number(txtbx1)+Number(txtbx2);
if(!isNaN(allsum) && allsum !=Infinity)
{
Form.txtsum.value = allsum;
}
return CheckZero()
}
on page load
txtsum.Attributes.Add("onblur", "return CalculateSum()");