js实现实时计算结果

<script type="text/javascript">
function cal(ida,idb,idc)
{
var aa=Number(document.getElementById(ida).value);
var bb=Number(document.getElementById(idb).value);
var cc=aa*bb;
document.getElementById(idc).value=cc;
}
</script>

<asp:TextBox ID="TextBox3" runat="server" onkeyup="cal('TextBox3','TextBox4','TextBox5')" ></asp:TextBox>
<asp:TextBox ID="TextBox4" runat="server" onkeyup="cal('TextBox3','TextBox4','TextBox5')" ></asp:TextBox>
<asp:TextBox ID="TextBox5" runat="server"></asp:TextBox>