How TO get datagrid textbox value in javascript

Help please
when at run time i enter some value in txtA and Move to next control it gives run time error object required on the design of form on "on blur" event

 Design Code
<asp:DataGrid ID="datagrid1" runat="server" DataSourceID="SqlDataSource1">
            <Columns>
                 <asp:TemplateColumn >
                    <ItemTemplate>
                        <asp:TextBox ID="txtA" runat="server" ></asp:TextBox>
                    </ItemTemplate>
                </asp:TemplateColumn>
                <asp:TemplateColumn >
                     <ItemTemplate>
                        <asp:TextBox ID="txtB" runat="server""></asp:TextBox>
                    </ItemTemplate>
                </asp:TemplateColumn>
                <asp:TemplateColumn>
                    <ItemTemplate>
                        <asp:TextBox ID="txtC" runat="server" Text='<%# DataBinder.Eval(Container, "DataItem.QTY") %>'
                           ></asp:TextBox>
                    </ItemTemplate>
                                   </asp:TemplateColumn>
            </Columns>
        </asp:DataGrid>

VB.NET CODE Behind Code
Private Sub datagrid1_ItemDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.DataGridItemEventArgs) Handles datagrid1.ItemDataBound
If e.Item.ItemType = ListItemType.AlternatingItem Or e.Item.ItemType = ListItemType.Item Then
Dim tempQtyTbClientId As String
Dim tempAmmtTbClientId As String
tempQtyTbClientId = CType(e.Item.FindControl("txtProgress"), TextBox).ClientID.ToString
tempAmmtTbClientId = CType(e.Item.FindControl("txtCummVolume"), TextBox).ClientID.ToString
CType(e.Item.FindControl("txtProgress"), TextBox).Attributes.Add("onblur", "fnValidateAmount(this, " & tempQtyTbClientId & ");")

End If

End Sub


Javascript Code

function fnValidateAmount()
{
var argu= fnValidateAmount.arguments;
s = (document.forms(0).elements("txtA").value)
n = (document.forms(0).elements("txtC").value)
if ((document.forms(0).elements("txtA").value!= "") && (document.forms(0).elements("txtC").value!=""))
{
a = (eval(document.forms(0).elements("txtA").value)) + (eval(document.forms(0).elements("txtC").value))
document.forms(0).elements("txtB").value = a.toFixed(2);
(document.forms(0).elements("txtC").value) = n
}

if ((document.forms(0).elements("txtA").value == "") && (document.forms(0).elements("txtC").value!=""))
{
(document.forms(0).elements("txtB").value)=(document.forms(0).elements("txtC").value)
}

if ((document.forms(0).elements("txtA").value!="") && (document.forms(0).elements("txtC").value == ""))
{
(document.forms(0).elements("txtB").value) = (document.forms(0).elements("txtA").value)
}
return true
}