Grid row command is not waiting for javascript's alert response

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VB

ASP.NET

Javascript

Hi all,

In my grid view row command i want to delete the row with confirmation alert box(javascript alert ), upto alert box is working fine , but the problem is , The Row command is fired immediately that is not waiting for alert box
grid control
------------

<asp:TemplateField ItemStyle-Width="25px" ItemStyle-Height="15px">
                                   <ItemTemplate>
                                      
                                             <asp:ImageButton ID="lnkswi_del" runat="server" CommandName="Delete" ToolTip="Delete"
                                           CommandArgument="<%#CType(Container,GridViewRow).RowIndex%>" ImageUrl="~/Images/delete.png" CssClass="test" OnClientClick="return Confirmdelete();" />
                                   </ItemTemplate>
                               </asp:TemplateField>

javascript
-----------

function ConfirmDelete() {
            alertify.set({ buttonReverse: true });
            alertify.confirm('Are you sure you want to delete the record?', function (e) {
                if (e) {                 
                   
                    return true;
                } else {
                    return false;
                }
            });
        }

code behind
-----------

Protected Sub gridTran_RowCommand(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewCommandEventArgs) Handles gridTran.RowCommand

            If e.CommandName = "Delete" Then
                Dim index As Integer = Convert.ToInt32(e.CommandArgument)
                Dim gvRow As GridViewRow = gridTran.Rows(index)
                trxnno = gvRow.Cells(1).Text 
                deletemethod() // this method will delete the row
            End If

after click the alert box ok only the row command event should fire , but its not

help me resolve this

and also , i did it through rowdatabound event, ther also the same problem is occuring

Protected Sub gridTran_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles gridTran.RowDataBound
        If (e.Row.RowType = DataControlRowType.DataRow) Then
            Dim theDeleteButton As ImageButton = CType(e.Row.Cells(0).FindControl("lnkswi_del"), ImageButton)
            theDeleteButton.OnClientClick = "return ConfirmDelete();"
        End If
    End Sub

thanks in advance
Velsamy