Using javascript for checkall in gridview

script language="javascript" type="text/javascript"
function selectall(status)
{
var gridview=document.getElementById("gridview1");
for(var i=1;i
Columns>


asp:TemplateField>
HeaderTemplate>
asp:CheckBox runat="Server" ID="chkall" onclick="javascript:selectall(this.checked);" />
/HeaderTemplate>
ItemTemplate>
asp:CheckBox runat="server" ID="chk1" />
/ItemTemplate>
/asp:TemplateField>
asp:BoundField DataField="ename" HeaderText="empname" ReadOnly="true" />
/Columns>
/asp:GridView>