Select all CheckBox in GridView ASP.Net

<script language="javascript" type="text/javascript">
function SelectAllCheckboxes(Chk)
{

var oItem = Chk.children;
var theBox= (Chk.type=="checkbox") ? Chk : Chk.children.item[0]; BoxState=theBox.checked;
elment=theBox.form.elements;

for(i=0;i<elm.length;i++)
if(elment[i].type=="checkbox" &&
elment[i].id!=theBox.id)
{
if(elment[i].checked!=BoxState)
elment[i].click();
}
}

<asp:TemplateField HeaderText="CheckAll">
<HeaderTemplate>

<input id="HeaderLevelCheckBox" runat="server" onclick="javascript:SelectAllCheckboxes(this);" type="checkbox" />
</HeaderTemplate>
<HeaderStyle HorizontalAlign="Left" />
<ItemTemplate>
<asp:CheckBox ID="RowLevelCheckBox" runat="server" />
</ItemTemplate>
</asp:TemplateField>