Select All in GridView is not working

I have a .net Gridview in which "Select All" checkbox has been added to select all the rows available in grid. Below is my Grid code

<asp:TemplateColumn>
  <HeaderTemplate>
  <input id="chkAll" type="checkbox" onclick="CheckAllDataGridCheckBoxes('chkItem',this.checked)">
   </HeaderTemplate>
   <ItemTemplate>
   <asp:CheckBox ID="chkItem" runat="server"></asp:CheckBox>
   </ItemTemplate>

and here is my javascript code

<script type="text/javascript">
    function CheckAllDataGridCheckBoxes(aspCheckBoxID, checkVal)
    {
        re = new RegExp(':' + aspCheckBoxID + '$')  //generated control name starts with a colon
        for(i = 0; i < Form1.elements.length; i++)
        {
            elm = document.forms[0].elements[i]
            if (elm.type == 'checkbox')
            {
                if (re.test(elm.name))
                    elm.checked = checkVal
            }
        }
    }
</script>

Now, the problem is that i am getting error at line for(i = 0; i < Form1.elements.length; i++) causing my rows remains unselected. What's wrong here?