Javascript function to select all checkboxes from gridview header checkbox

 function SelectheaderItemCheckbox(checkboxall) {
  
    if (checkboxall.checked) {
        var inputs = document.getElementById(dataGrid).getElementsByTagName('input');
     
        for (var i = 1; i < inputs.length; i++) {
            if (inputs[i].type == "checkbox") {
                inputs[i].checked = true;
            }
        }
    }
    else {
        var inputs = document.getElementById(dataGrid).getElementsByTagName('input');
        for (var i = 1; i < inputs.length; i++) {
            if (inputs[i].type == "checkbox") {
                inputs[i].checked = false;
            }
        }
    }

}
 function SelectchildCheckboxes(header) {

    var inputs = document.getElementById(dataGrid).getElementsByTagName('input');
    var countCheckboxItem = 0;
    var countCheckboxSelected = 0;
    for (var i = 1; i < inputs.length; i++) {
        if (inputs[i].type == "checkbox") {
            countCheckboxItem++;
            if (inputs[i].checked) {
                countCheckboxSelected++;
            }
        }
    }
    if (countCheckboxSelected == countCheckboxItem) {
        inputs[0].checked = true;
    }
    else {
        inputs[0].checked = false;
    }
}
<asp:GridView ID="GridViewItems" runat="server" AutoGenerateColumns="false" DataKeyNames="ItemId"
                    AllowPaging="true" PageSize="10" OnRowCommand="GridViewItems_RowCommand" OnPageIndexChanging="GridViewItems_PageIndexChanging">
                    <Columns>
                        <asp:TemplateField>
                            <HeaderTemplate>
                                <div class="TblChk">
                                    <asp:CheckBox ID="CheckBoxSelectAll" onclick="javascript:SelectheaderItemCheckbox(this)"
                                        class="FloatLeft" runat="server" />
                                </div>
                            </HeaderTemplate>
                            <ItemTemplate>
                                <asp:CheckBox ID="CheckBoxItem" onclick="javascript:SelectchildCheckboxes(this)"
                                    runat="server" />
                            </ItemTemplate>
                        </asp:TemplateField>
                                           </Columns>
                    <EmptyDataTemplate>
                        No Records Found....
                    </EmptyDataTemplate>
                </asp:GridView>