Validating checkboxes present inside gridview with Javascript

I have 2 checkboxes in a GridView. I want to validate them with JavaScript. This is my aspx code...

<asp:TemplateField HeaderText="IsExist">
            <ItemTemplate>
                <asp:CheckBox ID="chkExists" runat="server" Text="Exists" AutoPostBack="false" />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Not Exists In Update">
            <ItemTempl开发者_JAVA技巧ate>
                <asp:CheckBox ID="chkExistsInUpdate" runat="server" Text="NotExists" AutoPostBack="false"/>

            </ItemTemplate>
        </asp:TemplateField>

And my JavaScript condition is:

function check_one() {
            var obj = document.form1;
            if (obj.chkExists.checked == true || obj.chkExistsInUpdate.checked == true) {
                alert("Plese check only one checkbox...");
                return false;
            }
            else
                return true;
        }

But I don't know that how to access the checkboxes from GridView on the client side? Please suggest an example.

---

MyGridView = document.getElementById('<%= this.MyGridView.ClientID %>');
var Inputs = MyGridView.getElementsByTagName("input");
var chkBox = "chkExists";
for(var n = 0; n < Inputs.length; ++n)
     if(Inputs[n].type == 'checkbox' && 
        Inputs[n].id.indexOf(chkBox,0) >= 0 && 
        Inputs[n].checked)

      return true;   

Similarly you can check for the other checkbox too...