If you have a control on the control page, which has to be accessed in the master page using JavaScript, then here how to do so.
on the content page. create a text box has given below.
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<asp:Panel ID="panelContent" GroupingText="ContentPage Controls" runat="server">
<asp:TextBox ID="txtContent" runat="server"></asp:TextBox>
</asp:Panel>
</asp:Content>
Now access and control the page text box "txtContent" from the master pages.
<head runat="server">
<title></title>
<script type="text/javascript">
function accessControlContentPage() {
var txtCont = document.getElementById('<%= Page.Master.FindControl("ContentPlaceHolder1").FindControl("txtContent").ClientID %>');
txtCont.value = "I got populated using Master Page";
}
</script>
<asp:ContentPlaceHolder id="head" runat="server">
</asp:ContentPlaceHolder>
</head>
