How to get properties value of web user control using JavaScript.

I have created a user control.
WebUserControl.ascx
<script type="text/javascript">
    function btnShowPopUp_Click() {
        $find('popUp').show();
        return false;
    }
 
    function btnCancel_Click() {
        $find('popUp').hide();
        return false;
    }
 
    function btnOK_Click() {
        //I have to do some logic here but don't know
    }
</script>
<asp:Button ID="btnHidden" runat="server" Style="display: none;" />
<asp:Button ID="btnShowPopUp" runat="server" OnClientClick="return btnShowPopUp_Click();"
   ...