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();"Text="Enter Number" />
<asp:UpdatePanel ID="updatePanel1" runat="server">
    <ContentTemplate>
        <div style="background: #f8f8f8; border: solid 2px #080808; padding: 15px;">
            <asp:Label ID="lblNumber" Text="Number" runat="server" />
            <asp:TextBox ID="txtNumber" runat="server" />
            <br />
            <asp:Button ID="btnOK" runat="server" OnClientClick="return btnOK_Click();" Text="OK" />
            <asp:Button ID="btnCancel" runat="server" OnClientClick="return btnCancel_Click();"Text="Cancel" />
        </div>
    </ContentTemplate>
</asp:UpdatePanel>
<asp:ModalPopupExtender ID="modalPopUp1" runat="server" BehaviorID="popUp" PopupControlID="updatePanel1"
    TargetControlID="btnHidden" />

<pre lang="c#"> private string _assLabel = "";
    public string AssociatedLabelID
    {
        get
        {
            return _assLabel;
        }
        set
        {
            _assLabel = value;
        }
    }

    private string _assTextBox = "";
    public string AssociatedTextBoxID
    {
        get
        {
            return _assTextBox;
        }
        set
        {
            _assTextBox = value;
        }
    }

Default.aspx
<asp:Label ID="lblFirstNum" runat="server" Text="Enter First Number" />
        <asp:TextBox ID="txtFirstNum" runat="server" />
        <uc1:WebUserControl ID="wucFirst" AssociatedLabelID="lblFirstNum" AssociatedTextBoxID="txtFirstNum"  runat="server" />
        <br />
        <asp:Label ID="lblSecondNum" runat="server" Text="Enter Second Number" />
        <asp:TextBox ID="txtSecondNum" runat="server" />
        <uc1:WebUserControl ID="wucSecond" AssociatedLabelID="lblSecondNum" AssociatedTextBoxID="txtSecondNum"  runat="server" />

Now what I have to do is that when I first click 'wucFirst' it will show me PopUp which is in the user control.  When I enter some values in 'txtNumber' and click OK button I want to assign that value in AssociatedTextBoxID or if click 'wucSecond' and enter value in 'txtNumber' and click OK button I want to assign value in 'wucSecond's AssociatedTextBoxID.