JavaScript Send Checkbox Data from Popup to Main form

<script type="text/javascript">
    function selValue() {
        var val = '';
        var gridView = document.getElementById('<%=dgvData.ClientID %>');
        for (i = 1; i <= gridView.rows.length; i++) {
            var inputs = gridView.rows[i].getElementsByTagName('input');
            if (inputs != null) {
                if (inputs[0].type == "checkbox") {
                    if (inputs[0].checked) {
                        val = val + eval(gridView.rows[i].cells[2].value) + ','                    
                    }
                }
            }
        }
        window.opener.document.getElementById("txtText").value = val;
        window.close();
    }
</script>

--from main
    protected void btnSearch_Click(object sender, EventArgs e)
    {
        string myScript = "";
        myScript = "myRef = window.open('../../public/popUp_chkMulti.aspx?ID=ctl00_ContentMain_txtText', 'Mypop','left=400,top=200,width=400,height=300,toolbar=0,resizable=0,scrollbars=1');";
        ScriptManager.RegisterStartupScript(this, this.GetType(), Guid.NewGuid().ToString(), myScript, true);
        
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack == false)
        {
            hTextbox.Value = Request.QueryString["ID"];
            lbtOK.Attributes.Add("OnClick", "javascript:selValue('','" + hTextbox.Value + "') ");      
            
            BindGrid();
        }
    }

<script type="text/javascript">
function selValue(val,focus) {
    var gridview = document.getElementById('dgvData');
    var len = gridview.rows.length;
    for (i = 1; i < len; i++) {
        var input = gridview.rows[i].cells[0].getElementsByTagName('input'); 
        if(input[0].type == 'checkbox')
            if (input[0].checked == true) {
                val = val + gridview.rows[i].cells[1].innerHTML + ','; //OK
            }
        }

        if (val != '') {
            //alert(val);
            window.opener.document.getElementById(focus).value = val;
            window.close();
        } else {
            alert("ไม่มีรายการที่ถูกเลือก");
        }
}
</script>