jquery finder empty text values

<script>
    $(document).ready(function(){
    //cycle
        $(document).find(".na td").each(function () {
        //the element length which == 0
        var len = $(this).html().trim().length;
            if(len == 0) {
            //do something
                $(this).text("N/A");
                $(this).css("background","red");
            }
        })
    })
</script>