Retrieving Length of the Text area while user types in text area using JavaScript.

<script type = "text/javascript">

        function Display_Length(obj) 
        {
            document.getElementById("lbl_length").innerHTML = '(' + obj.value.length + ')';
        }

    </script>

//On onkeyup event,we have to call above function. 
<textarea id = "summary" runat = "server" style = "width: 500px; height: 80px;" 
onkeyup = "Display_Length(this);"></textarea> 
<asp:Label ID = "lbl_length" runat = "server" Text = ""></asp:Label>