How to calculate text length of Textarea in Jquery

<script type="text/javascript" src="js/jquery-1.8.0.js"></script>

<script type="text/javascript">

$(document).ready(function(){

var TotalLength = 50;

$("#txt-imgdisc").keyup(function(evt){

  var charCode = (evt.which) ? evt.which : event.keyCode;

var textlength = $(this).val().length;

if(textlength > TotalLength)

{

if(charCode === 8)

{

return true;

}

return false;

}

$("#spresult").text(TotalLength-textlength);

});

$("#txt-imgdisc").keydown(function(evt){

  var charCode = (evt.which) ? evt.which : event.keyCode;

var textlength = $(this).val().length;

if(textlength > TotalLength)

{

if(charCode === 8)

{

return true;

}

return false;

}

  $("#spresult").text(TotalLength-textlength);

  });

});

 </script>

<p style="color:red; font-weight:bold;">Enter Text Less then 50</p> <textarea id="txt-imgdisc" name="imgdisc" rows="5" cols="30" class="required require" maxlength="400" validate="Please enter the image discription."></textarea>

 <span id ="spresult" style="color:green; font-weight:bold;" >50</span>