textarea限制输入字数

<html>
    <head>
        <title> jquery实现textarea输入框限制字数</title>
        <meta http-equiv="content-type" content="text/html;charset=utf-8" />
        <script type="text/javascript" src="jquery.js"></script>
        <script type="text/javascript">
             $(function(){
              $("#weibo").keyup(function(){
               var len = $(this).val().length;
               if(len > 49){
                $(this).val($(this).val().substring(0,50));
               }
               var num = 50 - len;
               $("#word").text(num);
              });
             });
        </script>
        <style type="text/css">
            h6{color:blue;}
            textarea{resize:none;}
            #word{color:red;}
        </style>
    </head>
    <body>
        <h6>您可以输入<span>50</span>个字，现在剩余<span id="word">50</span>个</h6>
        <textarea name="con" id="weibo" cols="45" rows="6"></textarea>
    </body>
</html>