Jquery 验证数字

<script type="text/javascript"> 
        $(document).ready(function () {
            $("#error").hide();
            $("#txtStuAge").keyup(function () { 
                 var $val = $("#txtStuAge").val(); 
                var code; 
               for (var i = 0; i < $val.length; i++) { 
                        //charAt()获取指定位置字符串,charCodeAt()返回该字符串的编码 
                        //0的ASCII是48,9的ASCII是57 
                     var code = $val.charAt(i).charCodeAt(0); 
                     if (code < 48 || code > 57) { 
                        $("#error").show(); 
                        break; 
                    } 
                     else { 
                        $("#error").hide(); 
                    } 
                } 
           }); 
        }); 
     </script> 
前台： <asp:TextBox ID="txtStuAge" runat="server"></asp:TextBox><span id="error">请输入数字！</span>