jquery实现textarea输入字符控制(仿微博输入控制字符)

var text=$(".release textarea").val();//获取输入框的值 
var counter=text.length; //获取输入框的值 
$("#numtj em").text(500-counter); 
$(document).keyup(function() { 
var text=$(".release textarea").val(); 
var counter=text.length; 
$("#numtj em").text(500-counter); 
if(counter>500){ //如果字数大于500 
$(".public").addClass("public01").attr("disabled","true") //发布按钮禁用 
} 
else{ 
$(".public").removeClass("public01").removeAttr("disabled") //发布按钮启用 
} 
});

#numtj em { 
font-weight: 700; 
font-size: 22px; 
font-style: italic; 
font-family: Constantia, Georgia; 
color:#C00; 
}