textarea文字个数倒数，随时提示效果

<div class="family_v2">
<p class="nickname_v2">简介：</p>
<textarea id="content" name="sign" style="height:60px;overflow-y: hidden;"
onkeyup="changeLength(this,60)" class="nicknameBox_v2 brief_box_v2">
</textarea>
<div class="limit_num_v2">
<h3>60</h3>
</div>
</div>

//验证textarea的长度
function changeLength(obj,lg){
var len = $(obj).val();
$(obj).next().find("h3").text(lg-len.length);
if(len.length>=lg){
$(obj).next().find("h3").text(0);
$(obj).val(len.substring(0,lg));
}
}