超出字符后将会弹出提示的js脚本如下：

<script language="javascript" type="text/javascript">
function chkmaxsms(vobj1,vmax) {
var str=vobj1.value;
var strlen=str.length;

if(strlen>vmax) {
alert('字数超过限制');
eval(vobj1.value=str.substr(0,vmax));
}
}
</script>
<textarea name="textfield" onkeyup="javascript:chkmaxsms(this,'10')">