限制文本框输入的字节数的函数以及用法

函数:
<script language="javascript">
var lastcount=0;
var maxvalue=80;
function countstrbyte(message,total,used,remain){//字节统计
var bytecount=0;
var strvalue=message.value;
var strlength=message.value.length;
var marvalue=total.value;
if (lastcount!=strlength){//在此判断,减少循环次数
  for(i=0;i<strlength;i++){
    bytecount=(strvalue.charCodeAt(i)<=256)?bytecount+1:bytecount+2;
   if (bytecount>maxvalue){
      message.value=strvalue.substring(0,i);
   alert("留言内容最多不能超过80个字节!\n注意:一个汉字为两个字符.\n如果是黑客攻击非常欢迎,你攻击我修复大家一起进步^-^");
   bytecount=maxvalue;
   break;
   } 
  }
   used.value=bytecount;
   remain.value=maxvalue-bytecount;
   lastcount=strlength;
 }
}
</script>

文本框:

<textarea cols="50" name="nr" rows="5" class="text" onFocus="this.className='textedit';this.select()"
 onKeyDown="countstrbyte(this.form.nr,this.form.total,this.form.used,this.form.remain);"
 onKeyUp="countstrbyte(this.form.nr,this.form.total,this.form.used,this.form.remain);"></textarea>

底部显示内容:

<center>
最多允许输入<input name="total" type="text" disabled id="total" value="80" size="2" class="text" onFocus="this.className='textedit';this.select();">个字节&nbsp;
以用<input name="used" type="text" disabled id="used" value="0" size="2" class="text" onFocus="this.className='textedit';this.select();">个字节&nbsp;
剩余<input name="remain" type="text" disabled id="remain" value="80" size="2" class="text" onFocus="this.className='textedit';this.select();">个字节&nbsp;
<span class="style2"><strong>一个汉字两个字节</strong></span>
</center>