textarea自动下拉(带计数)图

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textarea自动下拉(带计数)
<div id="b"> </div>
<textarea cols="30" rows="5" id="a">
</textarea>
<script language="javascript">
document.onkeydown=function()
{
str=a.value.replace(/[^x00-xff]/g,"aa")
b.innerHTML=str.length+1
if(str.length+1>150)
{
a.style.overflow="visible"
}
}
</script>