限定textarea里的最大字符数

<script language="javascript">
function textcontrol(content) {
if (content.length > 255) document.zb.Content.value=document.zb.Content.value.substring(0, 255);
}
</script>

<textarea name="Content" cols="50" rows="10" onPropertyChange="textcontrol(form1.content.value)"></textarea>