TextArea限定输入长度

<script type="text/javascript">
/*
* 限制textarea文本域输入的字符个数
* @id        textarea表单ID
* @count 要限制的最大字符数
*/
function limitChars(id, count)
{
    var obj = document.getElementById(id);
    if (obj.value.length > count)
    {
        obj.value = obj.value.substr(0, count);
document.getElementById("overCountTips").innerHTML="最多只能输入"+count+"个字符";
    }else{
   document.getElementById("overCountTips").innerHTML="";
}
}


</script>


<textarea name="description" cols="50" rows="3" id="description" onKeyDown="limitChars('description', 20)" onchange="limitChars('description', 20)" onpropertychange="limitChars('description', 20)"><div class="STYLE4" id="overCountTips" style="display:"></div>