比较平滑的js控制textaera的高度

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 
<title>佐佐制造 --textarea---addheight</title> 
<style type="text/css"> 
/* Reset style */ 
* { margin:0; padding:0; word-break:break-all; } 
body { background:#FFF; color:#333; font:12px/1.4em Helvetica, Arial, sans-serif; } 
h1, h2, h3, h4, h5, h6 { font-size:1em; } 
a { color:#333; text-decoration:none; } 
    a:hover { text-decoration:underline; } 
ul, li { list-style:none; } 
fieldset, img { border:none; } 
input, textarea, select, button { font:12px Helvetica, Arial, sans-serif; } 
.button { padding:0 10px; height: 22px; border: 1px solid; border-color: #EEE #999 #999 #EEE; background: #DDD; color: #333; line-height: 20px; cursor: pointer; margin-left:2px; } 
.submit { padding:0 10px; height: 22px; border: 1px solid; border-color: #DDD #CC8305 #CC8305 #DDD; background: #FFA200; color: #FFF; line-height: 20px; letter-spacing: 1px; cursor: pointer; } 
/* edit style */ 
#edit { width:300px; margin:50px; } 
    .edit_caption { width:100%; overflow:hidden; margin-bottom:1px; overflow:hidden; } 
    .edit_caption span { display:block; float:left; margin:0 1px; padding:4px 10px; background:#DDD; cursor:pointer; } 
    .edit_caption span:hover { background:#EEE; } 
    #edit textarea { width:300px; height:80px; margin:0; padding:0; } 
</style> 
<script type="text/javascript"> 
var addH=1; 
function addHeight() { 
  if (!document.getElementById('comment_content')) return flase; 
    var comment = document.getElementById('comment_content'); 
    var nowH = parseInt(comment.style.height); 
    if (nowH < 250) {     
        nowH+=2; 
        comment.style.height=nowH+"px"; 
        addH++; 
        if (addH > 25){  
            addH=1; 
        } else { 
        window.setTimeout("addHeight()","10"); 
        } 
    } 
} 
function minHeight() { 
  if (!document.getElementById('comment_content')) return flase; 
    var comment = document.getElementById('comment_content'); 
    var nowH = parseInt(comment.style.height); 
    if (nowH > 50) { 
        nowH-=2; 
        comment.style.height=nowH+"px"; 
        addH++; 
        if (addH > 25){  
            addH=1; 
        } else { 
        window.setTimeout("minHeight()","10"); 
        } 
    } 
} 
</script> 
</head> 
<body> 
 
     
        <span class="tool_bigger" onclick="addHeight()">放大</span> 
        <span class="tool_smaller" onclick="minHeight()">缩小</span> 
     
    <textarea id="comment_content" tabindex="1" rows="8" name="comment_content" style="height:100px"></textarea> 
 
</body> 
</html>

[Ctrl+A 全选注:如需引入外部Js需刷新才能执行]