让文本框textarea自动适应内容的高度

<script>
var agt = navigator.userAgent.toLowerCase();
var is_op = (agt.indexOf("opera") != -1);
var is_ie = (agt.indexOf("msie") != -1) && document.all && !is_op;
function ResizeTextarea(a,row){
if(!a){return}
if(!row)
row=5;
var b=a.value.split("\n");
var c=is_ie?1:0;
c+=b.length;
var d=a.cols;
if(d<=20){d=40}
for(var e=0;e<b.length;e++){
if(b[e].length>=d){
c+=Math.ceil(b[e].length/d)
}
}
c=Math.max(c,row);
if(c!=a.rows){
a.rows=c;
}
}
</script>
<textarea style="overflow: hidden;  font-family: Verdana,Arial; font-style: normal;  font-size: 13px; line-height: normal; " rows="4" cols="30" onfocus="javascript:ResizeTextarea(this,4);"
onclick="javascript:ResizeTextarea(this,4);" onkeyup="javascript:ResizeTextarea(this,4);"></textarea>