如圖，一開始元素的透明度是30，鼠標移上的時候，透明度慢慢增加，到透明度100停止。鼠標移出，透明度慢慢減少，減少到30.
要點一：因為無法直接獲取和改變透明度的值，可以把透明度值賦給一個變量，讓變量變化，最后把變量的值再賦給元素的透明值。
var alpha=30;
要點二：判斷目標值和目前透明值，來判定是正向速度還是負向速度。
if(target > alpha){
speed = 2;
}else{
speed = -2;
}
要點三：如果透明值達到目標值，關掉定時器，否則透明值繼續變化。最后把值賦給元素，因為透明度有兼容問題,所以要寫上兩個寫法。
if(alpha == target){
clearInterval(timer);
}
else{
alpha = alpha + speed;
run.style.filter = 'alpha(opacity='+alpha+')';
run.style.opacity = alpha/100;
document.title = alpha;
}
最后，上代碼
<!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>無標題文檔</title>
<style>
body{margin:0; padding:0;}
#run{width:100px; height:100px; background:#06c; position:absolute; border:1px solid #000; opacity:0.3; filter:alpha(opacity=30);}
</style>
<script>
window.onload = function(){
var run = document.getElementById("run");
var btn = document.getElementById("btn");
var speed = 1;
var timer = null;
var alpha=30;
run.onmouseover = function(){
startrun(100);
}
run.onmouseout = function(){
startrun(30);
}
function startrun(target){
clearInterval(timer);
timer = setInterval(function(){
if(target > alpha){
speed = 2;
}else{
speed = -2;
}
if(alpha == target){
clearInterval(timer);
}
else{
alpha = alpha + speed;
run.style.filter = 'alpha(opacity='+alpha+')';
run.style.opacity = alpha/100;
document.title = alpha;
}
},30)
}
}
</script>
</head>
<body>
<div id="run"></div>
</body>
</html>