<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>share</title>
<style>
body,div,span{
margin: 0;
padding: 0;
}
#div1{
width: 200px;
height: 200px;
background: red;
filter: alpha(opacity:30);
opacity: 0.3;
}
</style>
<script>
window.onload=function(){
var oDiv=document.getElementById("div1");
oDiv.onmouseover=function(){
startChange(100);
}
oDiv.onmouseout=function(){
startChange(30);
}
}
var timer=null;
var alpha=30;
function startChange(iTarget){
var oDiv=document.getElementById("div1");
clearInterval(timer);
timer=setInterval(function(){
var speed=0;
if(alpha>30){
speed=-10;
}else{
speed=10;
}
if(alpha==iTarget){
clearInterval(timer);
}else{
alpha=alpha+speed;
oDiv.style.filter="alpha(opacity:"+alpha+")";
oDiv.style.opacity=alpha/100;
}
},30);
}
</script>
</head>
<body>
<div id="div1"></div>
</body>
</html>把if（alpha>30）{}改成if（alpha>iTarget){}
起初你的alpha是30，之后自增10，alpha是40，过了30毫秒再调用一次函数，此时alpha〉30，speed为-10，之后alpha自减10，又变成30，所以总是循环再0.3和0.4之间