用js实现同一个页面多个渐变效果

如下效果，下面的5个元素，随便一个，鼠标移上去透明度渐渐增加，鼠标移出，透明度渐渐减小。

效果预览：

要点一：

var speed = 0;
if(target>obj.alpha){
speed = 5;
}else{
speed = -5;
}

根据目标值和当时值的对比，来决定是正向还是负向速度。

要点二：

for(i=0; i<runs_li.length; i++){
runs_li[i].timer = null;
runs_li[i].alpha = 30;
runs_li[i].onmouseover = function(){
startrun(this,100);
}
runs_li[i].onmouseout = function(){
startrun(this,30);
}
}

给每一个元素加上各自的透明度值，各自的透明度变化分开。

最后，上代码：

<!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>无标题文档</title>
<style>
body,ul,li{margin:0; padding:0;}
#runs{width:300px; margin:10px auto;}
#runs li{width:80px; height:80px; background:#06c; list-style:none; float:left; margin:10px; display:inline; filter:alpha(opacity=30); opacity:0.3;}
</style>
<script>
window.onload = function(){
  var runs = document.getElementById("runs");
  var runs_li = runs.getElementsByTagName("li");
  var i=0; 
  for(i=0; i<runs_li.length; i++){
    runs_li[i].timer = null;
    runs_li[i].alpha = 30;
    runs_li[i].onmouseover = function(){
      startrun(this,100);
    }
    runs_li[i].onmouseout = function(){
      startrun(this,30);
    }
  }
}

function startrun(obj,target){
  clearInterval(obj.timer);
  obj.timer = setInterval(function(){
    var speed = 0;
    if(target>obj.alpha){
      speed = 5;
    }else{
      speed = -5;
    }
    
    if(obj.alpha == target){
      clearInterval(obj.timer);
    }else{
      obj.alpha = obj.alpha + speed;
      obj.style.filter = "alpha(opacity="+obj.alpha+")";
      obj.style.opacity = obj.alpha/100;
    }
  
  },30)
}

</script>
</head>

<body>
<ul id="runs">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
  <li>5</li>
</ul>
</body>
</html>