setInterval函数的用法与setTimeout完全一致，区别仅仅在于setInterval指定某个任务每隔一段时间就执行一次，也就是无限次的定时执行
var time=setInerval(function(){
console.log(2);
},1000)
通过setInterval方法实现网页动画
<!DOCTUPE html>
<html lang = "en" >
<head>
<meta charset="UTF-8">
<meta name="Viewport" content="width=device-width,initial-scale=1.0">
<title>Document</title>
<style>
#someDiv{
width:100px;
height:100px;
background:red;
}
</style>
</head>
<body>
<div id = "someDiv"></div>
<script>
var div = dociment.getElementById('someDiv');
var opacity =1;
var fader = setInterval(function(){
opacity-=0.05;
if(opacity>0){
div.style.opacity=opacity;
}else{
clearInterval(fader);
}
},30);
</script>
</body>
</html>
