setInterval(“fun()”,1000)
setInterval(fun, 1000);每隔1秒钟，就去执行一次 fun 这个函数（最常用）
setInterval( function(){} , 1000 )
setInterval(fun(),1000) 错误的
倒计时：<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
*{
margin: 0;
padding: 0;
}
#box{
width: 100%;
height: 50px;
margin: 0 auto;
background-color: lightpink;
}
#txt{
line-height: 50px;
text-align: center;
font-size: 30px;
color: deeppink;
font-family: "微软雅黑";
}
</style>
<script>
window.onload = function(){
var box = document.getElementById(‘box‘);
var txt = document.getElementById(‘txt‘);
setInterval(fun,1000);
function fun(){
var date = new Date();
var wuyi = new Date(‘2018/5/1 00:00:00‘);
var second =parseInt((wuyi.getTime()-date.getTime())/1000);
var day =parseInt(second/3600/24) ;
var h = parseInt(second/3600%24);
var min =parseInt(second/60%60);
var s= parseInt(second%60);
day<10?‘0‘+day:day;
h<10?h=‘0‘+h:h;
min<10?min=‘0‘+min:min;
s<10?s=‘0‘+s:s;
txt.innerHTML = ‘距离五一还有‘+day+‘天‘+h+‘小时‘+min+‘分‘+s+‘秒‘;
}
}
</script>
</head>
<body>
<div id="box">
<p id="txt"></p>
</div>
</body>
</html>