<!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" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<title>Document</title>
<script>
//js ,每隔1秒钟，时间自动 变化1秒
window.onload=function()
{
setInterval(fn,1000) //每隔1000毫秒（1秒）执行一次fn函数
var demo=document.getElementById("demo");
var num=1; //一定写在外面，是全局变量
function fn()
{
num++;
demo.innerHTML=num;
}
}
</script>
</head>
<body>
<div id="demo"></div>
</body>
</html>
