<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
</head>
<body>
<input type="button" value="换背景吧" />
<input type="button" value="停" />
<script>
var aBtn = document.getElementsByTagName('input');
var arrUrl = [ 'img/1.jpg', 'img/2.jpg', 'img/3.jpg', 'img/4.jpg' ];
var num = 0;
var timer = null;
var oBody = document.body;
aBtn[0].onclick = function (){
clearInterval( timer ); // null、未定义
//定时器是由用户控制的，要先关后开，防止用户手速过快连续点击
timer = setInterval(function(){
oBody.style.background = 'url('+ arrUrl[num] +')';
num++;
num%=arrUrl.length;
}, 1000);
};
aBtn[1].onclick = function (){
clearInterval( timer );
};
// oBody.style.background = 'url('+ arrUrl[num] +')';
</script>
</body>
</html>