使用JS显示倒计时数字时钟效果

!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>javascript实现的倒计时时钟/title>
style>
body,div{margin:0;padding:0;}
body{color:#fff;font:16px/1.5 \5fae\8f6f\96c5\9ed1;}
#countdown{width:300px;text-align:center;background:#1a1a1a;margin:10px auto;padding:20px 0;}
input{border:0;width:283px;height:50px;cursor:pointer;margin-top:20px;background:url(https://www.jb51.net/jscss/demoimg/201210/btn-1.png) no-repeat;}
input.cancel{background-position:0 -50px;}
span{color:#000;width:80px;line-height:2;background:#fbfbfb;border:2px solid #b4b4b4;margin:0 10px;padding:0 10px;}
/style>
script>
window.onload = function ()
{
var oCountDown = document.getElementById("countdown");
var aInput = oCountDown.getElementsByTagName("input")[0];
var timer = null;
aInput.onclick = function ()
{
this.className == "" ? (timer = setInterval(updateTime, 1000), updateTime()) : (clearInterval(timer));
this.className = this.className == '' ? "cancel" : '';
};
function format(a)
{
return a.toString().replace(/^(\d)$/,'0$1')
}
function updateTime ()
{
var aSpan = oCountDown.getElementsByTagName("span");
var oRemain = aSpan[0].innerHTML.replace(/^0/,'') * 60 + parseInt(aSpan[1].innerHTML.replace(/^0/,''));
if(oRemain = 0)
{
clearInterval(timer);
return
}
oRemain--;
aSpan[0].innerHTML = format(parseInt(oRemain / 60));
oRemain %= 60;
aSpan[1].innerHTML = format(parseInt(oRemain));
}
}
/script>
/head>
body>
div id="countdown">
span>01/span>分钟span>40/span>秒
input type="button" value="" />
/div>
/body>
/html>