JS: countdown effect

Before doing the countdown, let me talk about a method setInterval(). This method can call a function repeatedly, so the countdown effect can be achieved with this method

1.要做倒计时，首先是要获取当前时间，要知道最终时间是什么时候才可以倒计时
2.用最终时间减去当前时间，可以知道还剩下多少时间到达目标时间
3.用数学方法就可以得知倒计时的时分秒(具体看代码)
4.要注意的是，因为setInterval()是在一秒后才会触发，所以一开始的时间并不是倒计时时间，所以可以将setInterval()这个方法先调用一次.
实现效果如下

Insert picture description here
code show as below

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .hour{
            height: 40px;
            width: 40px;
            background: black;
            text-align: center;
            color: white;
            float: left;
            margin-left: 2px;
        }
        .minute{
            height: 40px;
            width: 40px;
            background: black;
            text-align: center;
            color: white;
            float: left;
            margin-left: 2px;
        }
        .second{
            height: 40px;
            width: 40px;
            background: black;
            text-align: center;
            color: white;
            float: left;
            margin-left: 2px;
        }
    </style>
</head>
<body>
    <div>
        <span class="hour">1</span>
        <span class="minute">2</span>
        <span class="second">3</span>
    </div>
    <script>
        var hours=document.querySelector('.hour');
        var minutes=document.querySelector('.minute');
        var seconds=document.querySelector('.second');
        var inputTime=+new Date('2020-9-11 12:00:00');
        countDate();
        setInterval(countDate,1000);
        function countDate(){
            var nowTime=+new Date();
            var times=(inputTime-nowTime)/1000;
            var h =parseInt(times/60/60%24);
            var m =parseInt(times/60%60);
            var s =parseInt(times%60);
            h=h<10 ?'0 '+h:h;
            m=m<10 ?'0 '+m:m;
            s=s<10 ?'0 '+s:s;
            hours.innerHTML=h;
            minutes.innerHTML=m;
            seconds.innerHTML=s;
        }
    </script>
</body>
</html>

JS: countdown effect

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