JS倒计时

//方法
    function countDownTimerCallback(a) {
        var h = addZero(Math.floor(a / 3600));
        m = addZero(Math.floor(a / 60) % 60),
        s = addZero(Math.floor(a) % 60);
        d = h + "时" + m + "分" + s + "秒";
        document.getElementById("countdown").innerHTML = "时间还有" + d
    }

    function addZero(n){
        if(n < 10){
            n = '0'+n;
        }
        return n;
    };
   //倒计开始
    var t = 60;
    var thisSetInterval = setInterval(function(){
        if(t>=0){
            countDownTimerCallback(t);
            t--
        }else{
            clearInterval(thisSetInterval);
        }
    },1000);

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