js实现活动倒计时

let startTime = 1527647143949; // 开始时间
    var time = new Countdown('timer',startTime);
    function Countdown (el,startTime) {
        this.startTime = startTime || '';
        this.el = el || '';
        // 轮询计算时间
        this.loop = function () {
            var that = this;
            setInterval(function(){
                that.init();
            },1000);
        };
        // 格式化时分秒
        this.formatDuring = function (mss) {
            var hours = parseInt((mss % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
            var minutes = parseInt((mss % (1000 * 60 * 60)) / (1000 * 60));
            var seconds = parseInt((mss % (1000 * 60)) / 1000);
            return hours + ": " + (minutes < 10 ? '0'+minutes : minutes) + ": " + (seconds < 10 ? '0'+seconds : seconds);
        };
        // 初始化倒计时
        this.init = function () {
            var endTime = this.startTime+(24*60*60*1000); // 结束时间 
            var timeLeft = endTime - new Date().getTime(); // 剩余时间
            document.getElementById(this.el).innerHTML = this.formatDuring(timeLeft);
            this.loop();
        };
        this.init();
    };