js的日期倒计时在日常项目中还是比较常用的，活动、拼团功能最为常见，先分析最要是处理好日期函数，然后一个页面上有多个倒计时情况下的方法；

来，先看下 拼多多 的拼单倒计时的效果：

那么，先上个代码吧：

案例效果：

<style>
 .mytime{ line-height: 40px; width: 300px; margin: 0 auto;}
 </style>

<div class="mytime jsTime" data-time="2019-04-01 16:58:00">时间1</div>
<div class="mytime jsTime" data-time="2019-04-04 18:00:02">时间2</div>
<div class="mytime jsTime" data-time="2019-04-05 19:01:31">时间3</div>
<div class="mytime jsTime" data-time="2019-04-06 05:05:15">时间4</div>
<div class="mytime jsTime" data-time="2019-04-07 09:01:43">时间5</div>
<hr>
<div class="mytime jsTime2" data-time="2019-04-08 16:30:05">时间1</div>
<div class="mytime jsTime2" data-time="2019-04-09 14:01:22">时间2</div>
<div class="mytime jsTime2" data-time="2019-04-10 18:06:25">时间3</div>
<div class="mytime jsTime2" data-time="2019-04-11 22:07:19">时间4</div>
<div class="mytime jsTime2" data-time="2019-04-12 23:12:38">时间5</div>

<!-- 方法1 -->
<script>
 const countdown = {
 domList : document.querySelectorAll('.jsTime'),
 formatNumber(n){
 // return n.toString().padStart(2, '0'); // 用padStart方法要注意兼容问题
 n = n.toString();
 return n[1] ? n : '0' + n;
 },
 setTime(dom){
 //获取设置的时间 如：2019-3-28 14:00:00 ios系统得加正则.replace(/\-/g, '/');
 const leftTime = new Date(dom.getAttribute('data-time').replace(/\-/g, '/')) - new Date();
 if (leftTime >= 0) {
 const d = Math.floor(leftTime / 1000 / 60 / 60 / 24);
 const h = Math.floor(leftTime / 1000 / 60 / 60 % 24);
 const m = Math.floor(leftTime / 1000 / 60 % 60);
 const s = Math.floor(leftTime / 1000 % 60);
 dom.innerHTML = `剩余${ d > 0 ? d + '天' : '' }${ [h, m, s].map(this.formatNumber).join(':') }`;
 } else {
 clearInterval(dom.$timer);
 dom.innerHTML = '拼团已结束';
 }
 },
 start(){
 this.domList.forEach((dom) => {
 this.setTime(dom);
 dom.$timer = setInterval(() => {
 this.setTime(dom);
 }, 1e3);
 });
 },
 };
 countdown.start();
</script>

<!-- 方法2 -->
<script>
 //时间格式处理
 const formatNumber = n => {
 n = n.toString();
 return n[1] ? n : '0' + n;
 };
 //团购倒计时
 const teamCountTime = (obj) => {
 var timer = null;
 function fn(){
 //获取设置的时间 如：2019-3-28 14:00:00 ios系统得加正则.replace(/\-/g, '/');
 var setTime = obj.getAttribute('data-time').replace(/\-/g, '/');
 //获取当前时间
 var date = new Date(),
 now = date.getTime(),
 endDate = new Date(setTime),
 end = endDate.getTime();
 //时间差
 var leftTime = end - now;
 //d,h,m,s 天时分秒
 var d, h,m,s;
 var otime = '';
 if (leftTime >= 0) {
 d = Math.floor(leftTime / 1000 / 60 / 60 / 24);
 h = Math.floor(leftTime / 1000 / 60 / 60 % 24);
 m = Math.floor(leftTime / 1000 / 60 % 60);
 s = Math.floor(leftTime / 1000 % 60);
 if (d <= 0) {
 otime = [h, m, s].map(formatNumber).join(':');
 } else {
 otime = d + '天' + [h, m, s].map(formatNumber).join(':');
 }
 obj.innerHTML = '剩余' + otime;
 //
 timer = setTimeout(fn, 1e3);
 } else {
 clearTimeout(timer);
 obj.innerHTML = '拼团已结束';
 }
 }
 fn();
 };
 let jsTimes = document.querySelectorAll('.jsTime2');
 jsTimes.forEach((obj) => {
 teamCountTime(obj);
 });
</script>

具体代码可访问本人github