js简单时分秒倒计时

效果：

javascript：

    <script type="text/javascript">
        function countTime() {
            //获取当前时间
            var date = new Date();
            var now = date.getTime();
            //设置截止时间
            var endDate = new Date("2018-10-25 00:00:00");
            var end = endDate.getTime();
            //时间差
            var differTime = end - now;
            //定义变量,h,m,s保存倒计时的时间
            var h, m, s;
            if (differTime >= 0) {
                h = Math.floor(differTime / 1000 / 60 / 60);
                m = Math.floor(differTime / 1000 / 60 % 60);
                s = Math.floor(differTime / 1000 % 60);
                h = h < 10 ? ("0" + h) : h;
                m = m < 10 ? ("0" + m) : m;
                s = s < 10 ? ("0" + s) : s;
                document.getElementById("_h").innerHTML = h + "时";
                document.getElementById("_m").innerHTML = m + "分";
                document.getElementById("_s").innerHTML = s + "秒";
                setTimeout(countTime, 1000);

            } else {
                document.getElementById("_h").innerHTML = "00时";
                document.getElementById("_m").innerHTML = "00分";
                document.getElementById("_s").innerHTML = "00秒";
            }

        }
    </script>

HTML:　

<body onload="countTime()">
    <div class="timer">
        <span id="_h"></span>
        <span id="_m"></span>
        <span id="_s"></span>
    </div>
</body>