js写倒计时_Math

<span class="countDown"></span>
<script>
    function countDown(second, select) {
        let oSpan = document.querySelector("." + select);
        if (!!second) {
            let val;
            second >= 0 ? val = second : val = Math.abs(second);
            let countDownTime = setInterval(() => {
                if (val < 0) {
                    clearInterval(countDownTime);
                    val = "计时结束";
                }
                oSpan.innerHTML = val;
                --val;
            }, 1000);
        } else if (!!second === false) {
            console.log("请传参！");
        }
    }

    countDown(3, "countDown");


</script>