分享一个简单的倒计时封装

var maxtime = 30 * 60; //一个小时，按秒计算，自己调整!
function CountDown() {
    if (maxtime >= 0) {
        let  minutes = Math.floor(maxtime / 60);
        let seconds = Math.floor(maxtime % 60);
        let msg = "距离结束还有" + minutes + "分" + seconds + "秒";
        document.all["timer"].innerHTML = msg;
        if (maxtime == 5 * 60)alert("还剩5分钟");
            --maxtime;
    } else{
        clearInterval(timer);
        alert("时间到，结束!");
    }
}
let timer = setInterval(()=>{
    CountDown()
}, 1000);