简单的时钟 还需要多余解释了?

<div id="dateBox">
    <div class="time">10:09:45</div>
    <div class="date">2020-08-01 星期六</div>
</div>

<script>
    var time = document.getElementsByClassName('time')[0];
    var d = document.getElementsByClassName('date')[0];

    function getWeek(n) {
        var day = n.getDay();
        var week = '';
        switch (day) {
            case 0:
                week = "星期日";
                break;
            case 1:
                week = "星期一";
                break;
            case 2:
                week = "星期二";
                break;
            case 3:
                week = "星期三";
                break;
            case 4:
                week = "星期四";
                break;
            case 5:
                week = "星期五";
                break;
            case 6:
                week = "星期六";
                break;
            default:
                week = "未知";
                break;
        }
        return week;
    }

    function showInfo() {
        var now = new Date();
        // 1. 设置时分秒
        // padStart()有兼容性问题
        var hour = now.getHours().toString().padStart(2, '0');
        var minute = now.getMinutes().toString();
        if (minute < 10) {
            minute = '0' + minute;
        }
        var second = now.getSeconds().toString();
        second = second < 10 ? '0' + second : second;
        time.textContent = `${hour}:${minute}:${second}`;
        // 2. 设置年月日
        var year = now.getFullYear();
        var month = (now.getMonth() + 1).toString();
        month = month < 10 ? '0' + month : month;
        var date = now.getDate().toString();
        date = date < 10 ? '0' + date : date;
        var week = getWeek(now);
        d.textContent = `${year}-${month}-${date} ${week}`;
    }

    showInfo();

    // setInterval()是一种定时器，每间隔一段时间去执行一次回调函数。
    // 第一个参数是回调函数，第二个参数是间隔时间，单位毫秒。
    // window.setInterval(function(){
    //     showInfo();
    // }, 1000);

    // window.setInterval(showInfo, 1000); // 推荐
    window.setInterval("showInfo()", 1000);
    
</script>