判断某天是周几

直接上代码
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>

<body>
    <script type="text/javascript">
    //判断某天是周几 IE9以上支持
    function getMyDay(date) {
        var week;
        var num = date.getDay();
        switch (num) {
            case 0:
                week = '周日';
                break;
            case 1:
                week = "周一";
                break;
            case 2:
                week = '周二';
                break;
            case 3:
                week = "周三";
                break;
            case 4:
                week = "周四";
                break;
            case 5:
                week = '周五';
                break;
            case 6:
                week = "周六";
                break;
        }
        return week;
    }
    var w1 = getMyDay(new Date("2018-01-30"));
    document.write(w1);
    </script>
</body>

</html>