function theWeek() {
    var totalDays = 0;
    now = new Date();
    years = now.getYear()
    if (years < 1000)
        years += 1900
    var days = new Array(12);
    days[0] = 31;
    days[2] = 31;
    days[3] = 30;
    days[4] = 31;
    days[5] = 30;
    days[6] = 31;
    days[7] = 31;
    days[8] = 30;
    days[9] = 31;
    days[10] = 30;
    days[11] = 31;
 
    //判断是否为闰年，针对2月的天数进行计算
    if (Math.round(now.getYear() / 4) == now.getYear() / 4) {
        days[1] = 29
    } else {
        days[1] = 28
    }

    if (now.getMonth() == 0) {
        totalDays = totalDays + now.getDate();
    } else {
        var curMonth = now.getMonth();
        for (var count = 1; count <= curMonth; count++) {
            totalDays = totalDays + days[count - 1];
        }
        totalDays = totalDays + now.getDate();
    }
    //得到第几周
    var week = Math.round(totalDays / 7);
    return week;
}

$('#week').text(theWeek());

以上获取到了本周是今年第几周，如何计算前面有多少周并且创建如下的DOM结构？

<ul id="before-week">
    ............
    <li>第29周</li>
    <li>第30周</li>
    <li>第31周（本周）</li>
</ul>