//定义
function getMonthWeek(a, b, c) {
    var date = new Date(a, b, c), w = date.getDay(), d = date.getDate();
    return Math.ceil(
        (d + 6 - w) / 7
    );
    }
//调用
var day=new Date();
lastCurDay = new Date(day.getFullYear(), day.getMonth() + 1, 0);//获取当前月最后一天时间
getMonthWeek(lastCurDay.getFullYear(), lastCurDay.getMonth(), lastCurDay.getDate())

可以为Date.prototype添加一个获取周数的方法

Date.prototype.getWeeks = function(){
   //do something
}

不优雅，但是可用

<script>
    alert(getWeeksInMonth( new Date() ))
    function getWeeksInMonth( date ) {
        var end = new Date( date.getFullYear(), date.getMonth() + 2, 0 );
        var start = new Date( date.getFullYear(), date.getMonth() + 1 , 1 );
        return getWeeksInYear( end ) - getWeeksInYear( start ) + 1;
    }
    // http://zerosixthree.se/snippets/get-week-of-the-year-with-jquery/
    function getWeeksInYear( date ) {
        var onejan = new Date( date.getFullYear(), 0, 1 );
        return Math.ceil( ( ( ( date - onejan ) / 86400000 ) + onejan.getDay() + 1 ) / 7 );
    }
</script>

Date.prototype.getWeekCountInCurrentMonth = function(){

    var date = new Date();
    
    date.setDate(1);
    
    var day = date.getDay() || 7;
    
    date.setMonth(date.getMonth() + 1);
    
    var endDay = date.getDay() || 7;
    
    if (day == endDay && day == 1){
        return 4;
    }else{
        return 5 + (day >= endDay && endDay != 1);
    }
}