JavaScript获取某年某月有多少天以及第一天是星期几

function getDaysWeekady(year,month)
{
    var date = new Date(year, month-1, 1);//月份是0-11
    var date2 = new Date(year, month, 1)
    var days = new Date(date2.getTime() - 864e5).getDate();
    console.log(days);
    console.log(weekday);
}