计算一个月有多少天

function monthHaveManyDays(year,month){
    var days;
    var monthArr = [4,6,9,11]
    if(month == 2){
        days = year % 4 == 0 && year % 100 != 0 || year % 400 == 0 ? 29 : 28
    }else if(monthArr.inculdes(month)){
        days = 30
    }else{
        days = 31
    }
    return days
}