get Last Day Of Month (javascript)

function getLastDayOfMonth(month,year){
    var day;
    switch(month)
        {
        case 1 :
        case 3 :
        case 5 :
        case 7 :
        case 8 :
        case 10:
        case 12:
            day = 31;
            break;
        case 4 :
        case 6 :
        case 9 :
        case 11:
               day = 30;
            break;
        case 2 :
            if( ( (year % 4 == 0) && ( year % 100 != 0) ) || (year % 400 == 0) )
                day = 29;
            else
                day = 28;
            break;

    }
    return day;
}