var month = 7;
var year = new Date().getFullYear();
switch(month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
alert(`${year}年${month}月有31天`);
break;
case 2:
var year = new Date().getFullYear();
if (year % 2 == 0 && year % 100 != 0 || year % 400 == 0) {
alert(`${year}年${month}月29天`);
} else {
alert(`${year}年${month}月28天`);
}
break;
default:
alert(`${year}年${month}月30天`);
break;
}