输入月份显示当月天数

var box3 = (parseInt(prompt("请输入一个月份")))
    if (box3 == 1 || box3 == 3 || box3 == 5 || box3 == 7 || box3 == 8 || box3 == 10 || box3 == 12) {
        alert('这个月有31天')
    } else if (box3 == 4 || box3 == 6 || box3 == 9 || box3 == 11) {
        alert('这个月有30天')
    } else if (box3 == 2) {
        alert('这个月有28天')
    } else {
        alert('请输入正确的月份')
    }