输入月份，显示当月天数，利用case穿透简化代码

var month =Number(prompt('请输入一个月份','比如3'));

            var smallMonthDay =30;

            var bigMonthDay =31;

            var leapMonth =28;

            switch(month){

                case 1:

                case 3:

                case 5:

                case 7:

                case 8:

                case 10:

                case 12:

                    alert(month + '月有' + bigMonthDay +'天')

                    break;

                case 4:

                case 5:

                case 9:

                case 12:

                    alert(month + '月有' + smallMonthDay + '天')

                    break;

                case 2:

                    alert(month + '月有' + leapMonth +'天')

                    break;

                default:

                    alert('输入有误')

                    break;

            }