<script>
var month = parseInt(prompt('请输入月份'))
switch(month) {
case 1:
case 3:
case 5:
case 7:
case 9:
case 10:
case 12:
console.log('31');
break;
case 2:
console.log('29');
break;
case 4:
case 6:
case 8:
case 11:
console.log('30');
break;
}
</script>