[felgall]

What about this to test the values in the easiest to follow way:

switch (aNum) {
case 1:
case 21:
case 31:
suffix = 'st';
break;
case 2:
case 22:
suffix = 'nd';
break;
case 3:
case 23:
suffix = 'rd';
break;
default:
suffix = 'th';
}