$$ returns an Elements instance, Elements is an array-like Class
anyway since you are basically filtering, you can tell Slick that you need an element with both class:
var randNumber = 12923;
if($$('.' + randNumber +'.XCLASS').length>0){
alert('XCLASS FOUND');
}else{
//dostuff
}
or you could just use one of the Elements methods, I think .some will be your best choice here:
var randNumber = 12923
var lookingForYou = $$('.' + randNumber);
alert(lookingForYou.some(function(el){
return el.hasClass('XCLASS');
}))
EDIT:
adding some links:
A better way to use Elements on MooTools blog
in my second example I used the some method, which, by looking at the source is not overloaded, but is just the one in Array.prototype.some:
Element.js source reference
Array.some on MDN