I figured it out:
function removeRooms(reserved){
var a1 = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17];
var a2 = reserved
var a = [], diff = [];
for ( var i = 0; i < a1.length; i++) {
a[a1[i]] = true ;
}
for ( var i2 = 0; i2 < a2.length; i2++) {
if (a[a2[i2]]) {
delete a[a2[i2]];
} else {
a[a2[i2]] = true ;
}
}
for ( var k in a) {
diff.push(k);
}
return diff;
}