naming convention opinion...

gistfile1.txt

I'm front-end'ing a site for a client that has a 'favourite' or 'star-ing' functionality. 

My question is, how to name whether the item has been starred or not.

Current logic in my CSS has things that are already .is-open, .is-closed, .is-active, .is-inactive, .is-visible and .is-hidden

I'm currently toying with 

.is-favourited and .is-Notfavourited but that's pretty crap

Maybe .is-listed, .is-unlisted ?

Thoughts?

.is-liked, .is-unliked
.is-starred, .is-unstarred
.is-favourited, .isnt-favourited

Why not just .is-starred, .is-unstarred

.megusta, .no-megusta

I jest, I'd do as @nouveller mentioned, is-liked, is-unliked - less characters for you, you know exactly what it is. Done.

I've never used this kind of convention but if it is what you are after why not:

.is-liked--true, .is-liked--false

EDIT: Although I would tend to just have defaults and then override them when it is true.

Why do you bother with the negative state at all? I would set the thing as closed / inactive / not favourited by default and make your css changes to show it as is-open / is-active / is-favourtied when those classes are added. Makes the js much easier as you are just toggling the one class (ex. is-active) instead of having to add one class and remove the other.

I'd try to reuse if I could.
<span class="star is-active"></span>

How will the un-starred and un-favourited items differ from being the standard presentation you would see for the item initially? Based on this answer, I would say gives you a reason for the negative classes.
I can see a need for is-closed/is-open as you might re present this give the current state is-open, is-closed (in an expanded navigation as an example).

So with that in mind I would go for the positive classes for these being;

.is-starred
.is-favourite or .is-fav

I would avoid using favourite in the class title just because the word has spelling variations.