How to test screen.width condition using karma and jasmine

for a while now I'm struggling with testing a simple if statement in my controller.

if (screen.width <= 768) {
                $location.hash('map');
                $anchorScroll();
            }

I found a way to spawn the browser with a different size but it didn't work the screen.width always stays the same. Is there a way to cover that part of the code?