Different menues based on browser-width

Hi, hopefully I am asking at the right place.

I’ve created two different main-menues, which worked fine and I am displaying one for desktop- and one for mobile-browsers using wp_is_mobile, which also works fine, but now
I am facing the problem, that I want to display the mobile-menu for desktop-browser also,
if “width < xxx px”.

I am not entirely sure, whether I have to get the browser-width via javascript and then paste the whole thing into an “elseif” or if there’s another, easier way.

I am also sorry, if I’ve described my problem poorly, English isn’t my native tongue.