Editing menu?

Hey there, I am looking into a way to change how the menu on my site works. Due to the length of some of the menus, tablet users don’t have access to pages at the bottom of the lists.
But should they be on a smaller device, the menu changes to the mobile version.

So I was wondering if it would be possible to do a “simple” statement like

<script>
if (ScreenResolution < 1080x1920){
return menuComputer
}
else {
return menuMobile
}
</script>

And if this is possible, where would I put it?

The screen res, and size would be sorted out properly, once the “perfect” resolution is found (i.e. the resolution that the menu trails off the screen)