var oldWidth;
setInterval(func, 1500);
function func(){
var windowWidth = window.innerWidth;
    if(oldWidth == windowWidth){
        return;
    }
    else{ // na chui to? tylko pierdoli czytelnosc.
        if(windowWidth > 1000){
       	//do something
		}
        else{
        //do something
        }
    }
    oldWidth = windowWidth;
}