The browsers provide us with 2 key APIs which allow us to add Responsive Javascript to our site, they are the window.matchMedia API and the window.onresize API.

resize.js

var resizeMethod = function(){
    if (document.body.clientWidth < 768) {
        console.log('mobile');
    }
    if (document.body.clientWidth > 768) {
        console.log('desktop');
    }
};

//Attach event for resizing
window.addEventListener("resize", resizeMethod, true);

//Prepare a MediaQueryList
var mql = window.matchMedia("(max-width:768px)");

//Add a listener to the MediaQueryList
mql.addListener(function(e){
        if(e.matches){
                console.log('enter mobile');
        }
});