[danik.js]

Для jQuery:

function myFunction() {
    var width  = $(window).width();
    if (width < 767) {
        alert('less than 767px');
    }
}
$(window).resize(myFunction);
myFunction();

VanillaJS:

function myFunction() {
    var width = window.innerWidth || document.documentElement.clientWidth;
    if (width < 767) {
        alert('less than 767px');
    }
}
window.onresize = myFunction;
myFunction();