Track browser size in real time, do stuff at certain breakpoints. A JavaScript media query, essentially. Borrowed from @aarongustafson.

browser-width.js

// watch browser width on resize
var browser_width = 0;
window.watchResize(function(){
	browser_width = window.innerWidth || document.body.offsetWidth;
});

// do stuff after breakpoint
window.watchResize(function(){
	var threshold = 400;
	if ( browser_width >= threshold ) {
		// do stuff when window is ≥ 400px wide
	}
});

watch-resize.js

(function( window ) {
	window.watchResize = function( callback ) {
		var resizing;
		callback.size = 0;
		function done() {
			var curr_size = window.innerWidth;
			clearTimeout( resizing );
			resizing = null;
			// only run on a true resize
			if ( callback.size != curr_size ) {
				callback();
				callback.size = curr_size;
			}
		}
		window.addEventListener( 'resize', function() {
			if ( resizing ) {
				clearTimeout( resizing );
				resizing = null;
			}
			resizing = setTimeout( done, 50 );
		});
		// init
		callback();
	};
}(window));

watch-resize.min.js

(function(a){a.watchResize=function(d){var c;d.size=0;function b(){var e=a.innerWidth;clearTimeout(c);c=null;if(d.size!=e){d();d.size=e}}a.addEventListener("resize",function(){if(c){clearTimeout(c);c=null}c=setTimeout(b,50)});d()}}(window));