Accurately (and reliably) return the width and height of the viewport

viewport.js

var viewport = function() {
  	var e = window, a = 'inner';
  	if (!( 'innerWidth' in window)){
		a = 'client';
	    	e = document.documentElement || document.body;
	}
	return { width : e[a+'Width'], height : e[a+'Height'] };
};