Cutting the mustard: http://timkadlec.com/2014/10/reducing-javascript-bloat-with-shoestring/

cutTheMustard.js

window.myApp = {};
/*! loadJS: load a JS file asynchronously. [c]2014 @scottjehl, Filament Group, Inc. (Based on http://goo.gl/REQGQ by Paul Irish). Licensed MIT */
function loadJS( src, cb ){
	"use strict";
	var ref = window.document.getElementsByTagName( "script" )[ 0 ];
	var script = window.document.createElement( "script" );
	script.src = src;
	script.async = true;
	ref.parentNode.insertBefore( script, ref );
	if (cb && typeof(cb) === "function") {
		script.onload = cb;
	}
	return script;
}

function loadJS(src,cb){...}

if ('querySelector' in document && 'addEventListener' in window) {
    window.myApp.cutsMustard = true;
    document.documentElement.className += " mustard";       
    loadJS("main.js");
}