async adding javascript

javascript async

var addScript = function(url,success){

	var script = document.createElement('script');
	script.src = url;
	var head = document.getElementsByTagName('head')[0], done=false;
	
	script.onload = script.onreadystatechange = function(){
		if (!done && (!this.readyState || this.readyState == 'loaded' || this.readyState == 'complete')) {
			done=true;
			success();
			script.onload = script.onreadystatechange = null;
			head.removeChild(script);
		}
	};
	
	head.appendChild(script);
};