async-loading.js

<script>
(function(doc, script) {
	var js,
		s = doc.getElementsByTagName(script)[0],
		load = function(src, callback, id) {
			if (d.getElementById(id)) return;
			js = doc.createElement(script);
			js.src = src;
			js.id = id;
			//js.async = true;
			if (typeof callback === 'function') {
				js.onload = function() {
					callback();
				}
			}
			s.parentNode.insertBefore(js, s);
		};
	
	load('url', function() {}, 'id-to-pass');
	load('url', function() {}, 'id-to-pass');
	load('url', function() {}, 'id-to-pass');
}(document, 'script'));
</script>