Basic loader - Loads a script only once

loadScript.js

loadedScript = {};
loadingScript = {};
pendingScriptSuccess = {};

function loadScript (url, success, async) {
	if(loadedScript[url]) {
		success();
		return;
	}

	pendingScriptSuccess[url] = _.isArray(pendingScriptSuccess[url]) ? pendingScriptSuccess[url].push(success) : [success];
	
	if(loadingScript[url]) {
		return;
	}
	loadedScript[url] = false;
	loadingScript[url] = true;

	async = async || true;
	$.ajax({
        crossDomain: true,
        dataType: "script",
        url: url,
        async: async,
        success: (function (url) {
        	return function (data) {
	        	loadedScript[url] = true;
	        	loadingScript[url] = false;
	        	for (var i=0, len=pendingScriptSuccess[url].length; i<len; i++) {
	        		if(pendingScriptSuccess[url][i])
	        			pendingScriptSuccess[url][i](data);
	        	}
	        	delete pendingScriptSuccess[url];
	        };
        })(url)
    })
}

main.js

(function ($) {

	//load dependencies 
	loadScript('underscore.js');

	//init when document loaded
	$( window ).load(init);

})(jQuery);

function init () {
  loadScript('underscore.js', function () {
    //use underscore - underscore.js is loaded only once
  });
}