Simple Asynchronous JS script loader. Not clever or modern - just works.

async-scriptloader.js

//js async loader

var $$ = $$ || {};

$$.asyncScriptLoader = function (url) {
    //get currently loaded scripts
    var allScripts = document.getElementsByTagName('script'),
        alreadyLoaded = false;
    function onLoad(){}; //url is parameter
    function onError(){}; //url is parameter
    
    function escapeRegExp(string){
        return string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
    };
    
    var patt = new RegExp(escapeRegExp(url)+'$');
    //is script to be loaded already there? (looks at end of script urls only)
    for(var i = 0, len = allScripts.length;i < len; i++) {
        if(patt.test(allScripts[i].src)) {
            alreadyLoaded = true;
            break;
        };
    };

    //if already loaded skip load, just do any callback
    if(!alreadyLoaded) {
        // Create a new script and setup the basics.
        var script = document.createElement("script");
        script.async = true;
        script.src = url;
        
        script.onerror = function(){
            onError(url);
            script.onload = script.onreadystatechange = script.onerror = undefined;
        };
   
        script.onload = function() {
            onLoad(url);
            // Clear it out to avoid getting called more than once or any memory leaks.
            script.onload = script.onreadystatechange = script.onerror = undefined;
        };
          
        script.onreadystatechange = function() {
            if ( "loaded" === script.readyState || "complete" === script.readyState ) {
                script.onload();
            };
        };
        // Attach the script tag to the page (before the first script) so the magic can happen.
        allScripts[0].parentNode.insertBefore(script, allScripts[0]);
    } else {
        //already loaded, do the success callback
        //needs this fn to return so any success handler can be attached so
        //delay the call by min timeout value.
        setTimeout(function(){
            onLoad();
        },0);
    };
    //export chain
    return {
        success: function(fn){
            onLoad = (typeof fn === 'function') ? fn : onLoad;
            return this;
        },
        error: function(fn){
            onError = (typeof fn === 'function') ? fn : onError;
            return this;
        }
    };
};