JS LoadScript

var loadScript = function(url, callback){

    var script = document.createElement("script")
    script.type = "text/javascript";
	script.async = true;
	script.defer = true;

    if (script.readyState){
        script.onreadystatechange = function(){
            if (script.readyState == "loaded" ||
                    script.readyState == "complete"){
                script.onreadystatechange = null;
                callback();
            }
        };
    } else {
        script.onload = function(){
            callback();
        };
    }

    script.src = url;
    document.getElementsByTagName("head")[0].appendChild(script);
}

var jsSrc = ("https:" == document.location.protocol ? "https://www" : "http://widget") + ".domain.com/widget.js";
loadScript(jsSrc, function(){
    console.log('done');
});