loadScript function - async

load.js

var loadScript = function(url, callback) {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    if (s.readyState) { // Internet Explorer
        s.onreadystatechange = function() {
            if (s.readyState == 'loded' || s.readyState == 'complete') {
                s.onreadystatechange = null;
                callback();
            }
        };
    }
    else { // other browsers
        s.onload = function() {
            callback();
        };
    }
    s.src = url;
    document.getElementsByTagName('head')[0].appendChild(s);
};

loadScript("https://cdnjs.cloudflare.com/ajax/libs/jquery/1.9.0/jquery.min.js", function() {
    alert('jQuery loaded!');
});