asynchronous javascript loading/executing

Question

In this post, asynchronous .js file loading syntax, someone said, \"If the async attribute is present, then the script will be executed asynchronously, as soon as it is avai

Answer 1

  (function(doc, script) {
    var js, 
        fjs = doc.getElementsByTagName(script)[0],
        add = function(url, id) {
            if (doc.getElementById(id)) {return;}
            js = doc.createElement(script);
            js.src = url;
            id && (js.id = id);
            fjs.parentNode.insertBefore(js, fjs);
        };

    // Google Analytics
    add(('https:' == location.protocol ? '//ssl' : '//www') + '.google-analytics.com/ga.js', 'ga');
    // Google+ button
    add('https://apis.google.com/js/plusone.js');
    // Facebook SDK
    add('//connect.facebook.net/en_US/all.js', 'facebook-jssdk');
    // Twitter SDK
    add('//platform.twitter.com/widgets.js', 'twitter-wjs');
}(document, 'script'));

source: https://css-tricks.com/thinking-async/