Optimised async loading of cross-domain scripts

snippet.js

/*
 * Updated to use the function-based method described in http://www.phpied.com/social-button-bffs/
 * Better handling of scripts without supplied ids.
 *
 * N.B. Be sure to include Google Analytics's _gaq and Facebook's fbAsyncInit prior to this function.
 */

(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'));

You could also add the LinkedIn SDK.

 // LinkedIn SDK 
 add('//platform.linkedin.com/in.js');

Here is an updated version including the updates of @mathiasbynens from his post on the similar topic + some adjustments.

JSFiddle: http://jsfiddle.net/ngryman/cEuxD/
Fork: https://gist.github.com/ngryman/5266324

• using all relative urls: google plus seems to work wo/ https, and gga now works w/ relative urls.
• not using document fragment as I find it a little bit overkill :)
• using the iframe version of the Facebook button (not in the docs anymore I think, but works).

(function(d, u) {
    var s = d.scripts[0],
        i = u.length, g;
    while (i--) {
        g = d.createElement('script');
        g.src = '//' + u[i] + '.js';
        s.parentNode.insertBefore(g, s);
    }
}(document, [
    // Google Analytics
    'google-analytics.com/ga',
    // Google+ button
    'apis.google.com/js/plusone',
    // Facebook SDK
    'connect.facebook.net/en_US/all',
    // Twitter SDK
    'platform.twitter.com/widgets'
]));

@zenopopovici - to set api_key, LinkedIn needs a callback. I forked this GIST a while ago and added callback method, you can preview it here: @3952471 . I know it's outdated, but I hope you'll be able to figure it out.

How can I add jQuery?

I would change the while statement to this

while (i--) {
    var  src = (u[i].lastIndexOf('http', u[i]) === -1) ? '//' + u[i] : u[i];

    g = d.createElement('script');
    g.src += (u[i].indexOf('.js', u[i].length - 3) === -1) ? src + '.js' : src;
    s.parentNode.insertBefore(g, s);
}

This makes the following things possible:

• it's possible to add scripts with ".js" at the end
• it's possible to add files with an absolute URL