async-callback.js

// Async load a script with a callback
(function () {
    function async(u, c, a) {
        var d = document;
        var t = 'script';
        var o = d.createElement(t);
        var s = d.getElementsByTagName(t)[0];
        o.src = u;

        if (typeof a === 'object') {
            for (var attr in a) {
                s.setAttribute(attr, a[attr]);
            }
        }

        if (c) {
            o.addEventListener('load', function (e) {
                c(null, e);
            }, false);
        }
        s.parentNode.insertBefore(o, s);
    }

    async('https://path.to/your/script.js', function () {
        // This will get called on load
    }, {
        // Any attributes that need adding to the script tag as key/value pairs
    });
})();