Javascript delayed loading function

defer.js

var defer = function (config) {
    var inc;
    if (config.id && !document.getElementById(config.id)) {
        inc = document.createElement('script');
        if (config.id) {
            inc.id = config.id;
        }
        inc.async = true;
        inc.src = config.url;
        inc.onload = config.callback || function () {};
        document.getElementsByTagName('head')[0].appendChild(inc);
    }
}