Edit in JSFiddle

function asyncData(url, callback) {
    var script = document.createElement('script');
    script.src = url;
    script.onload = function () {
        if (typeof callback == "function") callback();
        callback = null;
    };
    script.onreadystatechange = function () {
        if (script.readyState == 4 || script.readyState == "complete") {
            if (typeof callback == "function") callback();
            callback = null;
        }
    };
    document.getElementsByTagName('head')[0].appendChild(script);
}

// Test!
asyncData('https://apis.google.com/js/plusone.js?nocache=' + (new Date().getTime()), function () {
    alert('Loaded!');
});