load dynamic script with callback

gistfile1.js

function fetch(src, callback) {
    var script = document.createElement(nodeName),
        called;
    try {
        script.addEventListener("load", callback, false);
    } catch (x) {
        script.attachEvent("onreadystatechange", function() {
            if (!called && /loaded|complete/.test(script.readyState) {
                called = true;
                callback();
            }
        });
    }
    script.src = src;
    document.documentElement.firstChild.appendChild(script);
}