Script to load required external scripts before execution of user code - Cross browser compatible

crossGrease.user.js

// ==UserScript==
// @name         CrossGrease
// @namespace    https://gist.github.com/166MMX/
// @description  Script to load required external scripts before execution of user code - Cross browser compatible
// @downloadURL  https://gist.github.com/166MMX/3845101/raw
// @updateURL    https://gist.github.com/166MMX/3845101/raw
// @version      1.3
// ==/UserScript==

var crossGrease = function () {
};
crossGrease.require = function (urls, onLoad, onError) {
    var onFinishScope = {
        'injected':urls.length,
        'loaded':0,
        'errors':0,
        'onLoad':onLoad,
        'onError':onError
    };
    var onFinish = function () {
        this.loaded++;
        if (this.injected === this.loaded) {
            this.onLoad();
        }
        else if (this.injected === this.loaded + this.errors) {
            this.onError();
        }
    };
    var result = null;
    if (urls instanceof Array) {
        result = [];
        for (var i = 0, l = urls.length; l > i; i++) {
            result.push(crossGrease.inject(urls[i], function () {
                onFinish.call(onFinishScope);
            }, function () {
                onFinish.call(onFinishScope);
            }));
        }
    }
    else if ('string' === typeof urls) {
        result = crossGrease.inject(urls, onLoad, onError);
    }
    return result;
};
crossGrease.inject = function (url, onLoad, onError) {
    var dScript = null;
    if ('string' === typeof url) {
        dScript = document.createElement("script");
        dScript.setAttribute("src", url);
        if ('function' === typeof onLoad) {
            dScript.addEventListener("load", onLoad);
        }
        if ('function' === typeof onError) {
            dScript.addEventListener("error", onError);
        }
        document.body.appendChild(dScript);
    }
    return dScript;
};
crossGrease.execute = function (functionOrCode) {
    var scriptText = null, dScript = null;
    switch (typeof functionOrCode) {
        case 'function':
            scriptText = '(' + functionOrCode + ')();';
            break;
        case 'string':
            scriptText = functionOrCode;
            break;
        default:
            break;
    }
    if ('string' === typeof scriptText) {
        dScript = document.createElement("script");
        dScript.textContent = scriptText;
        document.body.appendChild(dScript);
    }
    return dScript;
};
crossGrease.requireAndExecute = function (urls, functionOrCode) {
    crossGrease.require(urls, function () {
        crossGrease.execute(functionOrCode);
    });
};
crossGrease.requireJQuery = function (version, functionOrCode) {
    if ('function' === jQuery && 'object' === jQuery.fn && 'string' === typeof jQuery.fn.jquery) {
        crossGrease.execute(functionOrCode);
    }
    else {
        var url = '//ajax.googleapis.com/ajax/libs/jquery/' + version + '/jquery.min.js';
        crossGrease.requireAndExecute(url, functionOrCode);
    }
};