Standalone getScript. This performs the same ability as jQuery.getScript, including the optional callback, but doesn't support the Promises implementation shared with all the other jQuery.ajax methods.

getScript.js

/**
 * Fetches and inserts a script into the page before the first
 * pre-existing script element, and optionally calls a callback
 * on completion.
 *
 * [TODO] Make this a module of its own so it can be used elsewhere.
 * 
 * @param  {String}   src      source of the script
 * @param  {Function} callback (optional) onload callback
 */
var getScript = function (src, callback) {
    var el = document.createElement('script');

    el.type = 'text/javascript';
    el.async = false;
    el.src = src;

    /**
     * Ensures callbacks work on older browsers by continuously
     * checking the readyState of the request. This is defined once
     * and reused on subsequent calls to getScript.
     * 
     * @param  {Element}   el      script element
     * @param  {Function} callback onload callback
     */
    getScript.ieCallback = getScript.ieCallback || function (el, callback) {
        if (el.readyState === 'loaded' || el.readyState === 'complete') {
            callback();
        } else {
            setTimeout(function () { getScript.ieCallback(el, callback); }, 100);
        }
    };

    if (typeof callback === 'function') {
        if (typeof el.addEventListener !== 'undefined') {
            el.addEventListener('load', callback, false);
        } else {
            el.onreadystatechange = function () {
                el.onreadystatechange = null;
                getScript.ieCallback(el, callback);
            };
        }
    }

    // This is defined once and reused on subsequeent calls to getScript
    getScript.firstScriptEl = getScript.firstScriptEl || document.getElementsByTagName('script')[0];
    getScript.firstScriptEl.parentNode.insertBefore(el, getScript.firstScriptEl);
};

LICENSE

  DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE 
                    Version 2, December 2004 

 Copyright (C) 2004 Sam Hocevar <sam@hocevar.net> 

 Everyone is permitted to copy and distribute verbatim or modified 
 copies of this license document, and changing it is allowed as long 
 as the name is changed. 

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE 
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION 

  0. You just DO WHAT THE FUCK YOU WANT TO.