JS ScriptLoader using ES6 Promises

scriptloader.js

/*!
 * ES6 ScriptLoader snippet
 * (c) Javier Aguilar mjolnic.com
 * License: MIT (http://www.opensource.org/licenses/mit-license.php)
 * Source: https://gist.github.com/mjolnic/93cc837dd2213ec0636a
 */
window.ScriptLoader = function () {
  /**
   *
   * @param {string} url
   * @param {object=} attr
   * @returns {Promise}
   */
  var loader = function (url, attr) {
    return new Promise(function (resolve, reject) {
      var script = window.document.createElement('script');
      script.src = url;
      script.async = true;
      script.crossOrigin = 'anonymous';
      attr = attr || {};

      for (var attrName in attr) {
        script[attrName] = attr[attrName];
      }

      script.addEventListener('load', function () {
        resolve(script);
      }, false);

      script.addEventListener('error', function () {
        reject(script);
      }, false);

      window.document.body.appendChild(script);
    });
  };

  /**
   * Loads scripts asynchronously
   * @param {string|string[]} urls
   * @param {object=} attr Other script tag attributes
   * @returns {Promise}
   */
  this.load = function (urls, attr) {
    if (!Array.isArray(urls)) {
      urls = [urls];
    }

    return Promise.all(urls.map(function (url) {
      return loader(url, attr);
    }));
  }

  /**
   * Loads scripts asynchronously. It supports multiple url arguments, so each one will be loaded right after the
   * previous is loaded. This is a way of chaining dependency loading.
   *
   * @param {string|string[]} urls, ...
   * @returns {Promise}
   */
  this.loadChain = function (urls) {
    var args = Array.isArray(arguments) ? arguments : Array.prototype.slice.call(arguments);
    var p = this.require(args.shift());
    var self = this;
    return args.length ? p.then(function () {
      self.requireChain(...args);
    }) : p;
  }
}

Sample usage:

var loader = new ScriptLoader();
loader.load([
    '//code.jquery.com/jquery-2.1.4.min.js',
    '//cdnjs.cloudflare.com/ajax/libs/knockout/3.4.0/knockout-min.js',
    '//cdnjs.cloudflare.com/ajax/libs/dropbox.js/0.10.3/dropbox.min.js'
]).then(function (scriptElements) {
    console.log(scriptElements.length + ' Scripts Loaded!');
    loader.load([
        '//maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js',
        'app.js'
    ]);
});

Nice, but how to prevent same script from loading multiple times?

@dziku86 this script is 5 years old I recommend searching other solutions