Simple promise-based script-loader

demo.js

const loader = new loadScript();
loader.load([
    '//apis.google.com/js/client:platform.js?onload=startApp',
    '//cdnjs.cloudflare.com/ajax/libs/dropbox.js/0.10.3/dropbox.min.js'
]).then(({length}) => {
    console.log(`${length} scripts loaded!`);
});

loadScript.js

window.loadScript = function () {
  /**
   *
   * @param {string} url
   * @param {object=} attr
   * @returns {Promise}
   */
  const loader = (url, attr) => new Promise((resolve, reject) => {
    const script = window.document.createElement('script');
    script.src = url;
    script.async = true;
    script.crossOrigin = 'anonymous';
    attr = attr || {};

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

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

    script.addEventListener('error', () => {
      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 = (urls, attr) => {
    if (!Array.isArray(urls)) {
      urls = [urls];
    }

    return Promise.all(urls.map(url => 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) {
    const args = Array.isArray(arguments) ? arguments : Array.prototype.slice.call(arguments);
    const p = this.require(args.shift());
    const self = this;
    return args.length ? p.then(() => {
      self.requireChain(...args);
    }) : p;
  }
}