native-shim.js

/**
 * @license
 * Copyright (c) 2016 The Polymer Project Authors. All rights reserved.
 * This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
 * The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
 * The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
 * Code distributed by Google as part of the polymer project is also
 * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 */

/**
 * This shim allows elements written in, or compiled to, ES5 to work on native
 * implementations of Custom Elements v1. It sets new.target to the value of
 * this.constructor so that the native HTMLElement constructor can access the
 * current under-construction element's definition.
 */
(function() {
  if (
    // No Reflect, no classes, no need for shim because native custom elements
    // require ES2015 classes or Reflect.
    window.Reflect === undefined ||
    window.customElements === undefined ||
    // The webcomponentsjs custom elements polyfill doesn't require
    // ES2015-compatible construction (`super()` or `Reflect.construct`).
    window.customElements.polyfillWrapFlushCallback
  ) {
    return;
  }
  const BuiltInHTMLElement = HTMLElement;
  /**
   * With jscompiler's RECOMMENDED_FLAGS the function name will be optimized away.
   * However, if we declare the function as a property on an object literal, and
   * use quotes for the property name, then closure will leave that much intact,
   * which is enough for the JS VM to correctly set Function.prototype.name.
   */
  const wrapperForTheName = {
    'HTMLElement': /** @this {!Object} */ function HTMLElement() {
      return Reflect.construct(
          BuiltInHTMLElement, [], /** @type {!Function} */ (this.constructor));
    }
  };
  window.HTMLElement = wrapperForTheName['HTMLElement'];
  HTMLElement.prototype = BuiltInHTMLElement.prototype;
  HTMLElement.prototype.constructor = HTMLElement;
  Object.setPrototypeOf(HTMLElement, BuiltInHTMLElement);
})();