Function install

src/polyfills/custom-elements.js:887–932

(win, ctor)

 * @param {!Function=} ctor
 */
export function install(win, ctor) {
  // Don't install in no-DOM environments e.g. worker.
  const shouldInstall = win.document;
  const hasCE = hasCustomElements(win);
  if (!shouldInstall || (hasCE && isPatched(win))) {
    return;
  }

  let install = true;
  let installWrapper = false;

  if (ctor && hasCE) {
    // If ctor is constructable without new, it's a function. That means it was
    // compiled down, and we need to do the minimal polyfill because all you
    // cannot extend HTMLElement without native classes.
    try {
      const {Reflect} = win;

      // "Construct" ctor using ES5 idioms
      // I'm not sure why, but Closure will complain at the
      // `Function.call.call()` below unless we cast to a Function instance
      // here.
      const instance = /** @type {!Function} */ (Object.create(ctor.prototype));

      // This will throw an error unless we're in a transpiled environment.
      // Native classes must be called as `new Ctor`, not `Ctor.call(instance)`.
      // We use `Function.call.call` because Closure is too smart for regular
      // `Ctor.call`.
      Function.call.call(ctor, instance);

      // If that didn't throw, we're transpiled.
      // Let's find out if we can wrap HTMLElement and avoid a full patch.
      installWrapper = !!Reflect?.construct;
    } catch (e) {
      // The ctor threw when we constructed it via ES5, so it's a real class.
      // We're ok to not install the polyfill.
      install = false;
    }
  }

  if (installWrapper) {
    wrapHTMLElement(win);
  } else if (install) {
    polyfill(win);
  }
}