/** Note that this method is not expected to be overridden. */
  @JsMethod
  @Override
  default <R extends @Nullable Object> IThenable<R> then(
      @JsOptional @Nullable IThenOnFulfilledCallbackFn<? super V, ? extends R> onFulfilled,
      @JsOptional @Nullable IThenOnRejectedCallbackFn<? extends R> onRejected) {
    return new Promise<V>(
            (resolve, reject) -> {
              Futures.addCallback(
                  this,