code: function PromiseHasUserDefinedRejectHandlerCheck(handler, deferred) {
    if (handler !== PromiseIdRejectHandler) {
      var combinedDeferred = GET_PRIVATE(handler, promiseCombinedDeferredSymbol);
      if (IS_UNDEFINED(combinedDeferred)) return true;
      if (PromiseHasUserDefinedRejectHandlerRecursive(combinedDeferred.promise)) {
        return true;
      }
    } else if (PromiseHasUserDefinedRejectHandlerRecursive(deferred.promise)) {
      return true;
    }
    return false;
  }
docs: Utility for debugger