Angular service - get value of a local variable

I've seen this question a lot but have yet to find an answer. I have an angular service I want to test that handles Undo\Redo. Here it is:

function UndoRedoService() {
  var undoStack = [], redoStack = [];

  this.add = function add(cmd) {
    undoStack.push(cmd);
  };
}

I want to test this method. In my test I want to be able to access the undoStack size. I don't want to extract it to another service since that would be an overkill. On the other hand, I don't want to make it public by adding it to "this."

Is there any way I can access this variable? I've tried monkey patching but couldn't access it like that either...

I'd appreciate any answer that isn't: "You shouldn't be testing local variable values"

Thanks! Uri