is there a particular reason why you set it to undefined instead of false when not showing?

I think that setting to undefined is better because as the value is not initialized at the constructor or so, the only possible values are "true" and "undefined". If you set to false, during the object lifecycle the possible values would be "true", "false" and "undefined" (before the updateErrorMsg is called)

But at the end, it is a matter of personal preference :)

Do you have to do a check if that property exist before looking at the true flag? I mean, do you need to do something like self.ctrl.hasOwnProperty('errorMessageVisible') because looking at the boolean value?