Observers: should I use values passed to the function, or can I use this.value safely instead?

[Tony Mobily]

I have code like this in my app:

      observers: [

        '_dataGenericObserver(userData.generic)',

      ],

      // If this.userData.generic.id is set, then the register tab is

      // NOT visible. In this case, the selected tab mustn't be "register"...

      _dataGenericObserver: function(){

        if (this.userData.generic.id){

          this.selected = 'login';

        } else {

          if (this.defaultRegister) {

            this.selected = 'register';

          } else {

            this.selected = 'login';

          }

        }

      },

Is this safe? Or should I *always* do this instead:

      observers: [

        '_dataGenericObserver(userData.generic)',

      ],

      // If this.userData.generic.id is set, then the register tab is

      // NOT visible. In this case, the selected tab mustn't be "register"...

      _dataGenericObserver: function(generic){

        if (generic.id){

          this.selected = 'login';

        } else {

          if (this.defaultRegister) {

            this.selected = 'register';

          } else {

            this.selected = 'login';

          }

        }

      },

...?

I noticed in some cases in other parts of the app that if I have an observer on `_someFunc(value.subvalue)`, and then have `_someFunc: function( subValue)`, in the function `subValue` is set, whereas `this.value.subValue` isn't. However, I am not able to replicate it - sorry.

Questions:

* Is it always recommended to use the values passed to the functions, rather than `this.*`?

* WHEN is it likely to happen, that a `subValue` is set as a function parameter, but NOT in `this.value.subValue`?