If you are writing a library, you can do things like:

CF.activity().onEntry= function(){
// inside this function, keyword 'this' is the activity.

this.x=234; // works same as declaring a var in activity script.

};

What this allows is for you to declare variables for an activity in the library and therefore hide things such that users cannot modify them or screw things up.

Same applies for onExit which is a cleanup function where you can remove things.

Also note that pages support this as well where you can do such things as installing FirmKey handlers, create/destroy dynamic widgets, etc...

CF.activity().page('MyWellKnownPage').onEntry = function()
{
// install Firm1 press handler when page is started.
GUI.widget('PS_FIRM1').onPress= function(){
System.print('Firm1 Pressed');
};
// create dynamic widgets if you wish. (this keyword references current page)
this.w = GUI.newPanel(); // hope my syntax is right.
}

CF.activity().page('MyWellKnownPage').onExit = function()
{
// remove Firm1 press handler when page is exited.
GUI.widget('PS_FIRM1').onPress= null;
GUI.deleteWidget(this.w); // this is likely incorrect but put here to illustrate point.
this.w = null;
}