Does JScript (ES3, in ScriptControl ActiveX object within VBA) support creation of non-enumerable parameters?

Question

If I define a property/function into an Object prototype (assuming that coercion will eventually lead to every variable accessing the property/function) as such ...

Object.prototype.doFunc = function(){ return 'this is ' + this;}
// OUTPUT - function(){ return 'this is ' + this;}
Object.prototype.doFunc.dontEnum = true
// OUTPUT - True

and then I run the for..in enumerator on an object, the doFunc function also comes up in the enumeration even though the object's .propertyIsEnumerable('doFunc') evaluates to a False

Any thoughts? (Again this is for ECMA Script 3/JScript that is native to the ScriptControl object in a VBA project.)