(Also see the discussion page for this proposal)

All that is needed is a way to set the magic internal DontEnum bit. Extend Object.prototype.propertyIsEnumerable() so you can query, set, or clear the internal DontEnum bit. The E parmeter has positive sense even though the ECMAScript spec DontEnum bit has negative sense.

   function propertyIsEnumerable(P, E=undefined):Boolean
   {
       var old = !DontEnum(P)
       if (!DontDelete(P)) {
          if (E is Boolean)
              DontEnum(P) = !E
       }
       return old
   }

Need to specify:

• Interaction with for-in, probably

Interaction with for-in should be well-specified by iterators and generators based on DontEnum.

— Brendan Eich 2007/05/29 10:33