Non-generic traps for non-generic objects (was: Overriding Map/etc with get/set hooks?)

Tab Atkins Jr. jackalmage at
Fri May 24 14:52:25 PDT 2013

On Fri, May 24, 2013 at 2:35 PM, Ron Buckton <rbuckton at> wrote:
> Another way to look at this is that there is no way to prevent a caller from using methods from the superclass on a subclass. In other OO languages, its much harder (or nearly impossible depending on the language) to forcibly call a superclass method against a subclass that has been overridden by the subclass.
> Tab wouldn't have this issue if there were a way to prevent an external caller from executing if the mapSubclass overrides `set`.

Sure, but that's attacking it on the wrong level, I think.

The problem is that the Map#set method grabs an *internal property*,
bypassing Proxies, etc., so you can't defend against it.  If I could
intercept access to [[MapData]] via proxy traps (like David suggests
in the OP), everything would be *perfect* - every single problem I
have would be resolved successfully, as would every additional
possibility that Jason brings up.

If Maps used a Map.MapDataSymbol, I could intercept it via proxies by
adding it to the symbol whitelist and do what I needed.  Of course,
the object represented by [[MapData]] is too abstract to *actually* do
that, but we could expose some interactions for it.


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