2012/10/4 Mark S. Miller <erig...@google.com> > > On Thu, Oct 4, 2012 at 4:36 AM, David Bruant <bruan...@gmail.com> wrote: > >> Hi, >> >> Currently, the wiki says: >> "Invariant check: check whether the target’s prototype and the trap >> result are identical." >> >> If the trap can only report one value, there is almost no point in having >> a trap. >> > > General principle: even when the successful result of the trap must be > identical to that same operation as performed on the target, making it a > trap has two uses and a corresponding hazard: > > * It allows the operation to be aborted by throwing an exception. > At a time getPrototypeOf wasn't a trap, it was not really an expected feature.
> * It allows the target to be modified first, in anticipation of the target > being queried at the end of the trap. > Do we really need to change the target prototype before reporting a different prototype? IIRC for performance reasons, it's been decided to not perform any inheritance-related invariant check, so which object is in the prototype or reported as such does not really matter since it provides no guarantee for set/get/has traps. For these traps, any lie can be told for not-own properties. In that context, being forced to return a given prototype is a bit too much in my opinion since it's not an information client code can really rely on for anything Things are a bit different for non-extensible objects from which we can have stronger expectations (since setting __proto__ doesn't work for them) David
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