Making operations on property descriptors more robust against Object.prototype hazards

Claude Pache claude.pache at
Sat Sep 13 05:30:34 PDT 2014

Le 13 sept. 2014 à 14:19, Claude Pache <claude.pache at> a écrit :

> Le 13 sept. 2014 à 11:47, Claude Pache <claude.pache at> a écrit :
>> Hi,
>> As noted some time ago, there is an issue with `Object.defineProperty` when it happens that someone has the facetious idea to give some value to `Object.prototype.get` [1].
>> While it is hardly an issue in day-to-day programming, I think that it is a good idea to make sure that specialized API (Reflect, proxy traps) are robust by design against that bug^H^H^H feature. Concretely, I propose:
>> (1) proxy.[[DefineOwnProperty]] — When a proxy defines the corresponding trap, it shall receive an object with no prototype as property descriptor.
>> (2) Reflect.getOwnPropertyDescriptor —That method shall return an object with no prototype, so that it can be readily used elsewhere, e.g. in Reflect.defineProperty.
>> —Claude
>> [1]
> One can also (either alternatively or in addition) correct the other end of the API: (1) in proxy.[[GetOwnProperty]], ignore non-own properties of the result of the trap, and (2) in Reflect.defineProperty, ignore non-own properties of the object descriptor. 
> The core problem is that the two internal operations that convert between an internal representation of a property descriptor on the one side, and a concrete object representing a property descriptor on the other side, are not the inverse one of the other, because of the interference of Object.prototype.
> —Claude

Sorry for the spam, but I thought that a concrete example is good to illustrate the issue. Currently, under the assumption that someone will write `Object.prototype.get = function() {}`, the following proxy:

    Proxy(target, {
        defineProperty: (target, prop, desc) => Reflect.defineProperty(target, prop, desc)
      , getOwnPropertyDescriptor: (target, prop) => Reflect.getOwnPropertyDescriptor(target, prop)

is broken, and the following code:

    desc = Reflect.getOwnPropertyDescriptor(source, prop)
    if (desc)
        status = Reflect.defineProperty(target, prop, desc)
is broken as well.


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