super.prop assignment can silently overwrite non-writable properties

Caitlin Potter caitpotter88 at gmail.com
Tue Apr 21 01:52:03 UTC 2015


>>If the prop property accessed by super.prop is an accessor, super.prop = x; should invoke its setter. super.prop should invoke its getter.
>It does.  This is about what happens when that property is a data property doesn't exist. What happens when we do  [[HomeObject]].[[GetPrototypeOf]]().[[Set]]('prop', x, this)

I don’t think the accessor case does work. `ownDesc` never refers to the property descriptor of the receiver when O[P] is a SuperReference, so if there’s an `this.prop` is an accessor, and `super.prop` doesn’t exist, the data descriptor path is taken.

> On Apr 20, 2015, at 9:37 PM, Allen Wirfs-Brock <allen at wirfs-brock.com> wrote:
> 
> 
> On Apr 20, 2015, at 6:21 PM, Mark Miller wrote:
> 
>> If the prop property accessed by super.prop is an accessor, super.prop = x; should invoke its setter. super.prop should invoke its getter.
> 
> It does.  This is about what happens when that property is a data property doesn't exist. What happens when we do  [[HomeObject]].[[GetPrototypeOf]]().[[Set]]('prop', x, this)
> 
> Allen
>    
> 
> 
> 
> 
> 
> 
> 
> 
>> 
>> On Tue, Apr 21, 2015 at 4:18 AM, Allen Wirfs-Brock <allen at wirfs-brock.com <mailto:allen at wirfs-brock.com>> wrote:
>> 
>> On Apr 20, 2015, at 12:39 PM, Jason Orendorff wrote:
>> 
>>> On Mon, Apr 20, 2015 at 12:44 PM, Allen Wirfs-Brock
>>> <allen at wirfs-brock.com <mailto:allen at wirfs-brock.com>> wrote:
>>>>> In the spec, 9.1.9 step 4.d.i. is where `super.prop = 2` ends up, with
>>>>> O=X.prototype.
>>>> 
>>>> 4.d.1 doesn't set the property, it just comes up with the property descriptor to use, if the `Receiver` does not already have a corresponding own property.
>>>> 
>>>> 5.c+5.e checks if the corresponding own property actually exists on the `Receiver`.
>>>> 
>>>> If it already exits then it does a [[DefineOwnProperty]] that only specifies the `value` attribute. This should respect the current `writable` attribute of the property and hence reject the attempt to change the value.
>>> 
>>> I agree with all of this, except I don't see where the attempt is
>>> rejected. Since the property is configurable, I think
>>> [[DefineOwnProperty]] succeeds.
>>> 
>>> The property is still non-writable afterwards. Only the value changes.
>>> 
>>> So this isn't breaking the object invariants: the property in question
>>> is configurable, so it's OK (I guess) to change the value. It's just
>>> surprising for assignment syntax to succeed in doing it.
>> 
>> I think it's bogus and needs to be corrected.  Not only does it allow (in weird cases for [[Set]] (ie, assignment) to change the value of a non-writable property.  It also means there are cases where [[Set]] will convert an accessor property to a data property.
>> 
>> In combination, I think this is a serious bug that needs to be fix in the final published ES6 spec.  The fix I propose is in 9.1.9 to replace Set 5.e as follows:
>> 
>> 5.e If existingDescriptor is not undefined, then
>>        i.   If IsAccessorDescript(existingDescript), return false.
>>        ii.  If existingDescriptor.[[Writable]] is false, return false.
>>        iii.  Let valueDesc be the PropertyDescriptor{[[Value]]: V}.
>>        iv.  Return Receiver.[[DefineOwnProperty]](P, valueDesc).
>> 
>> Lines 5.e.i and 5.e.ii are new additions.
>>  
>> Thoughts?
>> Allen
>> 
>> 
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>> 
>> 
>> 
>> -- 
>> Text by me above is hereby placed in the public domain
>> 
>>   Cheers,
>>   --MarkM
> 
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