super.prop assignment can silently overwrite non-writable properties

Allen Wirfs-Brock allen at wirfs-brock.com
Tue Apr 21 01:37:04 UTC 2015


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> 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> 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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