Object.freezing proxies should freeze or throw?

[Claude Pache]

> Le 8 août 2016 à 11:02, Claude Pache <claude.pache at gmail.com> a écrit :
> 
> Here is another test case, with [[GetOwnPropertyDescriptor]]:
> 
> ```js
> var target = Object.seal({x: 2});
> var proxy = new Proxy(target, {
>    getOwnPropertyDescriptor(o, p) {
>        var d = Reflect.getOwnPropertyDescriptor(o, p)
>        if (d && 'writable' in d)
>            d.writable = false
>        return d
>    }
> });
> 
> Object.getOwnPropertyDescriptor(proxy, 'x'); // { value: 2, writable: false, configurable: false }
> 
> Object.defineProperty(proxy, 'x', { value: 3 })
> 
> Object.getOwnPropertyDescriptor(proxy, 'x'); // { value: 3, writable: false, configurable: false }
> ```
> 
> IMHO, the most robust fix is the following: Both the [[DefineOwnProperty]] and the [[GetOwnPropertyDescriptor]] contain the following check:
> 
> * If IsCompatiblePropertyDescriptor(extensibleTarget, resultDesc, targetDesc) is false, throw a TypeError exception.
> 
> I think there should be also the following check (except when targetDesc is undefined, in which case another appropriate check is used):
> 
> * If IsCompatiblePropertyDescriptor(extensibleTarget, targetDesc, resultDesc) is false, throw a TypeError exception.
> 
> That would replace the weaker adhoc check about the [[Configurable]] attribute (search for settingConfigFalse) in those algorithms.
> 
> (Alternatively, in order to avoid double checks, define an AreBothWaysCompatiblePropertyDescriptors(extensibleTarget, desc1, desc2) abstract operation.)
> 
> —Claude
> 

Looking closer, it seems that using IsCompatiblePropertyDescriptor is in fact an overkill, because we probably don’t want the special case of conditionally mutable [[Writable]] attribute for nonconfigurable properties.

That is, I think that the following condition must hold: If either the target has a nonconfigurable property, or the proxy claims to have a nonconfigurable property, then every attribute of the property descriptor claimed by the proxy must be identical to the corresponding attribute of the property descriptor of the target.

—Claude