> Le 8 août 2016 à 11:02, Claude Pache <claude.pa...@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
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