below
On Dec 20, 2012, at 2:09 AM, Tom Van Cutsem wrote:
> [+jwalden]
>
> 2012/12/19 Allen Wirfs-Brock <[email protected]>
> ...
>
> Currently, that test is performed in step 5.b. by testing whether the
> "current" object we are visiting in the proto chain O is the Receiver object.
> At first sight, this is a rather weird way of deciding between update vs.
> addition. We can get away with this test because we've just queried the O
> object for an own property (ownDesc). Hence, if O === Receiver, then we know
> Receiver already has the same property and we must update, rather than add.
>
> In the presence of proxies, this test is no longer valid, as Receiver may be
> a proxy for O.
I don't think this really makes any difference. A an object and a Proxy for
the object are two distinct object and they need to always be treated as such.
The real problem with the SameAs test is that when Receiver and O are different
you can't really assume anything about how they relate to each other as
Reflect.set can be called with arbitrary values provided for its target and
receiver parameters.
>
> The right fix (at least, it feels like the obvious right fix to me), is not
> to test whether O === Receiver, but rather just let the algorithm explicitly
> test whether the Receiver object already defines an own property with key P,
> to decide between update vs. addition:
>
> replace step 5.b with:
> 5.b Let existingDesc be the result of calling the [[GetOwnProperty]] internal
> method of Receiver with argument P.
> 5.c If existingDesc is not undefined, then
> ... // same steps as before
Yes, here is the new step 5 as I have just updated it in my spec. draft (ignore
the 1, it's really 5)
If IsDataDescriptor(ownDesc) is true, then
If ownDesc.[[Writable]] is false, return false.
If Type(Receiver) is not Object, return false.
Let existingDescriptor be be the result of calling the [[GetOwnProperty]]
internal method of Receiver with argument P.
ReturnIfAbrupt(existingDescriptor).
If existingDescriptor is not undefined, then
Let valueDesc be the Property Descriptor {[[Value]]: V}.
Return the result of calling the [[DefineOwnProperty]] internal method of
Receiver with arguments P and valueDesc.
Else Receiver does not currently have a property P,
Return the result of performing CreateOwnDataProperty(Receiver, P, V).
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