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Per May 2016 TC39 consensus, Object.getOwnPropertyDescriptors is now stage 4! #582

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Per May 2016 TC39 consensus, Object.getOwnPropertyDescriptors is now stage 4! #582

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15 changes: 15 additions & 0 deletions spec.html
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Expand Up @@ -22683,6 +22683,21 @@ <h1>Object.getOwnPropertyDescriptor ( _O_, _P_ )</h1>
</emu-alg>
</emu-clause>

<emu-clause id="sec-object.getownpropertydescriptors">
<h1>Object.getOwnPropertyDescriptors ( _O_ )</h1>
<p>When the `getOwnPropertyDescriptors` function is called, the following steps are taken:</p>
<emu-alg>
1. Let _obj_ be ? ToObject(_O_).
1. Let _ownKeys_ be ? _obj_.[[OwnPropertyKeys]]().
1. Let _descriptors_ be ! ObjectCreate(%ObjectPrototype%).
1. Repeat, for each element _key_ of _ownKeys_ in List order,
1. Let _desc_ be ? _obj_.[[GetOwnProperty]](_key_).
1. Let _descriptor_ be ! FromPropertyDescriptor(_desc_).
1. Perform ! CreateDataProperty(_descriptors_, _key_, _descriptor_).
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Should we run this step also when descriptor is undefined? (Could happen if obj is a proxy, for instance.)

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I think that's perfectly fine. If a Proxy chooses to expose that it is a Proxy by providing a key in [[OwnPropertyKeys]] that does not have a corresponding descriptor in [[GetOwnProperty]], I think that intentional information leak should be propagated.

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what happens if you Object.getOwnpropertyDescriptor(proxy, key) and/or
Reflect.ownKeys(proxyObject) ?

On Wed, May 25, 2016 at 3:24 PM, Claude Pache [email protected]
wrote:

In spec.html
#582 (comment):

@@ -22683,6 +22683,21 @@

  •  <emu-clause id="sec-object.getownpropertydescriptors">

  •    <h1>Object.getOwnPropertyDescriptors ( _O_ )</h1>

  •    <p>When the `getOwnPropertyDescriptors` function is called, the following steps are taken:</p>

  •    <emu-alg>

  •      1. Let _obj_ be ? ToObject(_O_).

  •      1. Let _ownKeys_ be ? _obj_.[[OwnPropertyKeys]]().

  •      1. Let _descriptors_ be ! ObjectCreate(%ObjectPrototype%).

  •      1. Repeat, for each element _key_ of _ownKeys_ in List order,

  •        1. Let _desc_ be ? _obj_.[[GetOwnProperty]](_key_).

  •        1. Let _descriptor_ be ! FromPropertyDescriptor(_desc_).

  •        1. Perform ! CreateDataProperty(_descriptors_, _key_, _descriptor_).

Should we run this step also when descriptor is undefined? (Could
happen if obj is a proxy, for instance.)

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https://github.com/tc39/ecma262/pull/582/files/55ac18633a0e68ba24b1f62288e3c77ddece58bd#r64581739

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@ljharb ljharb May 25, 2016 •
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Reflect.ownKeys should return the same as [[OwnPropertyKeys]], and Object.getOwnPropertyDescriptor I believe will call [[GetOwnProperty]], so I think you'd get undefined for a key from a Proxy that had this inconsistent behavior.

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@claudepache claudepache May 27, 2016 •
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If Object.defineProperties accepts undefined in place of a property descriptor, it is probably fine for Object.getOwnPropertyDescriptors to produce undefined in place of a property descriptor, so that the following code will work:

Object.defineProperties(obj2, Object.getOwnPropertyDescriptors(obj1))

I’ve checked that Object.defineProperties does indeed accept undefined in place of a descriptor since ES6 (see step 5b), but the browsers still do not. (And test262 apparently forgot to be updated.)

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I guess my similarly-weak preference is that it should just pass through what the MOP gives it and not be responsible for normalizing or censoring.

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My weak preference is the same as @domenic's - also, this same "issue" already exists with

const obj = {};
Reflect.ownKeys(badProxy).forEach(x => {
  Object.defineProperty(obj, x, Reflect.getOwnPropertyDescriptor(badProxy, x));
});

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@bterlson bterlson Jun 1, 2016 •
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I don't see this as normalizing or censoring. To me, if GetOwnProperty returns undefined, the property doesn't exist. The fact that previously OwnKeys said the property was there is of no consequence - maybe it doesn't exist now because the proxy is confused, or maybe there were side effects that removed it.

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I don't see this as normalizing or censoring. To me, if GetOwnProperty returns undefined, the property doesn't exist.

I guess this is the core of our disagreement: if GetOwnProperty returns undefined, that to me means that its property descriptor is undefined. That can be true if the property doesn't exist, but it can also be true if the proxy wants to return undefined and the property does exist.

Basically, "what does it mean to exist"? :)

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@bterlson bterlson Jun 1, 2016 •
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There is no such thing as a property with an undefined property descriptor. Proxies may change their answers about whether a property exists, but if a property exists, it has a property descriptor.

1. Return _descriptors_.
</emu-alg>
</emu-clause>

<!-- es6num="19.1.2.7" -->
<emu-clause id="sec-object.getownpropertynames">
<h1>Object.getOwnPropertyNames ( _O_ )</h1>
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