General comments response (was Re: ES6 Rev13 Review: MOP-refactoring, symbols, proxies, Reflect module)

Allen Wirfs-Brock allen at
Sun Dec 30 15:18:02 PST 2012

On Dec 30, 2012, at 2:57 PM, Brandon Benvie wrote:

> Wouldn't it be possible to make a copy of the provided descriptor object and otherwise work the same as it is currently specified? This retains the ability to set and get custom descriptor attributes without making the stored record mutable, which seems like the primary concern.

There are no stored PD records (for ordinary objects, see The PD records that reference the original descriptor object as just transient values.   It is not stored by the ordinary [[DefineOwnProperty]] call.  A Proxy defineProperty trap might capture the descriptor object that is passed in or even mutate it.  But so what?.  If a caller of Object.defineOwnProperty actually cares about that possibility, it can always make sure it passes a fresh descriptor object.


> On Sun, Dec 30, 2012 at 5:13 PM, Allen Wirfs-Brock <allen at> wrote:
> > * I'm a bit uncomfortable with the removal of property descriptor normalization in the getOwnPropertyDescriptor/defineProperty traps. Especially for getOwnPropertyDescriptor I think it's a breaking change w.r.t. ES5.1.
> To summarize, the [[GetOwnProperty]] internal method of proxy objects is now (in the spec draft) specified to take the descriptor object returned from the  getOwnPropertyDescriptor trap and to package it up as an internal property descriptor record that is populated in the normal manner from the descriptor object.  The process of creating this PD record performs the same error checks that are performed (in ES5.1) by Object.defineOwnProperty.  In addition, the PD record now captures a reference to the original descriptor object.  If that PD record is subsequently used in a context where a descriptor object is required, the original captured descriptor object is used.  This is essential for expressing the semantics of Object.getOwnPropertyDescriptor in terms of the MOP while still enabling Proxies to extend of domain of property attributes.  Object.getOwnPropertyDescriptor(obj) calls [[GetOwnProperty]] on obj which yields a PD record.  It then calls the abstraction operation Fr
> omPropertyDescriptor to produce the descriptor object that is its return value.  If obj is an ordinary object, the PD record produced by ordinary [[GetOwnProperty]] does not have a captured descriptor object so FromPropertyDescriptor cons up a new descriptor object that is a direct reflection of the PD record.  If obj is a proxy then [[GetOwnProperty]] will return a PD record that captures the descriptor object that was returned by the trap.  FromPropertyDescriptor simply returns that captured descriptor object which becomes the value of the original Object.getOwnPropertyDescriptor call.
> This permits things like:
> Object.defineOwnProperty(pObj1,"foo", {method: func});  //define property on a proxy-based object, that have "method" properties
> console.log (Object.getOwnPropertyDescriptor(pObj1,"foo").method);    //we can retrieve the value of the method attribute (if the proxy supports it)
> Object.defineOwnProperty(pObj2,"foo",Object.getOwnProperty(pObj1, "foo"));  //copy a method properry from pObj1 to pObj2
> If descriptor object with extended attributes is applied to an ordinary object, it is always first internally converted to a PD record.  PD records only contain fields for the ordinary attributes, and any operations upon ordinary objects will have no visibility  of  the extended attributes.

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