On Thu, May 8, 2014 at 5:23 PM, Tom Van Cutsem <[email protected]> wrote:
> Allen, Mark and I discussed the [[Origin]] issue and came to the following > consensus: > > We remove [[Origin]] and revert to the originally specified behavior (< > http://wiki.ecmascript.org/doku.php?id=harmony:proxies_spec>) where the > descriptor returned by the proxy is coerced into a fresh, normalized, > completed, ordinary descriptor object. > > This ensures complete backward-compatibility with the ES5 behavior (i.e. > Object.getOwnPropertyDescriptor will always return a fresh, complete data > or accessor descriptor), and doesn't allow a proxy to play tricks with > descriptor objects. > > Allen's remaining concern is that this disallows proxies (or new exotic > objects) from inventing new types of descriptors, next to data and accessor > descriptors. Due to backwards-compat. constraints, we're basically stuck > with these two types of property descriptors forever. > > The originally specified Proxy behavior also included copying any > non-standard attributes provided by the proxy onto the fresh descriptor > object. However, if we're serious about keeping > Object.getOwnPropertyDescriptor backwards-compatible with existing ES5 > code, we may be better off by not augmenting descriptor objects with > non-standard attributes, even if this is unlikely to break existing code. > As Jason mentioned, if proxies want to introduce new per-property > attributes, they can provide other means of getting at that meta-data > rather than abusing the standard reflection API. > > So, the current proposal is to spec [[GetOwnProperty]] for Proxies such > that the descriptor returned by the trap is coerced into a fresh, > normalized, complete descriptor object, without copying custom attributes. > > Relevant bug seems to already have been filed by Andre: < > https://bugs.ecmascript.org/show_bug.cgi?id=2382> > Forgive me if this has already been discussed elsewhere, but the "Notes" section of [[GetOwnProperty]](P) lists several invariants that are similar in nature to the following (which I've just made up): - A property cannot be reported as configurable, if it does not exists as an own property of the target object or if it exists as a non-configurable own property of the target object. - A property cannot be reported as writable, if it does not exists as an own property of the target object or if it exists as a non-writable own property of the target object. - A property cannot be reported as enumerable, if it does not exists as an own property of the target object or if it exists as a non-enumerable own property of the target object. Then descriptors would allow user-invented descriptor properties, while still upholding the target's integrity. Rick
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