On Jul 1, 2011, at 11:37 AM, Andreas Gal wrote: > In our implementation we are using a generic object. It has some overhead, > and a specific internal "descriptor object" representation might be a bit > faster, but such magic objects that don't allow expando properties are > usually pretty surprising to programers. Most of the HTML5 DOM allows > arbitrary expando properties. I think we should stick to that theme.
IIRC Tom and Mark's http://traitsjs.org/ work relies on being able to extend property descriptors with .required and other ad-hoc ("expando") properties. This is important: it allows developers to blaze trails and tread paths that we later pave with standards. /be > > Andreas > > On Jul 1, 2011, at 5:54 AM, Andreas Rossberg wrote: > >> On 1 July 2011 12:12, Andreas Rossberg <[email protected]> wrote: >>> I believe there is some "type" confusion in the proxy proposal spec >>> wrt property descriptors and their reification into attributes >>> objects. >>> >>> 1. In a note on the def of [[DefineOwnProperty]] for proxies, the proposal >>> says: >>> >>> "The Desc argument to this trap is a property descriptor object >>> validated by ToPropertyDescriptor, except that it also retains any >>> non-standard attributes present in the original property descriptor >>> passed to Object.defineProperty. See the semantics of the modified >>> Object.defineProperty built-in, below." >>> >>> That seems fishy, since according to ES5 8.10: >>> >>> "Values of the Property Descriptor type are records composed of named >>> fields where each field‘s name is an attribute name and its value is a >>> corresponding attribute value as specified in 8.6.1." >>> >>> In particular, I take this to mean that property descriptors are not >>> objects (but abstract records), and that there cannot be any fields >>> whose name is not an attribute name. (In fact, in V8 we currently >>> encode property descriptors using objects, but the encoding is >>> different from the reified attributes object representation, and not >>> quite compatible with the idea of adding arbitrary other fields.) >> >> I forgot to say: step 5 of the definition invokes the defineProperty >> trap of the handler passing Desc as the second argument. But the >> handler expects a reified attributes object. >> >>> 2. In the modified definition of Object.defineProperty, the proposal >>> says in step 4.c: >>> >>> "Call the [[DefineOwnProperty]] internal method of O with arguments >>> name, descObj, and true." >>> >>> This is passing descObj, which in fact is _not_ a descriptor, but its >>> reification as an attributes object. >>> >>> /Andreas >>> >> _______________________________________________ >> es-discuss mailing list >> [email protected] >> https://mail.mozilla.org/listinfo/es-discuss > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
