2013/1/3 Allen Wirfs-Brock <[email protected]> > > On Jan 3, 2013, at 12:18 AM, Tom Van Cutsem wrote: > > I think it's perfectly obvious what it means for an exotic property to be > complete: that it is at least a complete data or accessor property (i.e. it > either has {value,writable,enumerable,configurable}, or it has > {get,set,enumerable,configurable} properties). It may further specify any > number of additional custom attributes. > > > You seem to be assuming that all exotic properties must be an incremental > variation of a data property or an accessor property. However, > historically this hasn't been the case. When accessor attributes were > added to ES they weren't just a incremental extension to the attributes of > a data property. If any new property forms are added to ES in the future, > they probably also won't be simply extended variations of data or accessor > properties. I would hope that proxies could be used to prototype any such > future extensions. > > As part of a general extension mechanism, I don't think we should > unnecessarily limit how a proxy handler chooses to define the > interpretation of its property descriptors. It's clear to me that such > limitations restrict the utility of proxies. What benefit to we get from > those limitations. > > > More generally, I think it is backwards-compatible to extend property > descriptors with extra attributes, but it's backwards-incompatible to allow > proxies to remove some of the standard attributes from property > descriptors. This breaks the assumptions of existing clients. > > > Since we never fully defined the invariants of > Object.getOwnPropertyDescriptor it isn't clear what assumptions existing > clients should be making. I think the most basic assumption that is > justified is that the result of gOPD can be used with Object.defineProperty > to define a similar property on a different object that is the *same kind*of > object as the original. >
Thanks, I feel this conversation has brought us to a higher-level point that I hope will allow others to join in. Your position is that ES5.1's property descriptors are really open-ended sets of attributes, and ES5.1 clients should not assume that the result of Object.getOwnPropertyDescriptor is either a data or an accessor property. It may return some third type of non-standard property. My position is that ES5.1, by codifying only two kinds of property descriptors, effectively does allow clients of Object.getOwnPropertyDescriptor to assume that the result is either a data or an accessor property. And my position is also that there most likely exists ES5.1 code that builds on this assumption. I also don't know of any DOM or other host objects that return a third type of property. >From your point of view, it's only natural that proxies would be an ES6 programmer's tool to define and experiment with entirely new types of properties. >From my point of view, it's problematic (as in: not backwards-compatible) that ES6+ would expose any new type of property via existing APIs at all (with proxies or even just directly in the spec itself). We will need some agreement on this higher-level point before we can make good decisions at the level of the Proxy API. What do others think? Cheers, Tom
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