Hi David,
We have a strawman for making the enumeration order well-specified:
http://wiki.ecmascript.org/doku.php?id=strawman:enumeration
Would that not be sufficient for the defineProperties case? I'd prefer that to
adding another trap.
Dave
On Mar 12, 2011, at 1:15 PM, David Bruant wrote:
> Hi,
>
> The thread entitled "iteration order for Object" made me realize that there
> is no defineProperties trap.
>
> Let's imagine I'd like to implement an object which would keep track of order
> in which own properties have been added/deleted to it. With the
> defineProperty and delete traps I can already track order of
> addition/deletion and reflect it on the enumerate/keys trap when required.
>
> However, as of ES5, if I do:
> Object.defineProperties(proxy, props)
> then the defineProperty trap is called for each own property of props with an
> order chosen by the ES implementation (it's the same order than the one used
> in for..in which is implementation-dependent). Consequently, I cannot
> guarantee the object user that these keys will be enumerated with the order
> s/he provided them in props.
>
> A defineProperties trap could allow me to throw an error saying roughly "I
> can't garantee my invariant with such a call, please tell me explicitely the
> order you want for your properties by using several Object.defineProperty".
>
> It would obviously be a derived trap. The implementation is quite
> straightforward (based on ES5 Object.defineProperties algorithm 15.2.3.7)
> with one subtility: when retrieving all own enumerable property names of the
> props object, if props is itself a proxy, then the keys trap of this proxy
> would be called.
>
> David
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