This doesn't answer the spec question, exactly, but v8 doesn't currently conform to ES6's requirements for [[OwnPropertyKeys]]: some objects return names in an implementation-dependent order (see https://code.google.com/p/v8/issues/detail?id=3056 for more details).
On Thu, Aug 20, 2015 at 9:54 AM, Jesse McCarthy < [email protected]> wrote: > I just want to confirm some things about property enumeration order of > plain Objects. I apologize that this has probably already been discussed > before, but it's hard to locate a clear answer. Please note that I'm solely > asking about ES2015 spec compliance, not what's in the wild. > > Given: > > ```js > var y = Object.create(null); > // Note that property names are all strings. > y.one = 1; > y.two = 2; > y.three = 3; > ``` > > This is my understanding of what's guaranteed (or not) about enumeration > order (in the context of the example given): > > 1. No guarantee of order > Anything that relies on or has property-order equivalence with > `[[Enumerate]]` or `EnumerableOwnNames` > 1b. for (x in y) > 1c. Object.keys(y) > > This is based on the statement in 9.1.11 [[Enumerate]] () step 1: > >> The mechanics and order of enumerating the properties is not specified >> > > Although it says that... > >> [[Enumerate]] must obtain the own property keys of the target object as >> if by calling its [[OwnPropertyKeys]] internal method. >> > > ...and `[[OwnPropertyKeys]]` specifies ordering, my reading is that > `[[Enumerate]]` doesn't guarantee that the iterator it returns will > preserve the order returned by `[[OwnPropertyKeys]]`. > > 2. Guarantee of insertion order (['one', 'two', 'three']) > Object.getOwnPropertyNames(y) > > Are those interpretations correct? > > Related: > In this thread... > https://esdiscuss.org/topic/nailing-object-property-order > > ...Bergi asked these pertinent questions that no one answered: > >> But why was the default object [[enumerate]] algorithm not specced to >> match the [[OwnPropertyKeys]] order then? >> ... >> Shouldn't we add a guarantee to [[enumerate]] that the subset of >> enumerated own properties comes in insertion order as well? >> > > This is partly in reference to http://stackoverflow.com/a/30244410/1034448 > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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