Property ordering of [[Enumerate]] / getOwnPropertyNames()
es-discuss-2015-02 at jessemccarthy.net
Thu Aug 20 16:54:57 UTC 2015
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.
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)
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'])
Are those interpretations correct?
In this thread...
...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
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