iteration order for Object

David Bruant bruant at
Mon Mar 14 09:42:00 PDT 2011

Le 14/03/2011 17:02, John Tamplin a écrit :
> On Mon, Mar 14, 2011 at 10:21 AM, Brendan Eich <brendan at
> <mailto:brendan at>> wrote:
>     Web developers find and exploit many de-facto standards.
>     Enumeration order being insertion order for non-arrays at least,
>     if not for all objects (arrays tend to be populated in index
>     order), is old as the hills and web content depends on it, as far
>     as I can tell. I'll do some experiments to try to get data on this.
> Aside from the JSON example of populating a dropdown list given (which
> I will agree is a real if contrived use case), there has been a lot of
> talk of "thousands of web developers" depending on preserving
> insertion order, but not one concrete example -- do you have one?
I gave one
It is theorical (sorry, not concrete like "there is code using it right
now"), but the point is that for objects (actual objects not arrays)
used as "dictionaries", numbers could be used as "alphabetic keys". When
the user writes objects as object literals in code, they might (not a
single proof since it has never been the case) appreciate if the JS
engine kept the order they have written the key in.

The order the user provides the keys in is the only bit of information
(I can think of) that the JS engine looses. But once again, if users
would have appreciate this feature, they have been forced (by spec and
implementations) to find other ways. So, unless we can reach all web
devs to ask "have you ever been disappointed of the implementation
for-in loop order?", you cannot really have facts on if they would use
the feature. Apparently, for the case of non-numeric properties, they
seem satisfied of the implementation which iterate them over

For more concrete numerical results, let's wait for Brendan to do his
experiments (or help him out if there is any way to do so?)

I think that in-memory objects (created as such) for-in enumeration and
JSON.parse created objects are two different concerns. Of course, they
should certainly be solved consistently, but they are different in my

I am a bit worried about JSON. JSON as an interchange format is defined
as unordered. "An object is an unordered collection of zero or more
name/value pairs". If people expect JSON as an interchange format to be
ordered, they are assuming something about the JSON parse method which
isn't itself standardized either.

One conceptual issue I see with ordering in the ES spec
JSON.parse-created objects is that the ECMA spec would explicitely say
"we consider that objects are ordered and we decide an order for
JSON.parse-created objects, (we decide of an order while there isn't in
the JSON format)". Once again, it's conceptual. I would be in favor of
adding an order, but it had to be said.
All of that said, it's a bit weird to say that an interchange format (so
something with a beginning, an end and a direction) has some "unordered"
part. The JSON grammar itself seem to contain an implicit order:
JSONMemberList :
    JSONMemberList , JSONMember

So I don't know. Does it really make sense to define JSON objects as an
unordered collection in the interchange format (in-memory objects are a
different story) or is it just an ECMAScript legacy?

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