On Dec 7, 2013, at 6:39 PM, Bjoern Hoehrmann wrote:
> * Allen Wirfs-Brock wrote:
>> On Dec 7, 2013, at 4:55 PM, John Cowan wrote:
>>> Allen Wirfs-Brock scripsit:
>>>
>>>> Similarly, the JSON texts:
>>>> {"1": 1, "2", 2}
>>>> and
>>>> {"2": 2, "1": 1}
>>>>
>>>> or the JSON texts:
>>>> {"a": 1, "a": 2}
>>>> and
>>>> {"a": 2, "a": 1}
>>>>
>>>> have an ordering of the object members that must be preserved by the
>>>> parser in order for downstream semantics to be applied.
>>>
>>> I cannot accept this statement without proof. Where in the ECMAscript
>>> definition does it say this?
>
>> In other words, ECMA-262 explicitly specifies that when multiple
>> occurrences of the same member name occurs in a JSON object, the value
>> associated with the last (right-most) occurrence is used. Order matters.
>>
>> A similar analysis applies to the first example.
>
> Your analysis does not demonstrate that `JSON.parse` preserves ordering.
> I am confident that even in the current ES6 draft `JSON.stringify` does
> not preserve ordering even if `JSON.parse` somehow did. It's based on
> `Object.keys` which does not define ordering as currently proposed. If
> you can re-create the key-value-pair order in your first example from
> the output of `JSON.parse` without depending on implementation-defined
> behavior, seeing the code for that would be most instructive.
You are correct that, ES5 does not define the for-in enumeration order. But it
does say that the Object.keys ordering must be the same as for-in enumeration
order. and there is a defacto standard for a partial enumeration order that all
browsers implement.
Quoting from
https://mail.mozilla.org/htdig/es-discuss/2009-October/010060.html
"The common behavior subset here is: for objects with no properties
that look like array indices, and no enumerable prototype properties,
for..in enumeration returns properties in insertion order. That
particular behavior is a de facto standard and required for Web
compatibility. A future standard should specify at least that much. "
Also https://mail.mozilla.org/pipermail/es-discuss/2010-December/012469.html
"We did identify one situation where enumeration order will be the same across
all major implementation that are currently in use (including IE6):
The enumeration order of an object's properties will be the order in which the
properties were added if all the following conditions hold:
The object has no inherited enumerable properties
The object has no array indexed properties
No properties have been deleted
No property has had its attributes modified or been changed from a data
property to an accessor property or visa versa
"
Also see https://mail.mozilla.org/pipermail/es-discuss/2011-March/012965.html
must other discussion history you can find in the es-discuss archives.
In practice, JavaScript implementation do have a standard enumeration order
that applies for the cases that most commonly when parsing and generating JSON
text. Application do depend upon that ordering.
Allen
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