Arrays with non-integer properties

liorean liorean at
Mon Oct 15 07:44:31 PDT 2007

On 15/10/2007, Serge LE HUITOUZE <slehuitouze at> wrote:
> More precisely, my question is about serialization.
> A chapter of the afore-mentionned standard is devoted to serializing an ECMA
> object in an XML form (a.k.a." EMMA form" in SISR wording). Along this chapter,
> interrogations come into play as to how some ECMA objects are serialized in
> "ECMA" format: It seems it's such an obvious (sub-)problem in the context of
> SISR standard that it is not even mentionned what governs this serialisation.
> I make the hypothesis that this format is the same as the way one writes a literal
> value in an ECMA program, though I can't get any confirmationon w3.voice forum...

The syntax of literals in ECMAScript 3 is far from capable of
representing the full set of object structures ECMAScript can contain.
Some examples would include:

- Function literals cannot represent the closure of a function object
- the scope chain goes missing when serialising.
- Host methods, objects and constructors may be serialisable but in
practice are not.
- Wrappers for primitives have no literal syntax at all.
- Circular references on objects are not allowed in literals. (But moz
has sharp variables for this.)
- Getter and setter functionality is not serialisable.
- Enumerability, removability, writability for properties are not serialisable.
- Prototype links are not serialisable.
- Array literals don't allow for properties with arbitrary non-uint32 names.
- Arrays may contain properties whose name is so high (e.g.
0xfffffffe) that serialising the entire array as a string with comma
separated elisions would require all of physical memory and then some.

Illustrating this last point - try this example (displaying the
results in a text node, so tries to serialise the array as a string):

    var array=[];

On my system it leads to:
- Saf eating all RAM, then paging to disk, and then crashing.
- Op eating memory up to a point (~1GiB in my test), then silently
fails without error.
- Ie stringify as "undefined" but otherwise behave reasonably.
- Moz... very slowly eats successively more and more ram, pegs the
CPU, and is otherwise entirely unresponsive. (Didn't want to wait it
out - in a quarter of an hour it's slowly grown from ~100MiB to ~300
MiB without showing any tendency to stop growing. But the growth curve
is markedly jagged, one second consuming 50 MiB more than the next

> [BTW, I don't know if ECMA's literal syntax is exactly the same as JSON, I'm
> interested in any clarification from you on this particular point.]

JSON is a subset of ECMAScript literals. It doesn't deal with
identifiers, reserved names, hexadecimal literals, optionally octal
literals, function literals, regexp literals etc.

> At some point of SISR chapter 7, more precisely item 5 in section 7.1, I find
> the following wording concerning EMMA (i.e. XML) serialization of ECMA arrays:
> "Any other properties of an Array object, for instance the keys of an associative
> array (e.g. a["prop"]), are subject to the same transformation rules as the
> regular properties of an object. In a sparse array, only those elements which
> hold defined values will be serialized."

That text is a bit wrong. It's the element and not the value that is
defined or not. A defined element can have undefined as it's value.
Serialisation should differentiate the two, because the elements that
are not defined can be elided in the serialisation while the elements
that are defined cannot be elided.

> Though this text describes the XML serialization, it clearly has implication on
> ECMA serialization as well: Indeed, it asserts as perfectly acceptable that an
> array object can have non-integer properties in addition to "regular" index
> properties.

Nothing surprising there. In fact, ECMAScript 3 itself does this: the
return arrays from regex matches have named properties.

> The question is then: How do you ECMA-serialize an array object having, in addition
> to index properties, non-integer properties?

There's no standard ECMAScript serialisation for this. It's not
covered by any literal syntax, you need a program snippet to represent

> >From my (short, but nevertheless painful) search in ECMA reference document,
> I was not able to find how you can specify literally an ECMA array with additional
> non-integer properties.

You cannot.

> The only answer I got on "w3.voice" was one suggesting to use the object notation to
> create "something" that mixes integer and non-integer properties. However, this "something"
> is nothing more than a regular object (though slightly unusual, since it has integer properites),
> not an array object with non-integer properties.
> It is thus obviously (at least, that's how I analyse it) not the way one should serialize
> such an array.

It cannot be serialised using JSON/ES3 literal syntax. Not if you want
to retain both the array-ness and the properties with non-array-ish
David "liorean" Andersson

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