for-in evaluation order

[Andreas Gal]

Note that almost all implementations break for-in enumeration order for dense arrays, for some definition of dense.

Example (firefox):

a = []
a[2] = 1
a[1] = 1
a[0] = 1
for (i in a) alert(i)

will alert 0, 1, 2 (in that order).

Now if you do
a["x"] = 1

you get 0, 1, 2, "x"

However, if we start out with x, we get this:

a = []
a["x"] = 1
a[2] = 1
a[1] = 1
a[0] = 1

Will alert x, 2, 1, 0

Even worse, this is likely to change in the future (we want to give all objects a dense, indexed part).

Most browser behave similar with respect to indexed properties. For good performance you really want to ignore enumeration order for array-like objects, but if the implementation falls back onto regular objects, you can suddenly observe true property addition order.

If we try to specify something, implementors will probably object, and each implementation will have specific objections depending on their particular array and object representation.

Andreas

On Dec 27, 2010, at 12:52 PM, Michael Day wrote:

> Thanks Boris, Dmitry,
> 
>>> Are objects implicitly ordered, by implementation convention?
>> For implementations that have to deal with the web, yes.
> 
> Okay, now I know what we have to do then :)
> 
> Might I suggest that this be added to the spec, if only in an informative note, to save future implementers the trouble?
> 
> Best regards,
> 
> Michael
> 
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