On Dec 8, 2009, at 1:10 PM, Mike Samuel wrote:
> 2009/12/8 Erik Corry <[email protected]>:
>> 2009/12/8 Mike Samuel <[email protected]>:
>>> It occurred to me after looking at the proxy strawman that it might
>>> help to nail down what "array-like" means in future drafts. It isn't
>>> directly related to the proxy stuff though so I thought I'd start a
>>> separate thread.
>>>
>>> I've seen quite a bit of library code that does something like
>>> if (isArrayLike(input)) {
>>> // iterate over properties [0,length) in increasing order
>>> } else {
>>> // Iterate over key value pairs
>>> }
>>
>> This looks fairly broken to me. If the object has enumerable
>> properties that aren't positive integers then they don't get iterated
>> over just because some heuristic says it's array-like. If the
>> heuristic says it's array-like then we iterate over portentially
>> billions of indexes even if it is very sparse.
>
> All true. And yet it is not uncommon. See the bottom of this email
> for a quick survey of a number of libraries' uses of the array-like
> concept.
>
[...]
>
> Prototype.js dodges this problem by adding an each method to
> Array.prototype and others. This obviously fails for cross-context
> Arrays, and also suffers from the large sparse array and missing
> non-array-index properties problems.
>
> It does switch on arrays in places, and does so inconsistently though
> -- in one case (unnecessarily?) filtering out arguments objects?
If you look just a bit below those lines, you will see that this whole `concat`
method is only assigned to `Array.prototype.concat` when
`CONCAT_ARGUMENTS_BUGGY` is truthy.
`CONCAT_ARGUMENTS_BUGGY` feature test looks like this:
(function() {
return [].concat(arguments)[0][0] !== 1;
})(1,2);
and is essentially a workaround to an "issue" with Opera, which, contrary to
specs, makes `arguments instanceof Array`, yet fails to implement proper
`concat` algorithm for arguments (as feature test demonstrates).
As far as `isArray` in Prototype.js, we just test for [[Class]] === "Array"
(which is good across frames, but still fails across windows in IE).
There's, of course, no silver bullet `isArray` solution for a library. It's all
about specific requirements, and as long as users do not fully understand
language specifics, there will be broken expectations; some will expect
"arrays" to inherit from `Array.prototype` (and so have any of
`Array.prototype` methods available); some will expect arrays to have special
[[Put]] and `length`behavior; and others will expect array-like host objects
(that neither inherit from Array, nor have special [[Put]] or [[Class]] ===
"Array") to be considered arrays as well.
It's all about context ;)
[...]
--
kangax
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