Mark S. Miller wrote:
> On Fri, Dec 11, 2009 at 2:27 AM, Mike Wilson <mike...@hotmail.com> wrote:
>
>> I think Breton mentions something important here; the desire
>> to actually detect if something is an array or arraylike to
>> be able to branch to different code that does completely
>> different things for array[likes] and objects.
[...]
>
> If we're looking for a convention that is
> * does not admit any legacy ES3R non-array non-host objects (to prevent
> false positives)
> * does easily allow ES5 programmers to define new array-like non-array
> objects
> * takes bounded-by-constant time (i.e., no iteration)
> * is a reasonably compatible compromise with the existing notions of
> array-like in legacy libraries as represented by previous examples in this
> thread
>
> then I suggest:
>
> function isArrayLike(obj) {
> var len;
> return !!(obj &&
> typeof obj === 'object' &&
> 'length' in obj &&
> !({}).propertyIsEnumerable.call(obj, 'length') &&
> (len = obj.length) >>> 0 === len);
> }
>
> Since getting 'length' may have side effects, this is written a bit weird so
> that this get only happens after earlier tests pass.
If you want to avoid side effects:
function isArrayLike(obj) {
if (!obj || typeof obj !== 'object') return false;
var desc = Object.getPropertyDescriptor(obj, 'length');
if (desc) {
var len = desc.value;
return !desc.enumerable && (len === undefined || len >>> 0 === len);
}
}
This allows any length getter without checking that it will return an array
index, but it still satisfies all of the above requirements.
However, I don't see why the check on the current value of length is
necessary. For it to make any difference, there would have to be a
*nonenumerable* length property on a non-function object, with a value
that is not an array index. How and why would that happen?
> And yes, I'm aware that this usage of Object.prototype.propertyIsEnumerable
> implies that catchalls must virtualize it in order for a proxy to be able to
> pass this test :(.
Same with Object.getPropertyDescriptor in the above.
--
David-Sarah Hopwood ⚥ http://davidsarah.livejournal.com
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