array like objects

Yehuda Katz wycats at
Mon Dec 7 17:40:18 PST 2009

Your strawman would support:

{0: "strawman", length: 0}

A better definition might be:

   o is an array like object if o[[Get]]('length') returns a Number one
greater than the largest numeric property, or 0 if no numeric properties

Yehuda Katz
Developer | Engine Yard
(ph) 718.877.1325

On Mon, Dec 7, 2009 at 5:33 PM, Mike Samuel <mikesamuel at> wrote:

> 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
>   }
> but different libraries defined array-like in different ways.
> Some ways I've seen:
>    (1) input instanceof Array
>    (2) Object.prototype.toString(input) === '[object Array]'
>    (3) input.length === (input.length >> 0)
> etc.
> The common thread with array like objects is that they are meant to be
> iterated over in series.
> It might simplify library code and reduce confusion among clients of
> these libraries if there is some consistent definition of series-ness.
> This committee might want to get involved since it could affect
> discussions on a few topics:
>   (1) key iteration order
>   (2) generators/iterators
>   (3) catchall proposals
>   (4) type systems
> One strawman definition for an array like object:
>    o is an array like object if o[[Get]]('length') returns a valid
> array index or one greater than the largest valid array index.
> The need to distinguish between the two in library code could be
> mooted if for (in) on arrays iterated over the array index properties
> of arrays in numeric oder order first, followed by other properties in
> insertion order, and host objects like NodeCollections followed suit.
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