array like objects

Garrett Smith dhtmlkitchen at
Sat Dec 12 11:21:45 PST 2009

On Fri, Dec 11, 2009 at 12:08 PM, Mike Wilson <mikewse at> wrote:
> Mark S. Miller wrote:
> If we're looking for a convention that is
> * does not admit any legacy ES3R non-array non-host objects (to prevent
> false positives)

No native ES objects?

> * does easily allow ES5 programmers to define new array-like non-array
> objects
> * takes bounded-by-constant time (i.e., no iteration)

*What* takes bounded-by-constant time? The object's existence would
not take any time.

> * is a reasonably compatible compromise with the existing notions of
> array-like in legacy libraries as represented by previous examples in this
> thread

Can you please clarify what the problem area is a little more?

> then I suggest:
> function isArrayLike(obj) {
>   var len;
>   return !!(obj &&
>             typeof obj === 'object' &&
>             'length' in obj &&
>             !({}), 'length') &&
>             (len = obj.length) >>> 0 === len);
> }

 Is looks like "array like" is defined as an an object where "length"
is non-enumerable and numeric. What about [[Get]], [[HasProperty]] for
numeric property names?

And why must the - length - property be an *own* property? Why could
length not be a getter in the prototype chain?

Indeed many Mozilla DOM collections work this way:

javascript: var cn = document.links; alert([cn.length,
({}) cn, "length")]);

Mozilla elerts "80, false"


More information about the es-discuss mailing list