array like objects

Brendan Eich brendan at
Sat Dec 12 12:05:52 PST 2009

On Dec 12, 2009, at 11:36 AM, Mike Wilson wrote:

> Mark S. Miller wrote:
> On Sat, Dec 12, 2009 at 10:36 AM, Mike Samuel <mikesamuel at>  
> wrote:
> On the String defect, we could repair that with
>    && ({}) !== '[object String]'
> Cons:
>   An extra function call in the likely case
>   Strings are arguable array-like
> Pros:
>   Strings are inconsistently indexable : (new String('foo'))[0] is
> undefined on IE 6
> Yes, this test would work. Before incorporating it, we should  
> discuss whether String wrapper objects should indeed be considered  
> array-like. My inclination is that they should not. Opinions?
> I don't think strings should trigger array-like, and this seems to  
> be the position held by most JS libraries (see f ex  
> dojo.isArrayLike). The normal use of these methods is to identify  
> things like Arguments and NodeLists, ie sequences of distinct  
> objects/values, not character sequences (text).

A String object is indexable (to get unit-length strings -- not  
characters) with length as fencepost above the maximum index.

A string or String object wrapping it has immutable .length and [i]  
for i < length properties, OTOH -- it is not frozen, but this raises  
the question: is a frozen Array instance no longer array-like?

Prototype's isArray wants 'splice' in obj && 'join' in obj, but that  
rules out many objects considered array-like, without monkey-patching.

We need a definition of array-like that matches common practice, if  
there is a common practice. Then we can see (without prejudgment)  
whether Strings satisfy it.

 From Mike Samuel's posts and my own quick reading, I don't see too  
much common practice. A more detailed survey of popular Ajax libraries  
would be great -- I'm out of time right now, but I'll try later today  
unless someone else does it first.

> Anyhow, whatever is decided here I think primitive strings and  
> String wrappers should be handled the same way in this respect.



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