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Re: Storage 'length' and enumeration

From: Ian Hickson <ian@hixie.ch>
Date: Thu, 30 Apr 2009 00:07:14 +0000 (UTC)
To: John J Barton <johnjbarton@johnjbarton.com>
Cc: public-webapps@w3.org
Message-ID: <Pine.LNX.4.62.0904300001490.829@hixie.dreamhostps.com>
On Wed, 29 Apr 2009, John J Barton wrote:
> So why call the property |length|? Wouldn't an enumerable JS host object 
> be just as fabulous with getNumberOfItems()?

All the other host objects use |length|.

> But the part that has me confused is maybe just me being dumb.  I just 
> have a hard time with:
>    sessionStorage[2] = "foo";  // set the key |2| to value "foo".
> then
>   var x = sessionStorage[2];  // null? "foo"? 
>   var y = sessionStorage[0]; // "2"
> I'm thinking: why do I have to think so hard about this? It should be 
> just an associative array.

You only have to think hard about this if you use numeric keys and use the 
implied accessors. Just use .setItem("2") and .getItem("2"), or use a 
non-numeric key, and then there's no confusion.

> Firebug shows objects from Firefox to developers. The appropriate 
> display format for objects depends on the character of the objects and 
> the needs of developers. For example, arrays are shown in square 
> brackets with the first few entries, ["foo", "bar", ...]. HTML Elements 
> are shown with their nodeName and id if any.  In this way developers can 
> quickly get an idea of the nature of the object, and perhaps drill down 
> for more information.
> How many display formats should be created? One for every kind of object 
> is simply impractical.  Even if time allowed to create formats for all 
> the built-in types, all the DOM types, all the library types (prototype, 
> jquery, dojo,...) etc, there would still be user types. So you have to 
> create categories of representations that cover the important cases.  
> Firebug has about thirty.
> Now given an object, how do we assign it to one of these thirty 
> representations?  The only possibility is to examine the properties of 
> the object.

That's no the only possibility by a long shot. In fact it's not even a 
particularly good option; I would recommend using "instanceof" and the 
like instead.

> Of course there are objects that are not arrays and yet have length, eg 
> Strings.

There are lots of objects that are not arrays (or should not be rendered 
as arrays) and yet have length. Collections, TimeRanges, <select> 
elements, Window objects, History, CanvasPixelArrays, etc.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
Received on Thursday, 30 April 2009 00:07:53 UTC

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