From: Phrogz <gavin@...>
Date: 2006-08-22T22:55:07+09:00
Subject: Re: Why Does Hash Apparently Reorder Its Internal Representation And Other Associated Ponderings

Hal Fulton wrote:
> Phrogz wrote:
> > And, for what it's worth, JavaScript's Object primitive is also an
> > associative array that retains insertion order for iterating keys,
> > while exhibiting performance characteristics of a hash.
>
> I find that interesting, since so many appear convinced that
> a so-called ordered hash would be "too slow."
>
> Do you know anything about the internal implementation?

I don't. I wouldn't call most JavaScript engines 'fast', but I suspect
that has to do with aspects other than the native Object type.

Without having tried it, I would think that preserving insertion order
would be a (small) memory size addition to each hash (as you've
mentioned before), a *tiny* slowdown in the speed to store a new entry,
and no performance change in accessing an entry.

Anyhow, because I'm too lazy to do so, you (or someone else) can get
source code for the SpiderMonkey[1] and Rhino[2] and check out how
insertion order is preserved on Object.

[1] http://www.mozilla.org/js/spidermonkey/ - the C-based JS engine
[2] http://www.mozilla.org/rhino/ - the Java-based JS engine