Am 02.08.2006 um 07:14 schrieb Sebastian Werner:
>>> Ah. Cool. Thank you for the information. But this doesn't explain
>>> why
>>> for some stuff arrays are faster. If arrays really are built up with
>>> hash maps why are they faster for common array actions like loops?
>>
>> Because you can use simple counter loops ("for (i=0;i<x;i++)") to
>> enumerate the keys (instead of "for (x in y)"). I think when you
>> iterate over an array with for/in, you get the same performance as
>> with any other object. I can't say whether there are some more
>> internal optimizations for arrays, but at least semantically they
>> behave like ordinary hash maps (with the exception of having an
>> automatically updated "length" property whenever numeric keys are
>> used).
>
> But nobody really knows it, or is this explained in any reference.
It's explained in the ECMAScript spec: http://www.ecma-
international.org/publications/files/ECMA-ST/Ecma-262.pdf (section
15.4, page 88). This section states that an array simply uses special
treatment for certain property names (namely numeric ones in a
certain range), but otherwise it's just an ordinary property
container like any other JavaScript object.
> Generally they could have a comparable API with completely different
> implementations.
From reading the spec, I don't think this is true.
> Mhh, I don't see why a counter should be faster than enumerate over
> the
> keys. I there a reason to think it must be slower?
Of course. I think it's fairly obvious that iterating over a
collection and reading the values out of it is slower than simply
adding to a variable.
Just for kicks, I performed a short test, and for/in was 7-8 times
slower than the counter loop:
var keyCacheLength = keyCache.length;
for (var i = 0; i < 2000; ++i) {
for (var j = 0; j < keyCacheLength; ++j) {
key = keyCache[j];
}
}
vs.
var keyCacheLength = keyCache.length;
for (var i = 0; i < 2000; ++i) {
for (j in keyCache) {
key = keyCache[j];
}
}
(The keyCache array has about 200 entries.)
Regards,
Andreas
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