Nick Fitzsimons wrote:
> 2009/7/20 Ariel Flesler <afles...@gmail.com>:
>> I'm really aware of how the different interpreters work, but it seems to me
>> that immutable literals (strings, numbers, regexps) are the first thing I'd
>> cache in a constant. Maybe current interpreters do that already internally ?
>>
> ECMA-262 3rd Edition section 7.8.5 [1] states:
>
> "A regular expression literal is an input element that is converted to
> a RegExp object (section 15.10)
> when it is scanned. The object is created before evaluation of the
> containing program or function begins.
> Evaluation of the literal produces a reference to that object; it does
> not create a new object."
This is changing in ECMAScript 5. Each evaluation of a regexp literal
will (in the future) create a new object. (I think IE already does
this, and they got this one right. The other browser vendors will change
their implementations to match.) So this is the perfect time for John
to be minimizing the number of literals in the program.
David Flanagan
> So inline regular expressions are not evaluated multiple times, they
> are turned into a RegExp object at compile time and become a reference
> to that object. This means that there is unlikely to be any benefit
> from assigning them to a variable, and if dereferencing that variable
> takes longer than simply dereferencing the RegExp object reference,
> then moving them to the variable would actually decrease performance.
>
> I would assume that this is something that all JS engines have
> implemented correctly, as it would take more work to _fail_ to
> implement it according to spec. Certainly, Microsoft's document
> "JScript Deviations from ES3" [2] has no mention of JScript's
> implementation violating this portion of the spec.
>
> RegExp objects created at runtime using the RegExp constructor would
> however benefit from caching; in fact I applied this technique in my
> own implementation of getElementsByClassName a couple of years ago.
>
> Cheers,
>
> Nick.
>
> [1] <http://www.ecma-international.org/publications/standards/Ecma-262.htm>
> [2]
> <http://wiki.ecmascript.org/lib/exe/fetch.php?id=resources%3Aresources&cache=cache&media=resources:jscriptdeviationsfromes3.pdf>
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