Finiteness of object properties set

David Bruant david.bruant at
Wed Sep 7 06:41:14 PDT 2011

Le 07/09/2011 15:29, Lasse Reichstein a écrit :
> On Wed, Sep 7, 2011 at 2:07 PM, David Bruant <david.bruant at 
> <mailto:david.bruant at>> wrote:
>     If this is really considered, the problem to solve is to express
>     an infinite set with a finite representation. RegExps may be a
>     solution. One big question is on RegExp expressivity: Can any
>     infinite string set be expressed with (a finite number of) JS
>     RegExps? 
> Nope. Any finite set of RegExps can be combined into a single RegExp, 
> so the language they recognize is still regular (or what the class of 
> languages recognized by JS RegExps really is, since it's bigger than 
> the regular languages, but doesn't include all context free langauges).
Very true. I had a doubt on whether RegExp were purely limited to 
regular languages. I have never really taken the time to check whether 
RegExps were just providing sugar for regular languages matching or if 
they were going further than that. But hopefully, you're right.

> As for proxies with arbitrarily many properties, I don't think there's 
> a way to prevent it (it's the catch-all effect).
Extensible proxies are not a problem. Non-extensible proxies are if the 
specs asks for non-extensible objects (then proxies) to have a finite 
set of properties. Current design and prototype implementation [1] seem 
to go in the direction of non-extensible proxies having a finite set of 
properties even though still "catching all". See L.262-273 where is 
defined this.fixedProps which is a regular object (finite number of 


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