Strengthening Function.prototype.toString

Juriy Zaytsev kangax at
Fri Sep 26 19:29:00 PDT 2008

On Sep 26, 2008, at 5:32 PM, Hallvord R. M. Steen wrote:

> On Fri, 26 Sep 2008 19:58:27 +0200, liorean <liorean at> wrote:
>> 2008/9/26 Erik Arvidsson <erik.arvidsson at>:
>>> I know that Opera on mobile phones used to return a string
>>> representation that did not reflect the original.
>> Yeah. Opera Mobile returned "[ECMAScript code]" or "[ecmascript
>> code]". This was contrary to the ES3 spec (must be parsable as a
>> function definition, IIRC) and also breaks the eval roundtripping by
>> throwing a parse error.
>> Anybody know if those issues have been fixed in more modern versions?
> No, not consistently across "modern versions". It's not likely to be
> properly "fixed" for a while yet. The reason is that on many platforms
> where memory is scarce, not enabling JS decompilation helps reduce  
> memory
> requirements.
> Unfortunately several major libraries (prototype in particular,  
> jQuery to
> a minor extent) have started (ab)using decompilation for various  
> purposes.
> Pages that rely on this tend to malfunction in Opera on low-memory
> devices. Some Opera versions make at least some of these pages work  
> again
> by returning "function(){[ecmascript code]}" instead but that's of  
> course
> a silly hack..
> (It would be great if ES-next could have a look at why these libraries
> rely on toString - hacks and make it possible to do equivalent  
> things with
> nicer language constructs. One of the reasons toString is used is  
> probably
> outside the group's scope and caused by typeOf bugs in IE: prototype's
> isFunction method uses toString in order to reliably detect whether
> something is a function reference. Another usage is apparently -  
> from my

That's what jQuery's `isFunction` used to do. Afaik, they reverted it  
to a simple `instanceof` check just recently. In prototype.js, on the  
other hand, we tried to work around cross-frame "issues" that arise  
when using `instanceof`. Doing a `typeof` check (of both an object and  
its call property) seems to be sufficient.

> memory - to look at the names of the named arguments and do something
> magic if the first argument is $super. I'm not up-to-date on whether  
> the
> next iteration has an elegant solution for this apparent requirement.)

Latest build does the same thing. That "something magic" is actually  
just creating a reference to a "super" method (same-named method of a  
parent class). Function decompilation, while non-standard, seems to be  
the best solution when implementing such inheritance mechanism. We  
would gladly consider alternative (more compliant) solutions, if any  

> -- 
> Hallvord R. M. Steen
> Core QA JavaScript tester, Opera Software
> Opera - simply the best Internet experience
> _______________________________________________
> Es-discuss mailing list
> Es-discuss at

Juriy Zaytsev

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