[webkit-dev] Question about Constructors in WebKit JS Bindings

Maciej Stachowiak mjs at apple.com
Mon Jun 22 23:07:48 PDT 2009

On Jun 22, 2009, at 10:07 PM, Adam Barth wrote:

> On Mon, Jun 22, 2009 at 9:58 PM, Peter Kasting<pkasting at google.com>  
> wrote:
>> On Mon, Jun 22, 2009 at 9:53 PM, Adam Barth <abarth at webkit.org>  
>> wrote:
>>> On Mon, Jun 22, 2009 at 7:04 PM, Maciej Stachowiak<mjs at apple.com>  
>>> wrote:
>>>> Your proposed alternative will have different behavior. It will  
>>>> use the
>>>> lexical global object of the calling JavaScript function, instead  
>>>> of the
>>>> global object originally associated with the Options constructor.
>>> Yes.  Almost everywhere you see this pattern it's incorrect.  We  
>>> have
>>> this bug a lot.
>> To be clear, are you saying the original pattern, or the "simpler"  
>> one Drew
>> proposes is typically the incorrect one?  (I assume the latter.)
> The latter one.  The V8 binding are better about this because we have
> v8::Context::GetCurrent(), which often gives you the context the JSC
> bindings have to manually manage.

I suspect most JS bindings in WebKit use the original more complicated  
pattern Drew cited, instead of the more succinct but incorrect one.

> Here's how the two bindings relate (if you look at both bindings and  
> wonder):
> v8::Context::GetEntered() ~~ exec->dynamicGlobalObject()
> v8::Context::GetCalling() ~~ exec->lexicalGlobalObject()
> v8::Context::GetCurrent() ~~ ????
> It might be good to add a similar "GetCurrent" concept to JSC
> (although, hopefully with a better name) so we can remove all this
> manual, error-prone caching / mark()ing.

What does the "current" context correspond to? How is it retrieved?  
Guessing wildly based on the names, I'm assuming host functions in v8  
have the equivalent of their own scope chain and thus in some sense  
their own lexical global object. We could do that for JSC, except that  
for most things it would be useless - it's only potentially useful for  
DOM-level explicit constructors, since JS-level ones are supposed to  
use the calling function's lexical global object as the basis for  
everything, and most other host functions don't need it at all.


More information about the webkit-dev mailing list