Subj : Re: Calling functions without the lookup
To   : Brian Barnes <ggadwa@voyager.net>
From : Brendan Eich <brendan@meer.net>
Date : Wed Jan 15 2003 09:31 am

Brian Barnes wrote:

> Brendan Eich wrote:
>
>> Brian Barnes wrote:
>>
>>> jsval        myfunc;
>>>
>>> JS_GetProperty(cx,obj,"BLECH",&myfunc);
>>> JS_CallFunctionValue(cx,obj,"BLECH",argc,&argv,&rval); 
>>
>>
>> You must mean
>>
>>  JS_CallFunctionValue(cx, obj,
>>                       (JSFunction *) JS_GetPrivate(cx, 
>> JSVAL_TO_OBJECT(myfunc),
>>                       argc, &argv, &rval);
>>
>> a jsval is not a JSFunction*, or even a tagged JSFunction*. 
>
>
> Need to not type so fast.


Me too! ;-)

>   What I meant was:
>
> JS_CallFunctionValue(cx,obj,myfunc,argc,&argv,&rval);
>
> JS_CallFunctionValue takes a jsval, not a JSFunction (from the code). 


Right -- I was clearly trying to show how to use JS_CallFunction.  But 
JS_CallFunctionValue is better for the case you're interested in.

> What I did was just to tear JS_CallFunctionName apart, so I could 
> avoid calling the same code (which is basically the JS_GetPrivate) 
> over and over again.
>
> So, trying again:
>
> jsval        myfunc;
> JS_GetProperty(cx,obj,"BLECH",&myfunc);
> JS_CallFunctionValue(cx,obj,myfunc,argc,&argv,&rval);
>
> Should be equivalent to JS_CallFunctionName with BLECH.  It's what the 
> JS_CallFunctionName does.  I'm calling into a compiled script and I 
> don't expect somebody to redefine the function (if they do, they will 
> get an error, as they should.)
>
> I always miss something super technical when talking with you Brendan! 
> I'm not sure why I need to cast to JSFunction or do the getprivate 
> instead of what I have above. 


You don't, I was just showing the JS_CallFunction alternative (but I 
forgot to change the name).  The short answer is you're doing the right 
thing.

But (more super-technical detail alert) you may not be able to use that 
API anyway, if you need to preserve the scope chain (linked by the 
parent slot) of the object tagged in myfunc.  If you extract that 
object's private data (a JSFunction*, call it fun), and then call that 
function via JS_CallFunction, the API will use fun->object as the 
callable object to invoke -- which means you'll get the scope chain that 
the compiler saw when it created fun.

That is not necessarily the right scope chain, if you are using 
JS_CloneFunctionObject, or if the function object you got from the 
obj.BLECH property was cloned for you automatically.  If it came from a 
nested function declared in JS, or even a top-level function in a script 
that you precompile in a null top-level object, or a "compile-time only" 
top-level object used for precompiling, if the script was executed 
against a different top-level, then the functions declared in it at 
top-level will be automatically cloned.  Capturing the value of such a 
function in script, and storing it in obj.BLECH, could lead to a 
situation where you can't use JS_CallFunction from native code to invoke 
the function with the right scope chain.

In other words, JSObject => JSFunction is many to one, while JSFunction 
=> JSObject (fun->object) is a 1:1 link back to the compiler-created 
function object.

I hope this helps.

/be

.