Subj : .prototype behaves two different ways (SpiderMonkey)
To   : netscape.public.mozilla.jseng
From : Jason Straathof <jstraathof@cannospamada.com>
Date : Tue Jan 14 2003 04:12 pm

Hello.

Here's what I'm trying to do, with the SpiderMonkey engine:

I want to be notified when a script sets properties on a function's 
prototype, which is useful for many reasons. In this case, I wish to 
update my user interface when certain "inherited" values change.

To do this, I am using the setProperty pointer of a JSClass instance, 
and it mostly works very well.

Consider (JavaScript):

     //---

     // Works on instances of my super class...
     var myInstance = new SuperClass();
     myInstance.color = "red"; // setProperty JSPropertyOp called nicely.

     // Define subclass...
     function SubClass() {
         // ... blah, blah ...
     }
     SubClass.prototype = new SuperClass();

     // Set prototype property...
     SubClass.prototype.color = "red";
             // No call to setProperty JSPropertyOp. Why?

     //---

The setProperty function is not called when I set any property of the 
prototype object - *in script*. If I perform a similar operation using 
the API, however, the thing works.

Consider (C code):

     //---

     // Variables...
     JSContext* cx;
     JSVal      result;
     JSObject*  classObj;
     JSObject*  protoObj;

     // ...

     JS_GetProperty(cx, JS_GetGlobalObject(cx), "SubClass", &result);
     classObj = JSVAL_TO_OBJECT(result);
     protoObj = JS_GetPrototype(cx, classObj);
     JS_SetProperty(cx, protoObj, "color", JSVAL_VOID);
             // setProperty JSPropertyOp called as I would expect.

     // ...

     //---

Can anyone tell me why this would be the case? Why does the prototype 
work in these two very distinct ways? Most importantly, how can I get 
the prototype to work the way I expect, as it does with the API, from 
within the script (which is where it really counts)?

If you are curious, I have also tried, within the script, assigning some 
variable the prototype, and performing the assignment there...

     var x = SubClass.prototype;
     x.color = "red"; // setProperty function not called.

Additionally, sending the prototype to a function where the assignment 
is performed does not work...

     function setStinkinColor(o, color) {
         o.color = color;
                 // JSPropertyOp not called when o is a prototype.
     }
     setStinkinColor(SubClass.prototype, "red");
             // doesn't call setProperty JSPropertyOp.

I am very confused. Any help will, of course, be greatly appreciated.

Jason.

.