Subj : Re: Using JS_InstanceOf with jsval's
To   : Mike Gibson <gibson@cs.utah.edu>
From : Brendan Eich <brendan@meer.net>
Date : Tue Jan 20 2004 01:54 pm

Mike Gibson wrote:

>I'm having problems getting JS_InstanceOf to return what I expect when
>I use jsval's returned from scripts.  I create s simple object model
>with the class "A" as the parent of "B".  JS_InstanceOf then tells me
>"new A();" is an instance of "B", which isn't correct.  Am I using it
>right?
>
>With the following JavaScript (test.js):
>
>  function A() { this.a = 1; }
>  function B() { this.b = 2; }
>  B.prototype = new A;
>
>I run the following C++ code (standard init code snipped):
>
>  // Compile and run the above JavaScript
>  ... snip ...
>
>  char *s;
>  jsval aval, bval;
>  s = "a = new A();";
>  ok = JS_EvaluateScript(cx, glob, s, strlen(s), "", 0, &aval);
>  s = "b = new B();";
>  ok = JS_EvaluateScript(cx, glob, s, strlen(s), "", 0, &bval);
>  
>  // this JS_Instance call returns true when it should return false
>  JS_InstanceOf(cx, JSVAL_TO_OBJECT(aval),
>                    JS_GetClass(JSVAL_TO_OBJECT(bval)), NULL));
>

The JS_InstanceOf API is six+ old, so it predates the instanceof 
operator by several years and (in spite of the name) does something more 
than a little bit different.  It is defined to return true if obj is an 
instance of the JSClass, and since user-defined constructors such as A 
and B all construct instances of the native Object class, the answer 
should be true here.

Here is some JS shell session copy/paste:

js>   function A() { this.a = 1; }
js>   function B() { this.b = 2; }
js>   B.prototype = new A;
[object Object]
js> var a = new A
js> var b = new B
js> b instanceof A
true
js> b instanceof B
true
js> a instanceof A
true
js> a instanceof B
false

This shows correct behavior, per ECMA-262 Edition 3.

If you want an API to tell what the instanceof operator's result would 
be, use this:

    JSObject *b = JSVAL_TO_OBJECT(bval);
    JSObject *B = JS_GetConstructor(cx, b);
    JSBool answer;
    JS_GET_CLASS(cx, B)->hasInstance(cx, B, JSVAL_TO_OBJECT(aval), &answer);

I've left out error handling, as usual with these sorts of sketches.  
Sorry there isn't a JS_HasInstance API; also, sorry we can't rename 
JS_InstanceOf, but that's a cost of business with long-lived C APIs.

/be

.