SpiderMonkey: r/o Error = 3 vs. var Error = 3;

I've made the Error constructor r/o to avoid users re-defining it
(JS_GetProperty w/ "Error" against global + JS_SetPropertyAttributes w/

This is executed in an ESContext which has a global object created by
JS_NewObject(jsc, class_with_no_constructor, NULL, NULL).
(class_with_no_constructor is essentially a static JSClass)

After which the standard classes are initialized against this global

The question is, why is it that executing "Error = 3" with the global
object as the scope results in no exception, whereas executing "var
Error = 3"  yield the following exception:

TypeError: redeclaration of const Error

? I would have assumed the opposite be true.  The result in both is
Error is not re-defined of course.

If the answer is read the ECMA spec go ahead and RTFM me :)

- Alex

6/7/2006 4:54:19 AM
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I've traced it down to js_SetProperty and the following code:

        if ((attrs & JSPROP_READONLY) ||
            (SCOPE_IS_SEALED(scope) && pobj == obj)) {
            JS_UNLOCK_SCOPE(cx, scope);
            if ((attrs & JSPROP_READONLY) && JS_VERSION_IS_ECMA(cx))
                return JS_TRUE;
            goto read_only_error;

and it's going in because attrs & JSPROP_READONLY is true.

interestingly, it seems if the version isn't ECMA, it would throw an
exception, otherwise it just silently returns w/o setting the property
value...interesting...and not what I'd expect (w/o reading ECMA *g*).

ah, and I see in a newer rev of this file this behavior is further tied
to JS_HAS_STRICT_OPTION.   Apparently this is an ECMA oddity.

now to start ECMA grepping

6/7/2006 5:18:12 AM
interesting, per Brendan

> Readonly properties, when set by assignment, are not changed, but no error or exception is thrown.


I'm guessing that if I created a new object proto'd on my global, an
executed "var Error = 3"  it would not throw an exception.

why the behavior is different between these two cases is pretty odd

6/7/2006 5:33:50 AM

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