Bug 197134 – Can't create a const attribute via the JSC C API

• Status: RESOLVED
• Resolution: WORKSFORME
• Component: JavaScriptCore

[Simon Fraser (smfr)]

There's no way that I could find to create a readonly, const attribute on a class via the JSC C API. I need a

I need the C equivalent of making an attribute with { JSC::PropertyAttribute::DontDelete | JSC::PropertyAttribute::ReadOnly | JSC::PropertyAttribute::ConstantInteger, NoIntrinsic }

This is for DumpRenderTree/WebKitTestRunner.

[Sam Weinig]

I think you can do this with using the staticValues array for a class definition:

static JSValueRef GetConstantValue(JSContextRef ctx, JSObjectRef, JSStringRef, JSValueRef*)
{
    return JSValueMakeNumber(ctx, 7);
}

JSStaticValue StaticValueArray[] = {
    { "constantValue", GetConstantValue, 0, kJSPropertyAttributeReadOnly | kJSPropertyAttributeDontDelete },
    { 0, 0, 0, 0 }
};

If memory serves me, JSC::PropertyAttribute::ConstantInteger is an optimization, and doesn't change behavior.

[Sam Weinig]

Or, if you don't want to go the callback route, you can create the properties after the object has been created.

JSObjectSetProperty(ctx, objectToAddPropertyTo, JSStringCreateWithUTF8CString("constantValue), JSValueMakeNumber(ctx, 7), kJSPropertyAttributeReadOnly | kJSPropertyAttributeDontDelete, &exceptionPtr);

[Saam Barati]

(In reply to Sam Weinig from comment #2)
> Or, if you don't want to go the callback route, you can create the
> properties after the object has been created.
> 
> JSObjectSetProperty(ctx, objectToAddPropertyTo,
> JSStringCreateWithUTF8CString("constantValue), JSValueMakeNumber(ctx, 7),
> kJSPropertyAttributeReadOnly | kJSPropertyAttributeDontDelete,
> &exceptionPtr);

I think this is what you want.

From the JS spec, these things mean:
- kJSPropertyAttributeReadOnly means you can't Put to the property. E.g, if you have kJSPropertyAttributeReadOnly property "foo", "x.foo = 42" won't succeed. This makes it a "writable: false" property.
- kJSPropertyAttributeDontDelete means the property descriptor is non-configurable. E.g, "delete x.foo" will fail. So will "Object.defineProperty(x, "foo", { ... })"

[Simon Fraser (smfr)]

OK, that seems to work.