Upgrading from Spidermonkey 1.8.5 to 45. How to get property name in getters/setters now they are JSNatives and tiny IDs have been removed?

Hi,

I'm trying to upgrade my embedding from 1.8.5 to 45 and one of the things that I'm struggling with is how property getters/setters now work in 45.

In 1.8.5 I used tiny IDs and JSPropertySpec to define properties and getters/setters for a class.
e.g. In 1.8.5 I had something like

enum part_tinyid {
    PART_HEADING,
    PART_PID,
    PART_SECID,
    PART_MID,
    ...
};

JSPropertySpec dj_Part_dynamic_props[] =
{
    {"heading", PART_HEADING,  flags,  dj_get_part_prop, dj_set_part_prop },
    {"pid",     PART_PID,      flags,  dj_get_part_prop, dj_set_part_prop },
    {"secid",   PART_SECID,    flags,  dj_get_part_prop, dj_set_part_prop },
    {"mid",     PART_MID,      flags,  dj_get_part_prop, dj_set_part_prop },
...
    { 0 }
};   


and then my getter was something like

JSBool dj_get_part_prop(JSContext *cx, JSObject *obj, jsid propid, jsval *vp)
{
    jsval id = JS_IdToValue(cx, propid, &id);
    int idint = JSVAL_TO_INT(id);

    switch (idint)
    {
        case PART_HEADING:
            ...
            break;

        case PART_PID:
            ...
            break;
    }

}

i.e. I had a single getter and a single setter for all of the properties and I could find which property I was getting/setting from propid

Now in Spidermonkey 45 Tiny IDs have been removed and dj_get_part_prop/dj_set_part_prop are JSNatives so I have something like:

JSPropertySpec dj_Part_dynamic_props[] =
{
    JS_PSGS("heading", dj_get_part_prop, dj_set_part_prop, flags),
    JS_PSGS("pid",     dj_get_part_prop, dj_set_part_prop, flags),
    JS_PSGS("secid",   dj_get_part_prop, dj_set_part_prop, flags),
    JS_PSGS("mid",     dj_get_part_prop, dj_set_part_prop, flags),
...
    JS_PS_END
}; 


JSBool dj_get_part_prop(JSContext *cx, unsigned int argc, JS::Value *vp)
{
    JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
    JS::RootedObject obj(cx, JS_THIS_OBJECT(cx, vp));
}

I can get the object by using JS_THIS_OBJECT and I can see that in the getter argc is 0, in the setter argc is 1 and args[0] contains the value of the property to set.
However, I can't see any way to find out *which* property I am getting/setting.

Am I being stupid here? How can I find out which property I am getting/setting? I know that tiny IDs have gone so I was expecting the property to be passed as a string or something.
Or is there no way of getting the property name and I have to use a different property getter/setter for each property?

If someone can help me I would be very grateful.

Thanks

Miles