When I'm moving from JavaScript engine SpiderMonkey version 1.7 to version 1.8.5, I am facing such informatiion:

Previous versions of SpiderMonkey supported two types of native functions: JSNative and JSFastNative. All support for the slow native function type has been removed, and the fast signature has been renamed to JSNative. Embeddings that defined JSNatives in previous versions must update all those functions to the new signature.

another words, JSNative is not JSNative already.

So I have such function for SP 1.7:

static JSNative GetCallback()
{
    return JSNative(JSFuncCallbak);
}

How does it have to look like for SP 1.8.5?