        // UNSAFE: We're using FFI, so we don't know how the pointer is used.
        // Hopefully, the JavaScript-side should clone that value synchronously.
        //
        // Also, we're transmuting so that &(f64, f64) is actually treated like
        // a *const u8 (immutable raw pointer to an u8) as it's what the JS
        // callback expects.  This should not matter: Rust types are not
        // communicated to JavaScript anyway.
        unsafe { onAttribute(attr_name, mem::transmute(self), 16); };
    }
}
