I stopped reading when the author said the word "joy" can't be used as an adjective. What about "joyful" or "joyous"? And I know it's not exactly the same word

I stopped reading when the author said the word "joy" can't be used as an adjective. What about "joyful" or "joyous"? And I know it's not exactly the same word but neither is "happy" and "happiness".