Joyfulness is the name for that very-happy feeling, or for the cheerful quality of a person, a day, a song, a place. It is not one short “yes!”; it is the way happiness fills something and stays there for a while. You can speak of a child’s joyfulness, or the joyfulness of a festival, or a joyfulness in the music that is likely to make other people happy too. It is a rather careful, slightly old-fashioned noun. In ordinary talk people usually say joy or happiness instead. You meet joyfulness more in stories, speeches, and writing that is trying to sound warm and full. Though actually, it does not point to a different feeling from joy — it just names the quality of being that way.