[W]hat we have seen and heard
we proclaim now to you,
so that you too may have fellowship with us;
for our fellowship is with the Father
and with his Son, Jesus Christ.
We are writing this so that our joy may be complete.
The NAB note on that last verse points to two other places St. John refers to completed joy. The first is from the Last Discourse (with a few extra verses for context):
"If you remain in me and my words remain in you, ask for whatever you want and it will be done for you. By this is my Father glorified, that you bear much fruit and become my disciples. As the Father loves me, so I also love you. Remain in my love. If you keep my commandments, you will remain in my love, just as I have kept my Father’s commandments and remain in his love.
"I have told you this so that my joy may be in you and your joy may be complete."