1 John 2:1 gives a different purpose for writing (to avoid sin), complementing the joy purpose in 1:4.

In 1 John 2:12, the purpose of writing is to assure forgiveness — the basis for the full joy mentioned here.

In 1 John 5:13, writing aims at assurance of eternal life — a key component of the full joy described here.

In John 15:11, Jesus says he spoke so that his joy may be in them and their joy full—directly echoed in John's purpose here.

John 16:24 promises that asking in Jesus' name brings fullness of joy—same concept of joy made complete through relationship.

In 2 Corinthians 1:24, Paul says he works with them for their joy—parallel to John writing for the same goal of complete joy.

Philippians 1:25 says Paul remains for their progress and joy in the faith—same apostolic aim as John's writing for complete joy.

2 John 1:12 uses the exact phrase 'so that our joy may be complete'—identical purpose to John's letter here.

In John 17:13, Jesus prays for His joy to be fulfilled in believers, directly matching John's purpose for writing.

Philippians 2:2 urges unity to fulfill Paul's joy, closely paralleling John's aim to complete joy through fellowship.

2 Timothy 1:4 desires to be filled with joy through Timothy, paralleling John's goal of full joy for his readers.