Background and release
On April 6, 2019, Twice announced the release of their fourth Japanese single titled "Happy Happy" at their final date of the #Dreamday Dome Tour in Nagoya. In April 2019, Happy Happy was also used in the 20th anniversary commercial for Qoo. The music video for Happy Happy was released on June 12, 2019 along with the music video for Breakthrough.
- "Happy Happy" - 3:27
- "The Best Thing I Ever Did (Japanese ver.)" - 3:33
- "Happy Happy" (collapsedone Remix) - 3:33
- "Happy Happy" (Instrumental) - 3:28