On August 15, Blockberry published the official tracklist for "[+ +]" (read as "plus plus") as their debut extended play as a full group. The extended play included "Favorite" and was released on August 20, 2018 alongside the title track, "Hi High". It started at number 2 on South Korea's Gaon Album Chart, and was the second best-selling debut album by a girl group in 2018. "[+ +]" was repackaged as "[X X]" (read as "multiple multiple") on February 19, 2019, with six more tracks, including a new single, "Butterfly". On August 17, 2019, Loona made their first appearance in the United States, performing at KCON 2019 in Los Angeles. On December 13, 2019, Loona released a single called "365" as an appreciation song for their fans.