What advantage did the japan have that made them certain that they would win the battle at midway?

Japan thought that they had control over the Pacific Ocean. Yamamoto planed to ambush the U.S. and perform an assault on Midway. Yamamoto also thought that the U.S. could not tolerate this ambush attack since it was so close to Pearl Harbor, Hawaii.

Your answer is that Japan was overconfident about having control over the Pacific Ocean.