Javier Baez has a quick bat and a lot to learn. The Cubs’ 20-year-old shortstop is unfazed by the competition of major-league training camps — he hit a home run and drove in three runs against the Milwaukee Brewers on Sunday. But big-league training camps aren’t fazed by him, ­either. He strikes out. He gets fooled by offspeed pitches. And the home run came against a fringe pitcher who might not be in the big leagues this season. He’s not lacking for confidence. ‘‘It’s the same game,’’ he said when asked about facing big-league pitchers. ‘‘I’ve got to get used to it.’’ He will. Or he should. Baez, the ninth overall pick in the 2011 major-league draft, is ranked the 16th-best prospect by both Baseball America and mlb.com. And so far, he doesn’t appear out of his league. ‘‘It feels great,’’ said Baez, who grew up in Puerto Rico but went to high school in Jacksonville, Fla. ‘‘I’m young. I’m already in the big-league camp. So it feels pretty good.’’ But his job in spring training with the Cubs is the same as any other 20-year-old kid in a big-league camp: observe, listen and learn. It takes more than a quick bat to get to the show. And the good news is that Baez seems to have a grasp of that concept. He’s honing his immense baseball skills, and learning how to be a professional.