Step 1

Stir reserved pineapple juice, vinegar, sugar, and ketchup in a saucepan over medium heat until blended, 2 to 3 minutes.

Step 2

Whisk cornstarch and water in a bowl until cornstarch is dissolved; stir into pineapple juice mixture.

Step 3

Stir pineapple and carrots into the sauce; bring to a boil, reduce heat to medium-low, and cook at a simmer until the sauce thickens, about 5 minutes.