In a medium saucepan, combine the pineapple juice, vinegar, sugar, ketchup, and salt. Stir well to dissolve the sugar as much as possible before heating.
Place the pan over medium heat and bring the mixture to a gentle boil, stirring occasionally to prevent sticking.
Once it starts bubbling steadily, stir in the cornstarch slurry into the pan while stirring continuously. Continue cooking for an additional 1-2 minutes, stirring constantly, until the sauce thickens and becomes glossy.
Remove the pan from the heat and let the sauce cool down completely to room temperature. Transfer to a clean jar or airtight container.
