Add Chicken
Place chicken in the bottom of the slow cooker.
Make the Sauce
In a bowl, whisk pineapple juice, soy sauce, brown sugar, ketchup (or BBQ sauce), and garlic until smooth.
Add Pineapple
Scatter pineapple chunks over the chicken.
Slow Cook
Pour sauce over everything. Cover and cook on LOW for 4–6 hours or HIGH for 2–3 hours, until chicken is tender and cooked through.
Thicken Sauce (Optional)
Mix cornstarch with cold water. Stir into slow cooker and cook an additional 20–30 minutes on HIGH.
Serve
Slice or shred chicken and spoon sauce generously over the top. Garnish if desired.