In a large skillet or in a wok, heat your oil to medium heat, and then add the carrot, tossing very frequently.
Allow the carrot to cook for about 4 minutes.
Then add your onion, and cook for another 4 minutes or so, and then add your garlic, cooking for another minute or so.
In a liquid measuring cup, add your soy sauce, brown sugar, juice, vinegar and cayenne.
Add your liquid mixture to the skillet and bring to a heavy simmer.
Add your shrimp, and cook until your shrimp is done (pink on both sides) approximately 3-4 minutes.
Don't overcook your shrimp.
At this time, you can throw in your pineapple chunks if you wish to use them.
While your shrimp is cooking, mix your cornstarch with approximately 4-5 tablespoons of water (just enough to make it a liquid mixture-you may need a little more or a little less because I never measure the amount of water I use).
Add the cornstarch mixture to your pan, and sir constantly until thick- this will not take very long at all.