1, With a sharp knife, slit each squid body lengthwise so it lays flat, then slice the body lengthwise into 2 or 3 pieces. In a bowl, combine the pieces with the tentacles and set aside.
2, In a bowl, stir together the cornstarch, 1 teaspoon salt, and ½ teaspoon pepper and set aside.
3, In a sauté pan, heat the 1 tablespoon oil over medium heat. When the oil is hot, add the pineapple slices and cook, turning once, for 3 to 4 minutes on each side, until golden brown. Transfer the pineapple slices to a cutting board, cut into bite-size pieces, then set aside on paper towels to drain.
4, Pour the oil to a depth of 2 inches into a wok or high-sided pot and heat over high heat to 375°F on a deep-frying thermometer. Line a shallow bowl with paper towels. While the oil heats, prepare the squid. Divide the squid into 3 equal portions. Dredge each portion in the cornstarch mixture, lifting it out with a fine-mesh sieve and shaking off the excess.
5, When the oil is ready, add one-third of the squid to the hot oil and fry for about 45 seconds, until golden brown and curled. Using a spider or slotted spoon, transfer the squid to the paper towel–lined bowl. Fry the the second and third batches the same way, adding them to the bowl and always allowing the oil to return to temperature between batches. Ten seconds before the last batch of squid is done, add the jalapeño chiles to the hot oil, then remove the chiles from the oil along with the squid.
6, In a large bowl, combine the fried squid and jalapeños, pineapple, toasted garlic, cilantro, and sugar and toss to combine. Season with salt and pepper and serve immediately.