Heat an air fryer to 400°F.
Working in batches if needed, place up to 1 pound frozen french fries in a single layer in the basket (they can be touching but shouldn’t overlap). Air fry, gently shaking the basket to toss the french fries halfway through, until golden-brown and crispy, 9 to 10 minutes for shoestring fries, 10 to 12 minutes for curly fries and steak fries, 12 to 14 minutes for waffle fries, and 18 to 20 minutes for straight-cut and crinkle fries.
Transfer to a serving bowl. Sprinkle with 1/4 teaspoon kosher salt if desired, and toss to combine.