This is the vanilla ItemStack. It is recommended that you use IItemStack whenever possible
They can be cast from each other, though.
It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.
Return Type: IIngredient
// ItemStack.asIIngredient() as IIngredient myItemStack.asIIngredient();
Return Type: IItemStack
// ItemStack.asIItemStack() as IItemStack myItemStack.asIItemStack();