// findByIndex :: Int -> [a] -> Maybe a

//           0  1  2
const arr = [2, 4, 6]

const testJust = findByIndex (2) (arr)
// => { val = 2 }

const testNothing = findByIndex (3) (arr)
// => { val = null }

// mapMaybe :: (a -> b) -> Maybe a -> Maybe b

const succ = x => x + 1

mapMaybe (succ) (testJust)
// => { val = 3 }

mapMaybe (succ) (testNothing)
// => { val = null }

// isJust :: Maybe a -> Bool

isJust (testJust)
// => true

isJust (testNothing)
// => false

// fromJust :: Maybe a -> a

fromJust (testJust)
// => 2

fromJust (testNothing)
// => ОШИБКА!

// withDefault  :: a -> Maybe a -> a

withDefault (0) (testJust)
// => 2

withDefault (0) (testNothing)
// => 0