3. implement Array.prototype.flat()

There is already Array.prototype.flat() in JavaScript (ES2019), which reduces the nesting of Array. Please implement your own.

const arr = [1, [2], [3, [4]]];flat(arr)// [1, 2, 3, [4]]flat(arr, 1)// [1, 2, 3, [4]]flat(arr, 2)// [1, 2, 3, 4]

follow up

Are you able to solve it both recursively and iteratively? How do you handle sparse array?