3. implement Array.prototype.flat()

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

Could you manage to implement your own one?

Here is an example to illustrate

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?