8Implement a function to flatten a nested array

Implement flatten(array, depth = Infinity). It returns a new array with nested arrays flattened into it, up to depth levels, like Array.prototype.flat(). Do not use flat() itself.

flatten([1, [2, [3, [4]]], 5]); // [1, 2, 3, 4, 5]
flatten([1, [2, [3, [4]]], 5], 1); // [1, 2, [3, [4]], 5]

Requirements

• Without depth, flatten completely.
• depth = 0 returns a shallow copy.
• Do not change the input.
• Skip holes in sparse arrays, like flat() does.
• Only arrays are flattened: strings and array-like objects stay as they are.