flatten

Flattens an array up to the specified depth.

• Use recursion, decrementing depth by 1 for each level of depth.
• Use Array.prototype.reduce() and Array.prototype.concat() to merge elements or arrays.
• Base case, for depth equal to 1 stops recursion.
• Omit the second argument, depth, to flatten only to a depth of 1 (single flatten).

const flatten = (arr, depth = 1) =>
  arr.reduce(
    (a, v) =>
      a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v),
    []
  );

flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]