flatten.js

flatten.js

const flatten =
  (arr, depth = Infinity) =>
    arr.reduce(
      (list, v) =>
        list.concat(
          depth > 0 ?
            (depth > 1 && Array.isArray(v) ?
                flatten(v, depth - 1) :
                v
            ) :
            [v]
        )
      , []);

flatten([[0, 1], 2, 3, [4, [5, 6, 7], [8, [9, [10, [11, 12], 13]]]]], 0);
// [[0,1],2,3,[4,[5,6,7],[8,[9,[10,[11,12],13]]]]]

flatten([[0, 1], 2, 3, [4, [5, 6, 7], [8, [9, [10, [11, 12], 13]]]]], 1);
// [0,1,2,3,4,[5,6,7],[8,[9,[10,[11,12],13]]]]

flatten([[0, 1], 2, 3, [4, [5, 6, 7], [8, [9, [10, [11, 12], 13]]]]], 2);
// [0,1,2,3,4,5,6,7,8,[9,[10,[11,12],13]]]

flatten([[0, 1], 2, 3, [4, [5, 6, 7], [8, [9, [10, [11, 12], 13]]]]], 3);
// [0,1,2,3,4,5,6,7,8,9,[10,[11,12],13]]

flatten([[0, 1], 2, 3, [4, [5, 6, 7], [8, [9, [10, [11, 12], 13]]]]], 4);
// [0,1,2,3,4,5,6,7,8,9,10,[11,12],13]

flatten([[0, 1], 2, 3, [4, [5, 6, 7], [8, [9, [10, [11, 12], 13]]]]], 5);
// [0,1,2,3,4,5,6,7,8,9,10,11,12,13]