const arr = [1, 2, [3, 4, [5, 6]]]
arr.flat(Infinity)
// [1, 2, 3, 4, 5, 6]

  function handlerArr(arr, _rst) {
    const rst = _rst || [];
    arr.forEach(ele => {
      typeof ele === 'object' ? handlerArr(ele, rst) : rst.push(ele);
    });
    return rst;
  }

function flatten(arr, depth = 1) {
    if (!depth || !Array.isArray(arr)) return arr
    return arr.reduce((prev, mem) => 
        prev.concat(flatten(mem, depth - 1)), [])
}

const flatArr = (arr, deep = false) =>
  arr.reduce((prev, cur) => {
    if (deep && Array.isArray(cur)) {
      return prev.concat(flatArr(cur, deep));
    } else {
      return prev.concat(cur);
    }
  }, []);

const arr = [1, [2, [3, [4, [5], 6], 7], 8], 9];
const arr2 = [{}, [{}, {}, {}, [2, 3, 4, [5, 6, 7, [{}, {}]]]]];
console.log(flatArr(arr, true));

const flatArr = (arr) => Array.isArray(arr)
  ? arr.reduce( (a, b) => [...a, ...flatArr(b)] , [])
  : [arr]

flatArr([1, [[2], [3, [4]], 5]])