const flatten = (arr) => arr.flat(Infinity);

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

const flatten = (value) => {
  return value.reduce(
    (acc, curr) => acc.concat(
      Array.isArray(curr) ? flatten(curr) : curr
      ),
    [],
  );
}

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

const flatten = (arr) => {
  for (let i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      arr.splice(i, 1, ...arr[i]);
      i--;
    }
  }

  return arr;
}

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