Array.prototype.concat() doesn’t mutate the original array, it returns a new array. To fix the issue, we need to assign the return value of result.concat(flattenArray(item)) back to result.

function flattenArray(arr) {
  let result = [];
  arr.forEach((item) => {
    if (Array.isArray(item)) {
      result = result.concat(flattenArray(item));
    } else {
      result.push(item);
    }
  });
  return result;
}

console.log(flattenArray([1, [2, [3, [4]]]]));