What will be the output of the following recursive function?

function flattenArray(arr) {
  let flattened = [];
  arr.forEach(element => {
    if (Array.isArray(element)) {
      flattened.push(...flattenArray(element));
    } else {
      flattened.push(element);
    }
  });
  return flattened;
}
console.log(flattenArray([1, [2, 3], [4, [5, 6]]]));