const flatten = function(array, flattenArray = []) {
  for (let i = 0; i < array.length; i++) {
    const value = array[i];
    if (Array.isArray(value)) {
      flatten(value, flattenArray);
    } else {
      flattenArray.push(value);
    }
  }
  return flattenArray;
};

const a = [1, [2, [3, 4, [5, [6, 7]]]], 8, 9];
console.log(flatten(a)); // [1, 2, 3, 4, 5, 6, 7, 8, 9]