Flatten an arbitrarily nested array (JS)

array_flatten.js

/*
Write some code, that will flatten an array of arbitrarily nested arrays of
integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].
*/

var array_flatten = function (arr, flattened_arr = []) {
  if(!Array.isArray(arr)) {
    // Fow now, arbitrarily decide to just break out if input is invalid. Could handle other ways (return an empty array, for example), but this is OK for now. 
    console.error('Invalid input.');
    return;
  }
  for (let el of arr) {
    if (Array.isArray(el)) {
      array_flatten(el, flattened_arr);
    } else {
      flattened_arr.push(el);
    }
  }
  return flattened_arr;
}

// Some basic reality-check testing. Not necessarily the best, but definitely better than nothing.
console.assert(JSON.stringify(array_flatten([[1,2,[3]],4])) == '[1,2,3,4]', "Flatten deeply nested array");
console.assert(JSON.stringify(array_flatten([1,2,3,4])) == '[1,2,3,4]', "Does not mess with already flat array");
console.assert(JSON.stringify(array_flatten(null)) === undefined, "Does not explode with null input");
console.assert(JSON.stringify(array_flatten([1,2,null,[3,null]])) === "[1,2,null,3,null]", "Does not explode with null elements");