'use strict';

// see if the array is already flattened
// returns true or false
const isArrayFlattened = function($array) {
  if (!$array) {
    return true;
  }
  for (const item of $array) {
    if (item instanceof Array) {
      return false;
    }
  }
  return true;
};

// main function to flatten the arrays. The use of Array.prototype.reduce is so that each iteration
// a (potential) layer of square brackets is removed.
// This is ensured by that [1].concat(2).concat(3) returns the same value as
// [1].concat([2, 3]), which is [1, 2, 3] for both
const flatten = function($array) {
  while (!isArrayFlattened($array)) {
    $array = $array.reduce((previous, item) => {
      return previous.concat(item);
    }, []);
  }
  return $array;
};

console.log(flatten([])); // []
console.log(flatten([1, [2], 3])); // [1, 2, 3]
console.log(flatten([[1, 2], 3])); // [1, 2, 3]
console.log(flatten([[[1], 2], 3])); // [1, 2, 3]
console.log(flatten([[[[1]], 2], 3])); // [1, 2, 3]
console.log(flatten([[1], [2], [3]])); // [1, 2, 3]
console.log(flatten([[[[1], [2]], [3]]])); // [1, 2, 3]