// recursive method
function flattenArray(arr) {
  var newArr = [],
      i = 0,
      arrLen = arr.length;

  for (i; i < arrLen; i++) {
    if (Array.isArray(arr[i])) {
      newArr = newArr.concat(flattenArray(arr[i]));
    } else {
      newArr.push(arr[i]);
    }
  }
  return newArr;
}

// ES6 reduce method
const flattenArrayNew = (arr) => arr.reduce(
  (a, b) => a.concat(
    Array.isArray(b) ? flattenArrayNew(b) : b
  ),
  []
);