function diff(arr1, arr2) {
  var newArr = [];
  //group and compare
  newArr = arr1.concat(arr2);
  // Same, same; but different.
  function checkDiff(arrItem){
    if(arr1.indexOf(arrItem) < 0 || arr2.indexOf(arrItem) < 0){
      return true;
    }
  }

  return newArr.filter(checkDiff);
}

diff([1, 2, 3, 5], [1, 2, 3, 4, 5]);