--description--

The mathematical term <dfn>symmetric difference</dfn> (△ or ⊕) of two sets is the set of elements which are in either of the two sets but not in both. For example, for sets A = {1, 2, 3} and B = {2, 3, 4}, A △ B = {1, 4}.

Symmetric difference is a binary operation, which means it operates on only two elements. So to evaluate an expression involving symmetric differences among three elements (A △ B △ C), you must complete one operation at a time. Thus, for sets A and B above, and C = {2, 3}, A △ B △ C = (A △ B) △ C = {1, 4} △ {2, 3} = {1, 2, 3, 4}.

--instructions--

Create a function that takes two or more arrays and returns an array of their symmetric difference. The returned array must contain only unique values (no duplicates).

--hints--

sym([1, 2, 3], [5, 2, 1, 4]) should return [3, 4, 5].

assert.sameMembers(sym([1, 2, 3], [5, 2, 1, 4]), [3, 4, 5]);

sym([1, 2, 3], [5, 2, 1, 4]) should contain only three elements.

assert.equal(sym([1, 2, 3], [5, 2, 1, 4]).length, 3);

sym([1, 2, 3, 3], [5, 2, 1, 4]) should return [3, 4, 5].

assert.sameMembers(sym([1, 2, 3, 3], [5, 2, 1, 4]), [3, 4, 5]);

sym([1, 2, 3, 3], [5, 2, 1, 4]) should contain only three elements.

assert.equal(sym([1, 2, 3, 3], [5, 2, 1, 4]).length, 3);

sym([1, 2, 3], [5, 2, 1, 4, 5]) should return [3, 4, 5].

assert.sameMembers(sym([1, 2, 3], [5, 2, 1, 4, 5]), [3, 4, 5]);

sym([1, 2, 3], [5, 2, 1, 4, 5]) should contain only three elements.

assert.equal(sym([1, 2, 3], [5, 2, 1, 4, 5]).length, 3);

sym([1, 2, 5], [2, 3, 5], [3, 4, 5]) should return [1, 4, 5].

assert.sameMembers(sym([1, 2, 5], [2, 3, 5], [3, 4, 5]), [1, 4, 5]);

sym([1, 2, 5], [2, 3, 5], [3, 4, 5]) should contain only three elements.

assert.equal(sym([1, 2, 5], [2, 3, 5], [3, 4, 5]).length, 3);

sym([1, 1, 2, 5], [2, 2, 3, 5], [3, 4, 5, 5]) should return [1, 4, 5].

assert.sameMembers(sym([1, 1, 2, 5], [2, 2, 3, 5], [3, 4, 5, 5]), [1, 4, 5]);

sym([1, 1, 2, 5], [2, 2, 3, 5], [3, 4, 5, 5]) should contain only three elements.

assert.equal(sym([1, 1, 2, 5], [2, 2, 3, 5], [3, 4, 5, 5]).length, 3);

sym([3, 3, 3, 2, 5], [2, 1, 5, 7], [3, 4, 6, 6], [1, 2, 3]) should return [2, 3, 4, 6, 7].

assert.sameMembers(
  sym([3, 3, 3, 2, 5], [2, 1, 5, 7], [3, 4, 6, 6], [1, 2, 3]),
  [2, 3, 4, 6, 7]
);

sym([3, 3, 3, 2, 5], [2, 1, 5, 7], [3, 4, 6, 6], [1, 2, 3]) should contain only five elements.

assert.equal(
  sym([3, 3, 3, 2, 5], [2, 1, 5, 7], [3, 4, 6, 6], [1, 2, 3]).length,
  5
);

sym([3, 3, 3, 2, 5], [2, 1, 5, 7], [3, 4, 6, 6], [1, 2, 3], [5, 3, 9, 8], [1]) should return [1, 2, 4, 5, 6, 7, 8, 9].

assert.sameMembers(
  sym(
    [3, 3, 3, 2, 5],
    [2, 1, 5, 7],
    [3, 4, 6, 6],
    [1, 2, 3],
    [5, 3, 9, 8],
    [1]
  ),
  [1, 2, 4, 5, 6, 7, 8, 9]
);

sym([3, 3, 3, 2, 5], [2, 1, 5, 7], [3, 4, 6, 6], [1, 2, 3], [5, 3, 9, 8], [1]) should contain only eight elements.

assert.equal(
  sym([3, 3, 3, 2, 5], [2, 1, 5, 7], [3, 4, 6, 6], [1, 2, 3], [5, 3, 9, 8], [1])
    .length,
  8
);

--seed--

--seed-contents--

function sym(args) {
  return args;
}

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

--solutions--

function sym() {
  var arrays = [].slice.call(arguments);
  return arrays.reduce(function (symDiff, arr) {
    return symDiff.concat(arr).filter(function (val, idx, theArr) {
      return theArr.indexOf(val) === idx
        && (symDiff.indexOf(val) === -1 || arr.indexOf(val) === -1);
    });
  });
}
sym([1, 2, 3], [5, 2, 1, 4]);