symmetricDifference

Returns the symmetric difference between two arrays, without filtering out duplicate values.

Create a Set from each array, then use Array.prototype.filter() on each of them to only keep values not contained in the other.

const symmetricDifference = (a, b) => {
  const sA = new Set(a),
    sB = new Set(b);
  return [...a.filter((x) => !sB.has(x)), ...b.filter((x) => !sA.has(x))];
};

symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]