Уникальная симметричная разность массива

const uniqueSymmetricDifference = (a, b) => [
  …new Set([
    …a.filter(v => !b.includes(v)),
    …b.filter(v => !a.includes(v)),
  ]),
];

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