### symmetricDifference

Returns the symmetric difference between two arrays.

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

```js
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))];
}
```

```js
symmetricDifference([1,2,3], [1,2,4]) -> [3,4]
```