### difference

Returns the difference between two arrays.

Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values not contained in `b`.

```js
const difference = (a, b) => { const s = new Set(b); return a.filter(x => !s.has(x)); };
```

```js
difference([1,2,3], [1,2,4]) // [3]
```