### Array intersection (Common values between two arrays)

Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values contained in `b`.

```js
const intersection = (a, b) => { const s = new Set(b); return a.filter(x => s.has(x)); }
// intersection([1,2,3], [4,3,2]) -> [2,3]
```