### deepFlatten

Deep flattens an array.

Use recursion.
Use `Array.concat()` with an empty array (`[]`) and the spread operator (`...`) to flatten an array.
Recursively flatten each element that is an array.

```js
const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));
```

```js
deepFlatten([1, [2], [[3], 4], 5]); // [1,2,3,4,5]
```