---
title: deepFlatten
tags: array,recursion,intermediate
---

Deep flattens an array.

- Use recursion.
- Use `Array.prototype.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]
```