---
title: Filter unique array values
tags: array
expertise: beginner
author: maciv
cover: blog_images/tulips-and-reeds.jpg
firstSeen: 2020-11-02T19:41:00+02:00
lastUpdated: 2020-11-02T19:41:00+02:00
---

Creates an array with the unique values filtered out.

- Use the `Set` constructor and the spread operator (`...`) to create an array of the unique values in `arr`.
- Use `Array.prototype.filter()` to create an array containing only the non-unique values.

```js
const filterUnique = arr =>
  [...new Set(arr)].filter(i => arr.indexOf(i) !== arr.lastIndexOf(i));
```

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