Add filterUnique

snippets/filterUnique.md

@@ -0,0 +1,18 @@
+---
+title: filterUnique
+tags: array,beginner
+---
+
+Creates an array with the unique values filtered out.
+
+- Use `new Set()` 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]
+```