fix naming

snippets/deep-flatten.md

@@ -0,0 +1,10 @@
+### Deep flatten 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));
+// deepFlatten([1,[2],[[3],4],5]) -> [1,2,3,4,5]
+```