Added linting, processed current snippets

snippets/flatten-array.md

@@ -3,6 +3,6 @@
 Use `Array.reduce()` to get all elements inside the array and `concat()` to flatten them.
 
 ```js
-const flatten = arr => arr.reduce( (a, v) => a.concat(v), []);
+const flatten = arr => arr.reduce((a, v) => a.concat(v), []);
 // flatten([1,[2],3,4]) -> [1,2,3,4]
 ```