Added longestString()

snippets/longestString.md

@@ -0,0 +1,15 @@
+### longestString
+
+Takes an array of strings and returns the longestString one.
+
+Uses the [rest operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters)
+to handle arrays as well as an indefinite amount of single arguments.
+
+```js
+const longestString = (...strings) => [...strings].reduce((a, b) => a.length > b.length ? a : b);
+```
+
+```js
+longestString('this', 'is', 'a', 'testcase') // 'testcase'
+longestString(['a', 'ab', 'abc']) // 'abc'
+```