Revert recent changes to last.md

Use `length` check instead to apply to strings and other array-likes.

snippets/last.md

@@ -5,10 +5,10 @@ tags: array,beginner
 
 Returns the last element in an array.
 
-Check if `arr` is an instance of an `Array`, use `arr.length - 1` to compute the index of the last element of the given array and return it, otherwise return `undefined`.
+Check if `arr` is truthy and has a `length` property, use `arr.length - 1` to compute the index of the last element of the given array and return it, otherwise return `undefined`.
 
 ```js
-const last = arr => (Array.isArray(arr) ? arr[arr.length - 1] : undefined);
+const last = arr => (arr && arr.length ? arr[arr.length - 1] : undefined);
 ```
 
 ```js