Make 'Anagrams of string' fully functional

Gets rid of mutable `.push` (and param reassignment) inside `.map`

snippets/anagrams-of-string-(with-duplicates).md

@@ -9,8 +9,7 @@ Base cases are for string `length` equal to `2` or `1`.
 const anagrams = str => {
   if(str.length <= 2)  return str.length === 2 ? [str, str[1] + str[0]] : [str];
   return str.split('').reduce( (acc, letter, i) => {
-    anagrams(str.slice(0, i) + str.slice(i + 1)).map( val => acc.push(letter + val) );
-    return acc;
+    return acc.concat(anagrams(str.slice(0, i) + str.slice(i + 1)).map( val => letter + val ));
   }, []);
 }
 // anagrams('abc') -> ['abc','acb','bac','bca','cab','cba']