Complete "longest common ..." examples (#100)

* no else return

* fix var ref

* fix importing/requiring dependencies

* complete longest common examples

03_recursion/ES6/01_countdown.js

@@ -1,11 +1,11 @@
-const countdown = (i) => {
+const countdown = i => {
   console.log(i);
   // base case
   if (i <= 0) {
-    return null;
+    return;
   }
+
   countdown(i - 1);
-  return null;
 };
 
 countdown(5);