Update deepMapKeys

Resolves #1139

snippets/deepMapKeys.md

@@ -10,14 +10,15 @@ Use `Object.keys(obj)` to iterate over the object's keys.
 Use `Array.prototype.reduce()` to create a new object with the same values and mapped keys using `fn`.
 
 ```js
-const deepMapKeys = (obj, f) =>
+const deepMapKeys = (obj, fn) =>
   Array.isArray(obj)
-    ? obj.map(val => deepMapKeys(val, f))
+    ? obj.map(val => deepMapKeys(val, fn))
     : typeof obj === 'object'
     ? Object.keys(obj).reduce((acc, current) => {
+        const key = fn(current);
         const val = obj[current];
-        acc[f(current)] =
+        acc[key] =
-          val !== null && typeof val === 'object' ? deepMapKeys(val, f) : (acc[f(current)] = val);
+          val !== null && typeof val === 'object' ? deepMapKeys(val, fn) : val;
         return acc;
       }, {})
     : obj;