Add transform

snippets/transform.md

@@ -0,0 +1,17 @@
+### transform
+
+Applies a function against an accumulator and each key in the object (from left to right).
+
+Use `Object.keys(obj)` to iterate over each key in the object, `Array.reduce()` to call the apply the specified function against the given accumulator.
+
+```js
+const transform = (obj, fn, acc) =>
+  Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);
+```
+
+```js
+transform({ 'a': 1, 'b': 2, 'c': 1 }, (r, v, k) => {
+  (r[v] || (r[v] = [])).push(k);
+  return r;
+}, {}); // { '1': ['a', 'c'], '2': ['b'] }
+```