Test migration to jest by hand

Apparently using regular expressions is way easier.

test/mapKeys/mapKeys.js

@@ -0,0 +1,6 @@
+const mapKeys = (obj, fn) =>
+Object.keys(obj).reduce((acc, k) => {
+acc[fn(obj[k], k, obj)] = obj[k];
+return acc;
+}, {
+module.exports = mapKeys;