Test migration to jest by hand

Apparently using regular expressions is way easier.

test/invertKeyValues/invertKeyValues.js

@@ -0,0 +1,8 @@
+const invertKeyValues = (obj, fn) =>
+Object.keys(obj).reduce((acc, key) => {
+const val = fn ? fn(obj[key]) : obj[key];
+acc[val] = acc[val] || [];
+acc[val].push(key);
+return acc;
+}, {
+module.exports = invertKeyValues;