Test migration to jest by hand

Apparently using regular expressions is way easier.

test/groupBy/groupBy.js

@@ -0,0 +1,6 @@
+const groupBy = (arr, fn) =>
+arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
+acc[val] = (acc[val] || []).concat(arr[i]);
+return acc;
+}, {
+module.exports = groupBy;