Test migration to jest by hand

Apparently using regular expressions is way easier.

test/countBy/countBy.js

@@ -2,5 +2,5 @@ const countBy = (arr, fn) =>
 arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
 acc[val] = (acc[val] || 0) + 1;
 return acc;
-}, {});
+}, {
 module.exports = countBy;