ran npm run tdd & generated TDD

test/groupBy/groupBy.js

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