ran npm run tdd

test/countBy/countBy.js

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