Updated the test system

test/mapKeys.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {mapKeys} = require('./_30s.js');
+
+test('mapKeys is a Function', () => {
+  expect(mapKeys).toBeInstanceOf(Function);
+});
+test('Maps keys', () => {
+  expect(mapKeys({ a: 1, b: 2 }, (val, key) => key + val)).toEqual({ a1: 1, b2: 2 });
+});