Updated the test system

test/averageBy.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {averageBy} = require('./_30s.js');
+
+test('averageBy is a Function', () => {
+  expect(averageBy).toBeInstanceOf(Function);
+});
+test('Produces the right result with a function', () => {
+  expect(averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n)).toBe(5);
+});
+test('Produces the right result with a property name', () => {
+  expect(averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n')).toBe(5);
+});