Updated the test system

test/maxDate.test.js

@@ -0,0 +1,15 @@
+const expect = require('expect');
+const {maxDate} = require('./_30s.js');
+
+test('maxDate is a Function', () => {
+  expect(maxDate).toBeInstanceOf(Function);
+});
+test('maxDate produces the maximum date', () => {
+  const array = [
+    new Date(2017, 4, 13),
+    new Date(2018, 2, 12),
+    new Date(2016, 0, 10),
+    new Date(2016, 0, 9)
+  ];
+  expect(maxDate(array)).toEqual(new Date(2018, 2, 12));
+});