Updated the test system

test/xProd.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {xProd} = require('./_30s.js');
+
+test('xProd is a Function', () => {
+  expect(xProd).toBeInstanceOf(Function);
+});
+test("xProd([1, 2], ['a', 'b']) returns [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]", () => {
+  expect(xProd([1, 2], ['a', 'b'])).toEqual([[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]);
+});