Updated the test system

test/memoize.test.js

@@ -0,0 +1,17 @@
+const expect = require('expect');
+const {memoize} = require('./_30s.js');
+
+test('memoize is a Function', () => {
+  expect(memoize).toBeInstanceOf(Function);
+});
+const f = x => x * x;
+const square = memoize(f);
+test('Function works properly', () => {
+  expect(square(2)).toBe(4);
+});
+test('Function works properly', () => {
+  expect(square(3)).toBe(9);
+});
+test('Cache stores values', () => {
+  expect(Array.from(square.cache)).toEqual([[2, 4], [3, 9]]);
+});