Updated the test system

test/when.test.js

@@ -0,0 +1,13 @@
+const expect = require('expect');
+const {when} = require('./_30s.js');
+
+test('when is a Function', () => {
+  expect(when).toBeInstanceOf(Function);
+});
+const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
+test('Returns the proper result', () => {
+  expect(doubleEvenNumbers(2)).toBe(4);
+});
+test('Returns the proper result', () => {
+  expect(doubleEvenNumbers(1)).toBe(1);
+});