Updated the test system

test/shuffle.test.js

@@ -0,0 +1,19 @@
+const expect = require('expect');
+const {shuffle} = require('./_30s.js');
+
+test('shuffle is a Function', () => {
+  expect(shuffle).toBeInstanceOf(Function);
+});
+const arr = [1, 2, 3, 4, 5, 6];
+test('Shuffles the array', () => {
+  expect(shuffle(arr)).not.toEqual(arr);
+});
+test('New array contains all original elements', () => {
+  expect(shuffle(arr).every(x => arr.includes(x))).toBeTruthy();
+});
+test('Works for empty arrays', () => {
+  expect(shuffle([])).toEqual([]);
+});
+test('Works for single-element arrays', () => {
+  expect(shuffle([1])).toEqual([1]);
+});