Updated the test system

test/sample.test.js

@@ -0,0 +1,16 @@
+const expect = require('expect');
+const {sample} = require('./_30s.js');
+
+test('sample is a Function', () => {
+  expect(sample).toBeInstanceOf(Function);
+});
+const arr = [3, 7, 9, 11];
+test('Returns a random element from the array', () => {
+  expect(arr.includes(sample(arr))).toBeTruthy();
+});
+test('Works for single-element arrays', () => {
+  expect(sample([1])).toBe(1);
+});
+test('Returns undefined for empty array', () => {
+  expect(sample([])).toBe(undefined);
+});