Updated the test system

test/everyNth.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {everyNth} = require('./_30s.js');
+
+test('everyNth is a Function', () => {
+  expect(everyNth).toBeInstanceOf(Function);
+});
+test('Returns every nth element in an array', () => {
+  expect(everyNth([1, 2, 3, 4, 5, 6], 2)).toEqual([2, 4, 6]);
+});