Updated the test system

test/nthElement.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {nthElement} = require('./_30s.js');
+
+test('nthElement is a Function', () => {
+  expect(nthElement).toBeInstanceOf(Function);
+});
+test('Returns the nth element of an array.', () => {
+  expect(nthElement(['a', 'b', 'c'], 1)).toBe('b');
+});
+test('Returns the nth element of an array.', () => {
+  expect(nthElement(['a', 'b', 'c'], -3)).toBe('a');
+});