Updated the test system

test/nthArg.test.js

@@ -0,0 +1,17 @@
+const expect = require('expect');
+const {nthArg} = require('./_30s.js');
+
+test('nthArg is a Function', () => {
+  expect(nthArg).toBeInstanceOf(Function);
+});
+const third = nthArg(2);
+test('Returns the nth argument', () => {
+  expect(third(1, 2, 3)).toBe(3);
+});
+test('Returns undefined if arguments too few', () => {
+  expect(third(1, 2)).toBe(undefined);
+});
+const last = nthArg(-1);
+test('Works for negative values', () => {
+  expect(last(1, 2, 3, 4, 5)).toBe(5);
+});