Updated the test system

test/offset.test.js

@@ -0,0 +1,24 @@
+const expect = require('expect');
+const {offset} = require('./_30s.js');
+
+test('offset is a Function', () => {
+  expect(offset).toBeInstanceOf(Function);
+});
+test('Offset of 0 returns the same array.', () => {
+  expect(offset([1, 2, 3, 4, 5], 0)).toEqual([1, 2, 3, 4, 5]);
+});
+test('Offset > 0 returns the offsetted array.', () => {
+  expect(offset([1, 2, 3, 4, 5], 2)).toEqual([3, 4, 5, 1, 2]);
+});
+test('Offset < 0 returns the reverse offsetted array.', () => {
+  expect(offset([1, 2, 3, 4, 5], -2)).toEqual([4, 5, 1, 2, 3]);
+});
+test('Offset greater than the length of the array returns the same array.', () => {
+  expect(offset([1, 2, 3, 4, 5], 6)).toEqual([1, 2, 3, 4, 5]);
+});
+test('Offset less than the negative length of the array returns the same array.', () => {
+  expect(offset([1, 2, 3, 4, 5], -6)).toEqual([1, 2, 3, 4, 5]);
+});
+test('Offsetting empty array returns an empty array.', () => {
+  expect(offset([], 3)).toEqual([]);
+});