Updated the test system

test/factorial.test.js

@@ -0,0 +1,21 @@
+const expect = require('expect');
+const {factorial} = require('./_30s.js');
+
+test('factorial is a Function', () => {
+  expect(factorial).toBeInstanceOf(Function);
+});
+test('Calculates the factorial of 720', () => {
+  expect(factorial(6)).toBe(720);
+});
+test('Calculates the factorial of 0', () => {
+  expect(factorial(0)).toBe(1);
+});
+test('Calculates the factorial of 1', () => {
+  expect(factorial(1)).toBe(1);
+});
+test('Calculates the factorial of 4', () => {
+  expect(factorial(4)).toBe(24);
+});
+test('Calculates the factorial of 10', () => {
+  expect(factorial(10)).toBe(3628800);
+});