Updated the test system

test/difference.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {difference} = require('./_30s.js');
+
+test('difference is a Function', () => {
+  expect(difference).toBeInstanceOf(Function);
+});
+test('Returns the difference between two arrays', () => {
+  expect(difference([1, 2, 3], [1, 2, 4])).toEqual([3]);
+});