Updated the test system

test/differenceBy.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {differenceBy} = require('./_30s.js');
+
+test('differenceBy is a Function', () => {
+  expect(differenceBy).toBeInstanceOf(Function);
+});
+test('Works using a native function and numbers', () => {
+  expect(differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor)).toEqual([1.2]);
+});
+test('Works with arrow function and objects', () => {
+  expect(differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x)).toEqual([{ x: 2 }]);
+});