Updated the test system

test/differenceWith.test.js

@@ -0,0 +1,11 @@
+const expect = require('expect');
+const {differenceWith} = require('./_30s.js');
+
+test('differenceWith is a Function', () => {
+  expect(differenceWith).toBeInstanceOf(Function);
+});
+test('Filters out all values from an array', () => {
+  expect(
+    differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b))
+  ).toEqual([1, 1.2]);
+});