Updated the test system

test/intersectionBy.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {intersectionBy} = require('./_30s.js');
+
+test('intersectionBy is a Function', () => {
+  expect(intersectionBy).toBeInstanceOf(Function);
+});
+test('Returns a list of elements that exist in both arrays, after applying the provided function to each array element of both', () => {
+  expect(intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor)).toEqual([2.1]);
+});