Updated the test system

test/sortedLastIndexBy.test.js

@@ -0,0 +1,9 @@
+const expect = require('expect');
+const {sortedLastIndexBy} = require('./_30s.js');
+
+test('sortedLastIndexBy is a Function', () => {
+  expect(sortedLastIndexBy).toBeInstanceOf(Function);
+});
+test('Returns the highest index to insert the element without messing up the list order', () => {
+  expect(sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x)).toBe(1);
+});