Updated the test system

test/groupBy.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {groupBy} = require('./_30s.js');
+
+test('groupBy is a Function', () => {
+  expect(groupBy).toBeInstanceOf(Function);
+});
+test('Groups the elements of an array based on the given function', () => {
+  expect(groupBy([6.1, 4.2, 6.3], Math.floor)).toEqual({ 4: [4.2], 6: [6.1, 6.3] });
+});
+test('Groups the elements of an array based on the given function', () => {
+  expect(groupBy(['one', 'two', 'three'], 'length')).toEqual({ 3: ['one', 'two'], 5: ['three'] });
+});