Updated the test system

test/isPlainObject.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {isPlainObject} = require('./_30s.js');
+
+test('isPlainObject is a Function', () => {
+  expect(isPlainObject).toBeInstanceOf(Function);
+});
+test('Returns true for a plain object', () => {
+  expect(isPlainObject({ a: 1 })).toBeTruthy();
+});
+test('Returns false for a Map (example of non-plain object)', () => {
+  expect(isPlainObject(new Map())).toBeFalsy();
+});