Updated the test system

test/isNull.test.js

@@ -0,0 +1,12 @@
+const expect = require('expect');
+const {isNull} = require('./_30s.js');
+
+test('isNull is a Function', () => {
+  expect(isNull).toBeInstanceOf(Function);
+});
+test('passed argument is a null', () => {
+  expect(isNull(null)).toBeTruthy();
+});
+test('passed argument is a null', () => {
+  expect(isNull(NaN)).toBeFalsy();
+});