Updated the test system

test/isString.test.js

@@ -0,0 +1,21 @@
+const expect = require('expect');
+const {isString} = require('./_30s.js');
+
+test('isString is a Function', () => {
+  expect(isString).toBeInstanceOf(Function);
+});
+test('foo is a string', () => {
+  expect(isString('foo')).toBeTruthy();
+});
+test('"10" is a string', () => {
+  expect(isString('10')).toBeTruthy();
+});
+test('Empty string is a string', () => {
+  expect(isString('')).toBeTruthy();
+});
+test('10 is not a string', () => {
+  expect(isString(10)).toBeFalsy();
+});
+test('true is not string', () => {
+  expect(isString(true)).toBeFalsy();
+});