Mastering React Test-Driven Development

Building middleware

The first step is to build the middleware that works with local storage:

1. Create the directories src/middleware and test/middleware, and then open the file test/middleware/localStorage.test.js. To make a start, let's define two spies, getItemSpy and setItemSpy, which will make up the new object. We have to use Object.defineProperty to set these spies because the window.localStorage property is write protected:

import { save } from '../../src/middleware/localStorage';describe('localStorage', () => {  const data = { a: 123 };  let getItemSpy = jest.fn();  let setItemSpy = jest.fn();  beforeEach(() => {    Object.defineProperty(window, 'localStorage', {      value: {        getItem: getItemSpy,        setItem: setItemSpy      }});  });});

2. Let's write ...