Mastering React Test-Driven Development

Setting the initial state

Let's make a start by building the reducer itself, before adding buttons to exercise the new functionality:

1. Open a new file named test/reducers/withUndoRedo.test.js and add the following setup and test, which specifies what should happen when we pass an undefined state to the reducer. This is equivalent to how we began testing our other reducers, but in this case, we pass the call through to the decorated reducer:

import { withUndoRedo } from '../../src/reducers/withUndoRedo';describe('withUndoRedo', () => {  let decoratedReducerSpy;  let reducer;  beforeEach(() => {    decoratedReducerSpy = jest.fn();    reducer = withUndoRedo(decoratedReducerSpy);  });  describe('when initializing state', () => { it('calls the decorated ...