Test-driving a reducer

In this section, we’ll test-drive a new reducer function, and then pull out some repeated code.

A reducer is a simple function that takes an action and the current store state as input and returns a new state object as output. Let’s build that now, as follows:

1. Create a new file (in a new directory) named test/reducers/customer.test.js. Add the following first test, which checks that if the reducer is invoked with an unknown action, our reducer should return a default state for our object. This is standard behavior for Redux reducers, so you should always start with a test like this:

   import { reducer } from "../../src/reducers/customer";
   describe("customer reducer", () => {
     it("returns a default state for an undefined existing state", () => {
       expect(reducer(undefined, {})).toEqual({
         customer: {},
         ...