Testing Reducers

The reducer is, again, a pure function! It’s quite easy to see what we need to validate actually, basically every case of our switch needs to have a test:

export default function mainReducer(state = initialState, action) {
  switch (action.type) {
    case 'CHANGE_LOCATION':
      return state.set('location', action.location);
    case 'SET_DATA':
      return state.set('data', fromJS(action.data));
    case 'SET_DATES':
      return state.set('dates', fromJS(action.dates));
    case 'SET_TEMPS':
      return state.set('temps', fromJS(action.temps));
    case 'SET_SELECTED_DATE':
      return state.setIn(['selected', 'date'], action.date);
    case 'SET_SELECTED_TEMP':
      return state.setIn(['selected', 'temp'], action.temp);
    default:
      return state;
  }
}

Let’s showcase this on the 'CHANGE_LOCATION' case, first create a reducer.test.js file in the __tests__ / directory, import the reducer and add the basic structure:

// __tests__/reducer.test.js
import mainReducer from '../reducer';

describe('mainReducer', function() {

});

The first branch of the switch statement we’ll test is the default one – if we don’t pass any state and an empty action in it should return the initial state. The thing is that the initialState is an immutable object, so we’ll need to import fromJS too:

// __tests__/reducer.test.js
import mainReducer from '../reducer';
import { fromJS } from 'immutable';

describe('mainReducer', function() {
  it('should return the initial state', function() {
    expect(mainReducer(undefined,

...