foodReducer_spec.js

const person = {
  name: 'ken',
  favoriteFood: []
}

function addFood (state, action) {
  return Object.assign({}, state, {
    favorite: state.favoriteFood.concat(action.data.food)
  })
}

function reducer (state = person, action) {
  switch (action.type) {
    case 'ADD_FOOD',
      return addFood(state, action)
    // if it doesnt match any action type it will 
    // just return the state
    default:
      return state
  }
}

describe('reducer', () => {
  it('handles ADD_FOOD', () => {
    const action = {
      type: 'ADD_FOOD',
      data: {
        food: 'apple'
      }
    }
    const result = reducer(person, action)
    expect(result).to.deep.equal({
      name: 'ken',
      favoriteFood: ['apple']
    })
  })
  // when I test the person object here it will come
  // out the same
  it('will not have a side effect', () => {
    expect(person).to.deep.equal({
      name: 'ken',
      favoriteFood: []
    })
  })
})