foodReducer.js

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

/*
  It's the same reducer function from above, I just extracted the operation for 
  'ADD_FOOD', turned into a function and named it addFood.
*/
function addFood (state, action) {
  return Object.assign({}, state, {
    favoriteFood: state.favoriteFood.concat(action.data.food)
  })
}
/* 
  state = person, means that the person object 
  is the default value for state. It will fall back to that value
  if I don't provide any value to it
*/ 
function reducer (state = person, action) {
  switch (action.type) {
    case 'ADD_FOOD':
      return addFood(state, action)
  }
}
/* 
  I've provided the person object as the state input,
  and an object for the action.
*/
const result = reducer(person, {
  type: 'ADD_FOOD',
  data: {
    food: 'apple'
  }
})

/*
The output:

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