redux_notes8.js

// an initial state
// `const` means the variable cannot be reassigned, it does not
// `const` DOES NOT mean the value it points to is immutable
const initialState = {
  visbilityFilter: "SHOW ALL",
  todos: []
}

// The naming convention is we call the reducer the same name as whatever it is updating.
// In this case, it is a reducer that updates the todo-app, so we just call it `todoApp`.
// Note the first time this reducer is called, the state is undefined, so
// we set a default value.  This setting of a default value is an ES 6 feature.
function todoApp(state=initialState, action){
  switch(action.type){
    case "SET_VISIBILITY_FILTER":
    // Object.assign is from ES6, it let's us not alter the state, instead we make a copy of it with the updated values
    // object immutability is the philosophy of Redux
      return Object.assign({}, state, {
        visibilityFilter: action.filter
      })
    case "ADD_TODO":
      return Object.assign({}, state, {
        todos: [
          // These three dots is called a `spread`, a new feature of ES6: 
          // http://odetocode.com/blogs/scott/archive/2014/09/02/features-of-es6-part-5-the-spread.aspx
          ...state.todos,
          {
            text: action.text,
            completed: false
          }
        ]
      })
    case "COMPLETE_TODO":
      return Object.assign({}, state, {
        todos: [
          ...state.todos.slice(0, action.index),
          Object.assign({}, state.todos[action.index], {
            completed: true
          }),
          ...state.todos.slice(action.index + 1)
        ]
      }) 
     default:
       return state
  }
}