redux_notes8.js

// the convention here is to name the reducer function the same
// name as the property in the state object that it is modifying
// this function modifies the visibilityFilter, hence it's
// named as such
function visibilityFilter(state="SHOW_ALL", action){
  // basically the same code and logic found in
  // the "SET_VISIBILITY_FILTER" case statement
}

function todos(state=[], action){
  // basically the same code and logic found in
  // the "UPDATE_TODO" and "ADD_TODO" case statements
}

// the convention for the "main reducer function" 
// is to name it with the "App" suffix
// In this case, we name it `todoApp`
function todoApp(state=initialState, action){
  return {
    visibilityFilter: visibilityFilter(state.visibilityFilter, action),
    todos: todos(state.todos, action)
  }
}

// A note of caution: In the above example, we decomposed the above reducer 
// into the constituent reducers `visibilityFilter` and `todos`. We were able
// to do this because these two reducers handled different parts of the state 
// object independently. This may not always be the case, so you gotta be careful.