Function reducer

examples/files/TodoListRedux.js:29–49

(state = initialState, action)

//   call reducer() with no state on startup, and we are expected to
//   return the initial state of the app in this case.
export const reducer = (state = initialState, action) => {
  const { todos } = state
  const { type, payload } = action

  switch (type) {
    case types.ADD: {
      return {
        ...state,
        todos: [payload, ...todos],
      }
    }
    case types.REMOVE: {
      return {
        ...state,
        todos: todos.filter((todo, i) => i !== payload),
      }
    }
  }

  return state
}