memoize-reselect.js

const getVisibleTodos = (state, props) => 
    switch (state.visibilityFilter) {
      case 'SHOW_COMPLETED': return state.todos.filter(todo => todo.completed)
      case 'SHOW_ACTIVE': return state.todos.filter(todo => !todo.completed)
      default: return todos
    }

const mapStateToProps = (state, props) => {
  return {
    todos: memoize(getVisibleTodos(state, props))
  }
}

const mapStateToProps = memoize( (state, props) => {
  return {
    todos: getVisibleTodos(state, props)
  }
})

const mapStateToProps = memoize( (state, props) => {
  return {
    todos: memoize(getVisibleTodos(state, props))
  }
})