important concepts

1. action
2. reducer
3. store
4. middleware
5. presentational/container component

blunders / pitfalls

1. forgot to export component class when defines component
2. incorrectly using arrow function resulted in the state being updated unexpectedly, for example

   [1,2,3].map(item => item * 2);

vs

   [1,2,3].map(item => {
   	item * 2;
   });

questions

1. how to design state tree
2. how to design component hierarchy
   • design presentational components
   • design container components
   • design other components
3. Is it reasonable that component and container contains each other? e.g. Footer contains FilterLink, while VisibleTodoList contains TodoList.