[Redux] Reducer Composition with combineReducers()

Previous, we do composition with objects:

const todoApp = (state = {}, action) => {
return {
todos: todos(
state.todos,
action
),
visibilityFilter: visibilityFilter(
state.visibilityFilter,
action
)
};
};

Since it is common options in Redux, there is a function call 'combineReducers':

const {combineReducers} = Redux;
combineReducers({
todos,   //ES6 shortcut syntax
visibilityFilter
});