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});