Reducer Composition

The reducer we send to Redux's createStore() method is a single function that returns the application's state object for any action that occurs within the application. That is a lot of responsibility for a single function. The way to keep the application maintainable is to split up the reducer into many smaller reducer functions that focus on a single part of the state. Reducer composition is about taking all of the small reducer functions and combine them into one function called the root reducer. Multiple reducer functions can be combined using Redux's combineReducers() function.