1. Implement multiple reducers in an application
2. Use the combineReducers() function to delegate different pieces of state to each reducer

So far we have been using a single reducer to return a new state when an action is dispatched. This works great for a small application where we only need our reducer to manage the state of one resource. However, as you will see, when working with multiple resources, placing all of this logic in one reducer function can quickly become unwieldy.

Enter combineReducers() to save the day! In this lab, we'll see how Redux's combineReducers() function lets us delegate different pieces of state to separate reducer functions.

We'll do this in the context of a book application that we'll use to keep track of programming books that we've read.

We want our app to do two things:

1. Keep track of all the books we've read: title, author, description.
2. Keep track of the authors who wrote these books.

Our app will need a state object that stores two types of information:

1. All our books, in an array
2. Our authors, also in an array

Each of these types of information--all our books, and the authors--should be represented on our store's state object. If we think of our store's state structure as a database, we can represent this as a belongs to/has many relationship, in that a book belongs to an author and an author has many books. So this means each author would have their own id, and each book would have an authorId as a foreign key.

With that, we can set the application state as:

{
  authors: //array of authors
  books: // array of books,
}

So our state object will have two top-level keys, each pointing to an array. For now, we'll use a single reducer to manage both of these resources.

// src/reducers/manageAuthorsAndBooks.js

export default function bookApp(
  state = {
    authors: [],
    books: []
  },
  action
) {
  let idx;
  switch (action.type) {
    case "ADD_BOOK":
      return {
        ...state,
        authors: [...state.authors],
        books: [...state.books, action.book]
      };
 
    case "REMOVE_BOOK":
      idx = state.books.findIndex(book => book.id === action.id);
      return {
        ...state,
        authors: [...state.authors],
        books: [...state.books.slice(0, idx), ...state.books.slice(idx + 1)]
      };
 
    case "ADD_AUTHOR":
      return {
        ...state,
        books: [...state.books],
        authors: [...state.authors, action.author]
      };
 
    case "REMOVE_AUTHOR":
      idx = state.authors.findIndex(author => author.id === action.id);
      return {
        ...state,
        books: [...state.books],
        authors: [...state.authors.slice(0, idx), ...state.authors.slice(idx + 1)]
      };
 
    default:
      return state;
  }
}

This is the current setup in src/reducers/manageAuthorsAndBooks.js, and if you boot up the application you'll see that it works. You can see, however, that with just two resources, the amount of code in our reducer is already getting pretty substantial. Moreover, placing the resources in the same reducer violates our goal of maintaining separation of concerns. By creating separate reducers for each resource in an application, we are better able to keep our code organized as our applications get more complicated.

NOTE: You may have noticed something in the reducer example: when we update one part of state, we're also using the spread operator on other parts. For example, in the "ADD_AUTHOR" case, we add action.author to the authors array, but we also use the spread operator to create a new book array. This is because both Object.assign() and the spread operator only create shallow copies of objects. If we leave out books: [...state.books], and just write the following:

return {
        ...state,
        authors: [...state.authors, action.author]
};

a new reference to the old state.books array will be used, not a new copy of the array. This is subtle, and can easily be overlooked, but by referencing the old state, we are no longer maintaining an immutable state. The official redux documentation goes into further detail on the benefits of immutability, discusses this exact issue, and provides further examples of how to properly use the spread operator to deeply copy nested data.

Redux's combineReducers() function will allow us to write separate reducers for each of the resources in our application. Each reducer is passed to the combineReducers() function, which will produce a single reducer such as the one we wrote above. We then pass that combined reducer to the store in src/index.js. Let's write some code, and then we'll walk through it below.

// src/reducers/manageAuthorsAndBooks.js

import { combineReducers } from "redux";

const rootReducer = combineReducers({
  authors: authorsReducer,
  books: booksReducer
});

export default rootReducer;

function booksReducer(state = [], action) {
  let idx;
  switch (action.type) {
    case "ADD_BOOK":
      return [...state, action.book];

    case "REMOVE_BOOK":
      idx = state.findIndex(book => book.id  === action.id)
      return [...state.slice(0, idx), ...state.slice(idx + 1)];

    default:
      return state;
  }
}

function authorsReducer(state = [], action) {
  let idx;
  switch (action.type) {
    case "ADD_AUTHOR":
      return [...state, action.author];

    case "REMOVE_AUTHOR":
      idx = state.findIndex(author => author.id  === action.id)
      return [...state.slice(0, idx), ...state.slice(idx + 1)];

    default:
      return state;
  }
}

There's a lot of code there, so let's unpack it a bit. At the very top you see the following lines:

import { combineReducers } from "redux";

const rootReducer = combineReducers({
  authors: authorsReducer,
  books: booksReducer
});

export default rootReducer;

Through combineReducer, we're telling Redux to produce a reducer which will return a state that has two keys: a key of books with a value equal to the return value of the booksReducer() and a key of authors with a value equal to the return value of the authorsReducer(). If you look at the booksReducer() and the authorsReducer() above, you will see that each returns a default state of an empty array. This will produce the same initial state that we originally specified when we built the combined reducer ourselves:

  state = {
    authors: [],
    books: []
  }

Note that because the rootReducer is the default export of manageAuthorsAndBooks.js, the app will continue to work with the code that's currently in the index.js file. However, if we like, we can update the file to reflect the new name:

// 
import { createStore } from "redux";
import rootReducer from "./reducers/manageAuthorsAndBooks";

const store = createStore(
  rootReducer,
  window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__()
);

As mentioned above, the initial state of the rootReducer is the same as the state we created when we had a single reducer for both resources. Here we are passing our rootReducer to the createStore method, so from the application's perspective nothing has changed.

Now if we examine the authorsReducer(), notice that this reducer only concerns itself with its own slice of the state. This makes sense. Remember that ultimately the array that the authorsReducer() returns will be the value associated with the key of authors in our application state object. Consequently, the authorsReducer() should only receive as its state argument the value of state.authors, in other words, the authors array. This means that we no longer need to retrieve the list of authors with a call to state.authors, but instead can access it simply by calling state.

function authorsReducer(state = [], action) {
  let idx;
  switch (action.type) {
    case "ADD_AUTHOR":
      return [...state, action.author];

    case "REMOVE_AUTHOR":
      idx = state.findIndex(author => author.id === action.id);
      return [...state.slice(0, idx), ...state.slice(idx + 1)];

    default:
      return state;
  }
}

The same applies to the booksReducer().

The combineReducers() function returns to us one large reducer that looks like the following:

function reducer(state = {
  authors: [],
  books: []
}, action) {
  let idx
  switch (action.type) {

    case "ADD_AUTHOR":
      return [...state, action.author]

    case 'REMOVE_AUTHOR':
      ...
  }
}

Because of this, we can dispatch actions the same way we always did: store.dispatch({ type: 'ADD_BOOK', { title: 'Snow Crash', author: 'Neal Stephenson' } }); will hit our switch statement in the reducer and add a new author.

One thing to note is that you can have more than one reducer respond to the same action if you want to. For example, in our application, when a user inputs information about a book, the user also inputs the author's name. It would be handy if, when a user submits a book with an author, that author is also added to our author array.

The action dispatched will be the same: store.dispatch({ type: 'ADD_BOOK', { title: 'Snow Crash', author: 'Neal Stephenson' } }); And our booksReducer stays the same as well:

function booksReducer(state = [], action) {
  let idx;
  switch (action.type) {
    case "ADD_BOOK":
      return [...state, action.book];

    case "REMOVE_BOOK":
      idx = state.findIndex(book => book.id === action.id);
      return [...state.slice(0, idx), ...state.slice(idx + 1)];

    default:
      return state;
  }
}

However, in authorsReducer, we can also include a switch case for "ADD_BOOK":

import uuid from "uuid"; 

function authorsReducer(state = [], action) {
  let idx;
  switch (action.type) {
    case "ADD_AUTHOR":
      return [...state, action.author];

    case "REMOVE_AUTHOR":
      idx = state.findIndex(author => author.id === action.id);
      return [...state.slice(0, idx), ...state.slice(idx + 1)];

    case "ADD_BOOK":
      let existingAuthor = state.filter(
        author => author.authorName === action.book.authorName
      );
      if (existingAuthor.length > 0) {
        return state;
      } else {
        return [...state, { authorName: action.book.authorName, id: uuid() }];
      }

    default:
      return state;
  }
}

In the new "ADD_BOOK" case, we're checking to see if any of the authorNames currently stored in state match the name dispatched from the BookInput component. If the name already exists, state is returned unchanged. If the name is not present, it is added to the authors array. Use the example above to modify the manageAuthorsAndBooks reducer and you can see the effect. We have two separate forms, one for adding just authors, and one that adds books and authors.

Note: We're using a useful package, uuid, to handle unique ID generation. With this refactor, since we are creating an author ID from within the reducer instead of in AuthorInput.js, we need to import it here as well.

For this codealong we have coded our two reducers in the same file, but it is common to separate each reducer into its own file. You could then import each reducer into a new file, something like reducers/rootReducer.js, where combineReducer is called. Or, alternatively, you could include combineReducer in your src/index.js file. For example:

import { combineReducers } from "redux";
import authorsReducer from './reducers/authorsReducer';
import booksReducer from './reducers/booksReducer';

const rootReducer = combineReducers({
  books: booksReducer,
  authors: authorsReducer
})

const store = createStore(rootReducer, window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__())

...

In React/Redux apps where we're using and storing many resources in our store, keeping reducers separated helps us organize code and separate concerns. Because combineReducers creates a single rootReducer for us, when an action is dispatched it can cause multiple reducers to modify their state, while still allowing us to keep all modifications to a particular resource within its own separate file.

• Implementing Combine Reducers from Scratch