Theory: Extra Reducers

Separating data by slices (really by reducers in Redux) leads to situations when we react to the same action in different parts of the repository. For example, if you delete a post, you must also delete its comments, which are in another slice.

In Redux, we solve it simply by working with the switch — adding a reaction to the desired action by its name. It is no longer possible in the Redux Toolkit because of the solid link between reducers and actions. It is the price we pay for reducing the code.

Redux Toolkit brings us the extraReducers mechanism to react to actions taking place in other slices. It works quite simply. We add the extraReducers property to the slice, through which you can set reactions (reducers) to external actions:

// Working with the actions we want to address, importing them from other slices
import { removePost } from '../postsSlice.js';

const postCommentsAdapter = createEntityAdapter();
const initialState = postCommentsAdapter.getInitialState();

const postCommentsSlice = createSlice({
  name: 'comments',
  initialState: initialState,
  reducers: {
    // Here you see the ordinary reducers
  },
  extraReducers: (builder) => {
    // Here you see the optional reducers
    // When you delete a post, you have to delete all its comments
    builder.addCase(removePost, (state, action) => {
      const postId = action.payload;
      // Select all comments you want to save
      const restEntities = Object.values(state.entities).filter((e) => e.postId !== postId);
      // The `setAll` removes the current entities and adds new ones
      postCommentsAdapter.setAll(state, restEntities);
    });
  },
});

// Somewhere in the application
dispatch(removePost(post.id));

The extra reducers are added as cases to the builder object, modifying it directly. So we don't need to give anything back. Moreover, the builder supports chains, which means we can call addCases one after another builder.addCase().addCase()....