success.js

// Action creator: returns success action
const successAction = postId => ({
  type: 'POST_DELETE_SUCCEEDED',
  postId,
});

// Action handlers: passing array of the reducers for this action type
//                  to apply sequence of the changes to the state tree
const onSuccess = {
  POST_DELETE_SUCCEEDED: [
    // 1. hide spinner
    (state, { postId }) =>
      state.update('processingPosts', processingPosts => processingPosts.delete(postId)),

    // 2. remove post entity
    {
      leaf: ['entities', 'posts'], // <= keypath to the leaf of the state
      reduce:
        (postsEntitiesState, { postId }) =>
          postsEntitiesState
            .updateIn(['index'], index => index.delete(postId))
            .updateIn(['entities'], entities => entities.delete(postId)),
    },
  ],
};