request.js

// Action creator: returns request action
const requestAction = postId => ({
  type: 'POST_DELETE_REQUESTED',
  postId,
});

// Action handler: reduces the state of the single leaf
const onRequest = {
  POST_DELETE_REQUESTED:
    (state, { postId }) =>
      state.update('processingPosts', processingPosts => processingPosts.add(postId)),
};