Concept for better reducers, with transparent meta properties for lists and id mappings.

xtPosts.hr.js

const xtPosts = makeState({
  // Give it a name for logs
  name: 'xtPosts',

  // Specify that we're using lists and byId mappings
  // maybe these aren't needed.
  list: true,
  byId: true,
  
  // Handle some redux actions
  actions: {
    // First arg is the state wrapper
    // second arg is the action payload
    // orig is the payload of FETCH_XT_FEED (original action)
    FETCH_XT_FEED_SUCCESS: (u, { posts, orig: { userId } }) =>
      // Update the list with the name being the first argument,
      // and the items being the second argument.
      // Usually lists are an array of id strings
      // This also sets a meta property for when the list was updated.
      u.setList(`postsByUser:${userId}`, posts.map(x => x.id))
      
      // Set the posts in an id:value mapping by returning tuples [id, value]
      // Updates the meta property for when the item was updated, and when it was first
      // stored.
      // We pass null as the first arg because this is nameless mapping.
      .setIdPairs(null, posts.map(x => [x.id, x])
                  
      // We're no longer loading the list for this user, so set it to being complete
      .setListLoading(`postsByUser:${userId}`, false),

    // Indicate that we're loading the list. We could also empty the list here with setList
    // but in this case we don't want that.
    FETCH_XT_FEED: (u, { userId }) => u.setListLoading(`postsByUser:${userId}`, true)),
  },
});