puyopuyotetr.is/app/javascript/mastodon/reducers/status_lists.js

import {
  FAVOURITED_STATUSES_FETCH_SUCCESS,
  FAVOURITED_STATUSES_EXPAND_SUCCESS,
} from '../actions/favourites';
import { Map as ImmutableMap, List as ImmutableList } from 'immutable';

const initialState = ImmutableMap({
  favourites: ImmutableMap({
    next: null,
    loaded: false,
    items: ImmutableList(),
  }),
});

const normalizeList = (state, listType, statuses, next) => {
  return state.update(listType, listMap => listMap.withMutations(map => {
    map.set('next', next);
    map.set('loaded', true);
    map.set('items', ImmutableList(statuses.map(item => item.id)));
  }));
};

const appendToList = (state, listType, statuses, next) => {
  return state.update(listType, listMap => listMap.withMutations(map => {
    map.set('next', next);
    map.set('items', map.get('items').concat(statuses.map(item => item.id)));
  }));
};

export default function statusLists(state = initialState, action) {
  switch(action.type) {
  case FAVOURITED_STATUSES_FETCH_SUCCESS:
    return normalizeList(state, 'favourites', action.statuses, action.next);
  case FAVOURITED_STATUSES_EXPAND_SUCCESS:
    return appendToList(state, 'favourites', action.statuses, action.next);
  default:
    return state;
  }
};