mastodon/app/assets/javascripts/components/reducers/status_lists.jsx

import {
  FAVOURITED_STATUSES_FETCH_SUCCESS,
  FAVOURITED_STATUSES_EXPAND_SUCCESS
} from '../actions/favourites';
import Immutable from 'immutable';

const initialState = Immutable.Map({
  favourites: Immutable.Map({
    next: null,
    loaded: false,
    items: Immutable.List()
  })
});

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', Immutable.List(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').push(...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;
  }
};