catstodon/app/javascript/mastodon/reducers/local_settings.js

import { LOCAL_SETTING_CHANGE } from '../actions/local_settings';
import { STORE_HYDRATE } from '../actions/store';
import Immutable from 'immutable';

const initialState = Immutable.Map({
  layout: 'auto',
});

const hydrate = (state, localSettings) => state.mergeDeep(localSettings);

export default function localSettings(state = initialState, action) {
  switch(action.type) {
  case STORE_HYDRATE:
    return hydrate(state, action.state.get('localSettings'));
  case LOCAL_SETTING_CHANGE:
    return state.setIn(action.key, action.value);
  default:
    return state;
  }
};