bigbuffet-rw/app/soapbox/reducers/chat_messages.js

import {
  CHAT_MESSAGES_FETCH_SUCCESS,
  CHAT_MESSAGE_SEND_SUCCESS,
} from 'soapbox/actions/chats';
import { Map as ImmutableMap, fromJS } from 'immutable';

const initialState = ImmutableMap();

export default function chatMessages(state = initialState, action) {
  switch(action.type) {
  case CHAT_MESSAGES_FETCH_SUCCESS:
    return state.set(action.chatId, fromJS(action.data));
  case CHAT_MESSAGE_SEND_SUCCESS:
    return state.set(action.chatId, state.get(action.chatId).insert(0, fromJS(action.data)));
  default:
    return state;
  }
};