simpleReducer.js

(state, action) => {
  switch (action.type) {
    case TOGGLE: {
      const count = state.count || 0;
      const isOn = count < 4 ? action.changes.on : false;

      return {
        ...state,
        ...action.changes,
        count: isOn ? count + 1 : count,
        on: isOn,
      };
    }
    default:
      return state;
  }
}