immutabe-store-reducer.js

import Store from 'immutable-store';

// Any mutations to an immutable-store returns the new mutated store as a result of the operation.
// More info at: https://github.com/christianalfoni/immutable-store
export default function (state = Store({todos: []}), action) {

  switch (action.type) {
  case ADD_TODO:
    return state.todos.push(action.payload);
  case REMOVE_TODO:
    return state.todos.splice(action.payload, 1);
  default:
    return state;
  }

}

In case you want to leverage existing persistent data structure, you can replace whole immutable-store with https://facebook.github.io/immutable-js/docs/#/Map/updateIn method.