ListView2.js

const App = new Application();
App.selectedTodos = new Backbone.Collection();

// List Route
const collection = new TodoCollection();
collection.fetch().then(() => {
    const listView = new ListView({
        collection, 
        selectedTodos: App.selectedTodos
    }).render();
});

// ListView
const ListView = Backbone.View.extend({
    initialize(options) {
        this.selectedTodos = options.selectedTodos
    },
    select(model) {
        this.selectedTodos.add(model);
    },
    unselect(model) {
        this.selectedTodos.remove(model);
    }
});