simple class EventEmitter

EventEmitter.js

class EventEmitter {
    constructor() {
        this.events = {};
    }

    on(type, listener) {
        this.events[type] = this.events[type] || [];
        this.events[type].push(listener);
    }

    emit(type, arg) {
        if (this.events[type]) {
            this.events[type].forEach(listener => listener(arg));
        }
    }
}

class Model extends EventEmitter {
    constructor(items = []) {
        super();
        this.items = items;
    }

    addItem(item) {
        this.items.push(item);
        this.emit('add', this.items);
        return item;
    }
}

class Controller {
    constructor(model, view) {
        this.model = model;
        this.view = view;
        
        view.on('add', this.addTodo.bind(this));
    }

    addTodo(title) {
        const item = this.model.addItem({
            id: Date.now(),
            title,
            completed: false
        });
    }
}