Javascript MVC Syntax

Question

I saw a Javascript MVC article here, and the model is defined as:

var ListModel = function (items) {
    this._items = items;
    this._selectedIndex = -1;

    this.itemAdded = new Event(this);
    this.itemRemoved = new Event(this);
    this.selectedIndexChanged = new Event(this);
};

ListModel.prototype = {

    getItems : function () {
        return [].concat(this._items);
    },

    addItem : function (item) {
        this._items.push(item);
        this.itemAdded.notify({item: item});
    },

    removeItemAt : function (index) {
        var item = this._items[index];
        this._items.splice(index, 1);
        this.itemRemoved.notify({item: item});
        if (index == this._selectedIndex)
            this.setSelectedIndex(-1);
    },

    getSelectedIndex : function () {
        return this._selectedIndex;
    },

    setSelectedIndex : function (index) {
        var previousIndex = this._selectedIndex;
        this._selectedIndex = index;
        this.selectedIndexChanged.notify({previous: previousIndex});
    }

};  

question 1. In Javascript, what does underscore means? e.g. this._items

question 2. in model, where does it use, how to use the following things:

this.itemAdded = new Event(this);
    this.itemRemoved = new Event(this);
    this.selectedIndexChanged = new Event(this);

Answer 1

The underscore is just convention, it only has meaning to indicate what was in somebodies head when they wrote it. In general people use an underscore to prefix method names that are meant to be private methods, meaning only use internally to a class, not used by other users.