Javascript MVC Syntax

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._selectedI开发者_开发技巧ndex;
    },

    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);

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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.

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The underscore doesnt mean anything, you can just use it in your variable names.

In this case it seems to be an indication that its supposed to be used for a private variable.