extend() using underscore vs. backbone [closed]

Answer 1

Backbone.Events.extend does not exist, so I will refer to Backbone.Model instead.

_.extend(target, mixin1, mixin2) is going to copy properties into the target object

Backbone.Model.extend is going to subclass Backbone.Model basically make a constructor (function) whose prototype has your provided properties. This will allow you to make instances of your new class

var Person = Backbone.Model.extend({name: 'yourName'});
var me = new Person();
alert(me.name);

while _.extend would fail

var Person = _.extend({name: 'yourName'}, Backbone.Model);
var me = new Person();  //error b/c Person is a regular object
alert(me.name);

In short Backbone.Model.extend creates a new constructor (function), while _.extend modifies an existing object;

var modified = {};
alert(modified === _.extend(modified, Backbone.Model)); //true
alert(modified === Backbone.Model.extend(modified)); //false