Are Backbone Models Singletons?

Question

I'm not a JavaScript expert so I might be doing something wrong here.

I have a simple Foo Backbone model with a default property bars which is an empty array. I create two instances of the model. I add "a" and "b" to the first model, "c" and "d" to the second model. When I print out the contents of bars with console.log(), it appears that both instances are the same object.

JsFiddle: http://jsfiddle.net/P7qsz/

Code:

var Foo = Backbone.Model.extend({
    defaults: {
        bars: []
    }
});

var foo = new Foo();
foo.get("bars").push("a");
foo.get("bars").push("b");
console.log(foo.get("bars"));

var foo2 = new Foo();
foo2.get("bars").push("c");
foo2.get("bars").push("d");
console.log(foo2.get("bars"));

In the console I see:

["a", "b", "c", "d"]
["a", "b", "c", "d"]

What am I doing wrong?

Answer 1

In javascript, Arrays (and objects) are passed as references.

Here, you assigned the same array reference to each models.

To fix it, you'll need to assign a new Array each time.

Backbone.Model.extend({
  initialize: function() {
    this.set("bars", []); // Here we set a new array
  }
});

Answer 2

They are not singletons.

console.log(foo === foo2) //false

What happens is that both instances shares the default object, and it has a refenrece to an array. So in the end all instances of this model have the same array.

Arrays and objets should never be declared in the default object, it should be in the constructor.

var Foo = Backbone.Model.extend({
    initialize: function () {
        this.set("bars", []);
    },
    defaults: {
        bars: null
    }
});