Backbone documentation in markdown format

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Backbone.js

![Backbone.js][1]

Backbone.js gives structure to web applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing API over a RESTful JSON interface.

The project is [hosted on GitHub][2], and the [annotated source code][3] is available, as well as an online [test suite][4], an [example application][5], a [list of tutorials][6] and a long list of real-world projects that use Backbone. Backbone is available for use under the [MIT software license][7].

You can report bugs and discuss features on the [GitHub issues page][8], on Freenode IRC in the #documentcloud channel, post questions to the [Google Group][9], add pages to the [wiki][10] or send tweets to [@documentcloud][11].

_ Backbone is an open-source component of [DocumentCloud][12]. _

Downloads & Dependencies (Right-click, and use "Save As")

Backbone's only hard dependency is [Underscore.js][13] ( >= 1.7.0). For RESTful persistence and DOM manipulation with Backbone.View, include [jQuery][14] ( >= 1.11.0), and [json2.js][15] for older Internet Explorer support. (Mimics of the Underscore and jQuery APIs, such as [Lo-Dash][16] and [Zepto][17], will also tend to work, with varying degrees of compatibility.)

Getting Started

When working on a web application that involves a lot of JavaScript, one of the first things you learn is to stop tying your data to the DOM. It's all too easy to create JavaScript applications that end up as tangled piles of jQuery selectors and callbacks, all trying frantically to keep data in sync between the HTML UI, your JavaScript logic, and the database on your server. For rich client-side applications, a more structured approach is often helpful.

With Backbone, you represent your data as Models, which can be created, validated, destroyed, and saved to the server. Whenever a UI action causes an attribute of a model to change, the model triggers a "change" event; all the Views that display the model's state can be notified of the change, so that they are able to respond accordingly, re-rendering themselves with the new information. In a finished Backbone app, you don't have to write the glue code that looks into the DOM to find an element with a specific id, and update the HTML manually — when the model changes, the views simply update themselves.

Philosophically, Backbone is an attempt to discover the minimal set of data-structuring (models and collections) and user interface (views and URLs) primitives that are generally useful when building web applications with JavaScript. In an ecosystem where overarching, decides-everything-for-you frameworks are commonplace, and many libraries require your site to be reorganized to suit their look, feel, and default behavior — Backbone should continue to be a tool that gives you the freedom to design the full experience of your web application.

If you're new here, and aren't yet quite sure what Backbone is for, start by browsing the list of Backbone-based projects.

Many of the code examples in this documentation are runnable, because Backbone is included on this page. Click the play button to execute them.

Models and Views

![Model-View Separation.][18]

The single most important thing that Backbone can help you with is keeping your business logic separate from your user interface. When the two are entangled, change is hard; when logic doesn't depend on UI, your interface becomes easier to work with.

Model

• Orchestrates data and business logic.
• Loads and saves from the server.
• Emits events when data changes.

View

• Listens for changes and renders UI.
• Handles user input and interactivity.
• Sends captured input to the model.

A Model manages an internal table of data attributes, and triggers "change" events when any of its data is modified. Models handle syncing data with a persistence layer — usually a REST API with a backing database. Design your models as the atomic reusable objects containing all of the helpful functions for manipulating their particular bit of data. Models should be able to be passed around throughout your app, and used anywhere that bit of data is needed.

A View is an atomic chunk of user interface. It often renders the data from a specific model, or number of models — but views can also be data-less chunks of UI that stand alone. Models should be generally unaware of views. Instead, views listen to the model "change" events, and react or re-render themselves appropriately.

Collections

![Model Collections.][19]

A Collection helps you deal with a group of related models, handling the loading and saving of new models to the server and providing helper functions for performing aggregations or computations against a list of models. Aside from their own events, collections also proxy through all of the events that occur to models within them, allowing you to listen in one place for any change that might happen to any model in the collection.

API Integration

Backbone is pre-configured to sync with a RESTful API. Simply create a new Collection with the url of your resource endpoint:

var Books = Backbone.Collection.extend({
  url: '/books'
});

The Collection and Model components together form a direct mapping of REST resources using the following methods:

GET  /books/ .... collection.fetch();
POST /books/ .... collection.create();
GET  /books/1 ... model.fetch();
PUT  /books/1 ... model.save();
DEL  /books/1 ... model.destroy();

When fetching raw JSON data from an API, a Collection will automatically populate itself with data formatted as an array, while a Model will automatically populate itself with data formatted as an object:

[{"id": 1}] ..... populates a Collection with one model.
{"id": 1} ....... populates a Model with one attribute.

However, it's fairly common to encounter APIs that return data in a different format than what Backbone expects. For example, consider fetching a Collection from an API that returns the real data array wrapped in metadata:

{
  "page": 1,
  "limit": 10,
  "total": 2,
  "books": [
    {"id": 1, "title": "Pride and Prejudice"},
    {"id": 4, "title": "The Great Gatsby"}
  ]
}

In the above example data, a Collection should populate using the "books" array rather than the root object structure. This difference is easily reconciled using a parse method that returns (or transforms) the desired portion of API data:

var Books = Backbone.Collection.extend({
  url: '/books',
  parse: function(data) {
    return data.books;
  }
});

View Rendering

![View rendering.][20]

Each View manages the rendering and user interaction within its own DOM element. If you're strict about not allowing views to reach outside of themselves, it helps keep your interface flexible — allowing views to be rendered in isolation in any place where they might be needed.

Backbone remains unopinionated about the process used to render View objects and their subviews into UI: you define how your models get translated into HTML (or SVG, or Canvas, or something even more exotic). It could be as prosaic as a simple [Underscore template][21], or as fancy as the [React virtual DOM][22]. Some basic approaches to rendering views can be found in the [Backbone primer][23].

Routing with URLs

![Routing][24]

In rich web applications, we still want to provide linkable, bookmarkable, and shareable URLs to meaningful locations within an app. Use the Router to update the browser URL whenever the user reaches a new "place" in your app that they might want to bookmark or share. Conversely, the Router detects changes to the URL — say, pressing the "Back" button — and can tell your application exactly where you are now.

Backbone.Events

Events is a module that can be mixed in to any object, giving the object the ability to bind and trigger custom named events. Events do not have to be declared before they are bound, and may take passed arguments. For example:

var object = {};

_.extend(object, Backbone.Events);

object.on("alert", function(msg) {
  alert("Triggered " + msg);
});

object.trigger("alert", "an event");

For example, to make a handy event dispatcher that can coordinate events among different areas of your application: var dispatcher = _.clone(Backbone.Events)

object.on(event, callback, [context])Alias: bind
Bind a callback function to an object. The callback will be invoked whenever the event is fired. If you have a large number of different events on a page, the convention is to use colons to namespace them: "poll:start", or "change:selection". The event string may also be a space-delimited list of several events...

book.on("change:title change:author", ...);

To supply a context value for this when the callback is invoked, pass the optional third argument: model.on('change', this.render, this)

Callbacks bound to the special "all" event will be triggered when any event occurs, and are passed the name of the event as the first argument. For example, to proxy all events from one object to another:

proxy.on("all", function(eventName) {
  object.trigger(eventName);
});

All Backbone event methods also support an event map syntax, as an alternative to positional arguments:

book.on({
  "change:title": titleView.update,
  "change:author": authorPane.update,
  "destroy": bookView.remove
});

object.off([event], [callback], [context])Alias: unbind
Remove a previously-bound callback function from an object. If no context is specified, all of the versions of the callback with different contexts will be removed. If no callback is specified, all callbacks for the event will be removed. If no event is specified, callbacks for all events will be removed.

// Removes just the `onChange` callback.
object.off("change", onChange);

// Removes all "change" callbacks.
object.off("change");

// Removes the `onChange` callback for all events.
object.off(null, onChange);

// Removes all callbacks for `context` for all events.
object.off(null, null, context);

// Removes all callbacks on `object`.
object.off();

Note that calling model.off(), for example, will indeed remove all events on the model — including events that Backbone uses for internal bookkeeping.

object.trigger(event, [*args])
Trigger callbacks for the given event, or space-delimited list of events. Subsequent arguments to trigger will be passed along to the event callbacks.

object.once(event, callback, [context])
Just like on, but causes the bound callback to fire only once before being removed. Handy for saying "the next time that X happens, do this". When multiple events are passed in using the space separated syntax, the event will fire once for every event you passed in, not once for a combination of all events

object.listenTo(other, event, callback)
Tell an object to listen to a particular event on an other object. The advantage of using this form, instead of other.on(event, callback, object), is that listenTo allows the object to keep track of the events, and they can be removed all at once later on. The callback will always be called with object as context.

view.listenTo(model, 'change', view.render);

object.stopListening([other], [event], [callback])
Tell an object to stop listening to events. Either call stopListening with no arguments to have the object remove all of its registered callbacks ... or be more precise by telling it to remove just the events it's listening to on a specific object, or a specific event, or just a specific callback.

view.stopListening();

view.stopListening(model);

object.listenToOnce(other, event, callback)
Just like listenTo, but causes the bound callback to fire only once before being removed.

Here's the complete list of built-in Backbone events, with arguments. You're also free to trigger your own events on Models, Collections and Views as you see fit. The Backbone object itself mixes in Events, and can be used to emit any global events that your application needs.

• "add" (model, collection, options) — when a model is added to a collection.
• "remove" (model, collection, options) — when a model is removed from a collection.
• "update" (collection, options) — single event triggered after any number of models have been added or removed from a collection.
• "reset" (collection, options) — when the collection's entire contents have been replaced.
• "sort" (collection, options) — when the collection has been re-sorted.
• "change" (model, options) — when a model's attributes have changed.
• "change:[attribute]" (model, value, options) — when a specific attribute has been updated.
• "destroy" (model, collection, options) — when a model is destroyed.
• "request" (model_or_collection, xhr, options) — when a model or collection has started a request to the server.
• "sync" (model_or_collection, resp, options) — when a model or collection has been successfully synced with the server.
• "error" (model_or_collection, resp, options) — when a model's or collection's request to the server has failed.
• "invalid" (model, error, options) — when a model's validation fails on the client.
• "route:[name]" (params) — Fired by the router when a specific route is matched.
• "route" (route, params) — Fired by the router when any route has been matched.
• "route" (router, route, params) — Fired by history when any route has been matched.
• "all" — this special event fires for any triggered event, passing the event name as the first argument.

Generally speaking, when calling a function that emits an event (model.set, collection.add, and so on...), if you'd like to prevent the event from being triggered, you may pass {silent: true} as an option. Note that this is rarely, perhaps even never, a good idea. Passing through a specific flag in the options for your event callback to look at, and choose to ignore, will usually work out better.

Backbone.Model

Models are the heart of any JavaScript application, containing the interactive data as well as a large part of the logic surrounding it: conversions, validations, computed properties, and access control. You extend Backbone.Model with your domain-specific methods, and Model provides a basic set of functionality for managing changes.

The following is a contrived example, but it demonstrates defining a model with a custom method, setting an attribute, and firing an event keyed to changes in that specific attribute. After running this code once, sidebar will be available in your browser's console, so you can play around with it.

var Sidebar = Backbone.Model.extend({
  promptColor: function() {
    var cssColor = prompt("Please enter a CSS color:");
    this.set({color: cssColor});
  }
});

window.sidebar = new Sidebar;

sidebar.on('change:color', function(model, color) {
  $('#sidebar').css({background: color});
});

sidebar.set({color: 'white'});

sidebar.promptColor();

Backbone.Model.extend(properties, [classProperties])
To create a Model class of your own, you extend Backbone.Model and provide instance properties, as well as optional classProperties to be attached directly to the constructor function.

extend correctly sets up the prototype chain, so subclasses created with extend can be further extended and subclassed as far as you like.

var Note = Backbone.Model.extend({

  initialize: function() { ... },

  author: function() { ... },

  coordinates: function() { ... },

  allowedToEdit: function(account) {
    return true;
  }

});

var PrivateNote = Note.extend({

  allowedToEdit: function(account) {
    return account.owns(this);
  }

});

Brief aside on super: JavaScript does not provide a simple way to call super — the function of the same name defined higher on the prototype chain. If you override a core function like set, or save, and you want to invoke the parent object's implementation, you'll have to explicitly call it, along these lines:

var Note = Backbone.Model.extend({
  set: function(attributes, options) {
    Backbone.Model.prototype.set.apply(this, arguments);
    ...
  }
});

new Model([attributes], [options])
When creating an instance of a model, you can pass in the initial values of the attributes, which will be set on the model. If you define an initialize function, it will be invoked when the model is created.

new Book({
  title: "One Thousand and One Nights",
  author: "Scheherazade"
});

In rare cases, if you're looking to get fancy, you may want to override constructor, which allows you to replace the actual constructor function for your model.

var Library = Backbone.Model.extend({
  constructor: function() {
    this.books = new Books();
    Backbone.Model.apply(this, arguments);
  },
  parse: function(data, options) {
    this.books.reset(data.books);
    return data.library;
  }
});

If you pass a {collection: ...} as the options, the model gains a collection property that will be used to indicate which collection the model belongs to, and is used to help compute the model's url. The model.collection property is normally created automatically when you first add a model to a collection. Note that the reverse is not true, as passing this option to the constructor will not automatically add the model to the collection. Useful, sometimes.

If {parse: true} is passed as an option, the attributes will first be converted by parse before being set on the model.

model.get(attribute)
Get the current value of an attribute from the model. For example: note.get("title")

model.set(attributes, [options])
Set a hash of attributes (one or many) on the model. If any of the attributes change the model's state, a "change" event will be triggered on the model. Change events for specific attributes are also triggered, and you can bind to those as well, for example: change:title, and change:content. You may also pass individual keys and values.

note.set({title: "March 20", content: "In his eyes she eclipses..."});

book.set("title", "A Scandal in Bohemia");

model.escape(attribute)
Similar to get, but returns the HTML-escaped version of a model's attribute. If you're interpolating data from the model into HTML, using escape to retrieve attributes will prevent [XSS][25] attacks.

var hacker = new Backbone.Model({
  name: "<script>alert('xss')</script>"
});

alert(hacker.escape('name'));

model.has(attribute)
Returns true if the attribute is set to a non-null or non-undefined value.

if (note.has("title")) {
  ...
}

model.unset(attribute, [options])
Remove an attribute by deleting it from the internal attributes hash. Fires a "change" event unless silent is passed as an option.

model.clear([options])
Removes all attributes from the model, including the id attribute. Fires a "change" event unless silent is passed as an option.

model.id
A special property of models, the id is an arbitrary string (integer id or UUID). If you set the id in the attributes hash, it will be copied onto the model as a direct property. Models can be retrieved by id from collections, and the id is used to generate model URLs by default.

model.idAttribute
A model's unique identifier is stored under the id attribute. If you're directly communicating with a backend (CouchDB, MongoDB) that uses a different unique key, you may set a Model's idAttribute to transparently map from that key to id.

var Meal = Backbone.Model.extend({
  idAttribute: "_id"
});

var cake = new Meal({ _id: 1, name: "Cake" });
alert("Cake id: " + cake.id);

model.cid
A special property of models, the cid or client id is a unique identifier automatically assigned to all models when they're first created. Client ids are handy when the model has not yet been saved to the server, and does not yet have its eventual true id, but already needs to be visible in the UI.

model.attributes
The attributes property is the internal hash containing the model's state — usually (but not necessarily) a form of the JSON object representing the model data on the server. It's often a straightforward serialization of a row from the database, but it could also be client-side computed state.

Please use set to update the attributes instead of modifying them directly. If you'd like to retrieve and munge a copy of the model's attributes, use _.clone(model.attributes) instead.

Due to the fact that Events accepts space separated lists of events, attribute names should not include spaces.

model.changed
The changed property is the internal hash containing all the attributes that have changed since its last set. Please do not update changed directly since its state is internally maintained by set. A copy of changed can be acquired from changedAttributes.

model.defaults or model.defaults()
The defaults hash (or function) can be used to specify the default attributes for your model. When creating an instance of the model, any unspecified attributes will be set to their default value.

var Meal = Backbone.Model.extend({
  defaults: {
    "appetizer":  "caesar salad",
    "entree":     "ravioli",
    "dessert":    "cheesecake"
  }
});

alert("Dessert will be " + (new Meal).get('dessert'));

Remember that in JavaScript, objects are passed by reference, so if you include an object as a default value, it will be shared among all instances. Instead, define defaults as a function.

model.toJSON([options])
Return a shallow copy of the model's attributes for JSON stringification. This can be used for persistence, serialization, or for augmentation before being sent to the server. The name of this method is a bit confusing, as it doesn't actually return a JSON string — but I'm afraid that it's the way that the [JavaScript API for JSON.stringify][26] works.

var artist = new Backbone.Model({
  firstName: "Wassily",
  lastName: "Kandinsky"
});

artist.set({birthday: "December 16, 1866"});

alert(JSON.stringify(artist));

model.sync(method, model, [options])
Uses Backbone.sync to persist the state of a model to the server. Can be overridden for custom behavior.

model.fetch([options])
Merges the model's state with attributes fetched from the server by delegating to Backbone.sync. Returns a [jqXHR][27]. Useful if the model has never been populated with data, or if you'd like to ensure that you have the latest server state. Triggers a "change" event if the server's state differs from the current attributes. fetch accepts success and error callbacks in the options hash, which are both passed (model, response, options) as arguments.

// Poll every 10 seconds to keep the channel model up-to-date.
setInterval(function() {
  channel.fetch();
}, 10000);

model.save([attributes], [options])
Save a model to your database (or alternative persistence layer), by delegating to Backbone.sync. Returns a [jqXHR][27] if validation is successful and false otherwise. The attributes hash (as in set) should contain the attributes you'd like to change — keys that aren't mentioned won't be altered — but, a complete representation of the resource will be sent to the server. As with set, you may pass individual keys and values instead of a hash. If the model has a validate method, and validation fails, the model will not be saved. If the model isNew, the save will be a "create" (HTTP POST), if the model already exists on the server, the save will be an "update" (HTTP PUT).

If instead, you'd only like the changed attributes to be sent to the server, call model.save(attrs, {patch: true}). You'll get an HTTP PATCH request to the server with just the passed-in attributes.

Calling save with new attributes will cause a "change" event immediately, a "request" event as the Ajax request begins to go to the server, and a "sync" event after the server has acknowledged the successful change. Pass {wait: true} if you'd like to wait for the server before setting the new attributes on the model.

In the following example, notice how our overridden version of Backbone.sync receives a "create" request the first time the model is saved and an "update" request the second time.

Backbone.sync = function(method, model) {
  alert(method + ": " + JSON.stringify(model));
  model.set('id', 1);
};

var book = new Backbone.Model({
  title: "The Rough Riders",
  author: "Theodore Roosevelt"
});

book.save();

book.save({author: "Teddy"});

save accepts success and error callbacks in the options hash, which will be passed the arguments (model, response, options). If a server-side validation fails, return a non-200 HTTP response code, along with an error response in text or JSON.

book.save("author", "F.D.R.", {error: function(){ ... }});

model.destroy([options])
Destroys the model on the server by delegating an HTTP DELETE request to Backbone.sync. Returns a [jqXHR][27] object, or false if the model isNew. Accepts success and error callbacks in the options hash, which will be passed (model, response, options). Triggers a "destroy" event on the model, which will bubble up through any collections that contain it, a "request" event as it begins the Ajax request to the server, and a "sync" event, after the server has successfully acknowledged the model's deletion. Pass {wait: true} if you'd like to wait for the server to respond before removing the model from the collection.

book.destroy({success: function(model, response) {
  ...
}});

Backbone proxies to Underscore.js to provide 9 object functions on Backbone.Model. They aren't all documented here, but you can take a look at the Underscore documentation for the full details…

user.pick('first_name', 'last_name', 'email');

chapters.keys().join(', ');

model.validate(attributes, options)
This method is left undefined and you're encouraged to override it with any custom validation logic you have that can be performed in JavaScript. By default save checks validate before setting any attributes but you may also tell set to validate the new attributes by passing {validate: true} as an option.
The validate method receives the model attributes as well as any options passed to set or save. If the attributes are valid, don't return anything from validate; if they are invalid return an error of your choosing. It can be as simple as a string error message to be displayed, or a complete error object that describes the error programmatically. If validate returns an error, save will not continue, and the model attributes will not be modified on the server. Failed validations trigger an "invalid" event, and set the validationError property on the model with the value returned by this method.

var Chapter = Backbone.Model.extend({
  validate: function(attrs, options) {
    if (attrs.end < attrs.start) {
      return "can't end before it starts";
    }
  }
});

var one = new Chapter({
  title : "Chapter One: The Beginning"
});

one.on("invalid", function(model, error) {
  alert(model.get("title") + " " + error);
});

one.save({
  start: 15,
  end:   10
});

"invalid" events are useful for providing coarse-grained error messages at the model or collection level.

model.validationError
The value returned by validate during the last failed validation.

model.isValid()
Run validate to check the model state.

var Chapter = Backbone.Model.extend({
  validate: function(attrs, options) {
    if (attrs.end < attrs.start) {
      return "can't end before it starts";
    }
  }
});

var one = new Chapter({
  title : "Chapter One: The Beginning"
});

one.set({
  start: 15,
  end:   10
});

if (!one.isValid()) {
  alert(one.get("title") + " " + one.validationError);
}

model.url()
Returns the relative URL where the model's resource would be located on the server. If your models are located somewhere else, override this method with the correct logic. Generates URLs of the form: "[collection.url]/[id]" by default, but you may override by specifying an explicit urlRoot if the model's collection shouldn't be taken into account.

Delegates to Collection#url to generate the URL, so make sure that you have it defined, or a urlRoot property, if all models of this class share a common root URL. A model with an id of 101, stored in a Backbone.Collection with a url of "/documents/7/notes", would have this URL: "/documents/7/notes/101"

model.urlRoot or model.urlRoot()
Specify a urlRoot if you're using a model outside of a collection, to enable the default url function to generate URLs based on the model id. "[urlRoot]/id"
Normally, you won't need to define this. Note that urlRoot may also be a function.

var Book = Backbone.Model.extend({urlRoot : '/books'});

var solaris = new Book({id: "1083-lem-solaris"});

alert(solaris.url());

model.parse(response, options)
parse is called whenever a model's data is returned by the server, in fetch, and save. The function is passed the raw response object, and should return the attributes hash to be set on the model. The default implementation is a no-op, simply passing through the JSON response. Override this if you need to work with a preexisting API, or better namespace your responses.

If you're working with a Rails backend that has a version prior to 3.1, you'll notice that its default to_json implementation includes a model's attributes under a namespace. To disable this behavior for seamless Backbone integration, set:

ActiveRecord::Base.include_root_in_json = false

model.clone()
Returns a new instance of the model with identical attributes.

model.isNew()
Has this model been saved to the server yet? If the model does not yet have an id, it is considered to be new.

model.hasChanged([attribute])
Has the model changed since its last set? If an attribute is passed, returns true if that specific attribute has changed.

Note that this method, and the following change-related ones, are only useful during the course of a "change" event.

book.on("change", function() {
  if (book.hasChanged("title")) {
    ...
  }
});

model.changedAttributes([attributes])
Retrieve a hash of only the model's attributes that have changed since the last set, or false if there are none. Optionally, an external attributes hash can be passed in, returning the attributes in that hash which differ from the model. This can be used to figure out which portions of a view should be updated, or what calls need to be made to sync the changes to the server.

model.previous(attribute)
During a "change" event, this method can be used to get the previous value of a changed attribute.

var bill = new Backbone.Model({
  name: "Bill Smith"
});

bill.on("change:name", function(model, name) {
  alert("Changed name from " + bill.previous("name") + " to " + name);
});

bill.set({name : "Bill Jones"});

model.previousAttributes()
Return a copy of the model's previous attributes. Useful for getting a diff between versions of a model, or getting back to a valid state after an error occurs.

Backbone.Collection

Collections are ordered sets of models. You can bind "change" events to be notified when any model in the collection has been modified, listen for "add" and "remove" events, fetch the collection from the server, and use a full suite of Underscore.js methods.

Any event that is triggered on a model in a collection will also be triggered on the collection directly, for convenience. This allows you to listen for changes to specific attributes in any model in a collection, for example: documents.on("change:selected", ...)

Backbone.Collection.extend(properties, [classProperties])
To create a Collection class of your own, extend Backbone.Collection, providing instance properties, as well as optional classProperties to be attached directly to the collection's constructor function.

collection.model
Override this property to specify the model class that the collection contains. If defined, you can pass raw attributes objects (and arrays) to add, create, and reset, and the attributes will be converted into a model of the proper type.

var Library = Backbone.Collection.extend({
  model: Book
});

A collection can also contain polymorphic models by overriding this property with a constructor that returns a model.

var Library = Backbone.Collection.extend({

  model: function(attrs, options) {
    if (condition) {
      return new PublicDocument(attrs, options);
    } else {
      return new PrivateDocument(attrs, options);
    }
  }

});

collection.modelId
Override this method to specify the attribute the collection will use to refer to its models in collection.get.
By default returns the idAttribute of the collection's model class or failing that, 'id'. If your collection uses polymorphic models and those models have an idAttribute other than id you must override this method with your own custom logic.

var Library = Backbone.Collection.extend({

  model: function(attrs, options) {
    if (condition) {
      return new PublicDocument(attrs, options);
    } else {
      return new PrivateDocument(attrs, options);
    }
  },

  modelId: function(attrs) {
    return attrs.private ? 'private_id' : 'public_id';
  }

});

new Backbone.Collection([models], [options])
When creating a Collection, you may choose to pass in the initial array of models. The collection's comparator may be included as an option. Passing false as the comparator option will prevent sorting. If you define an initialize function, it will be invoked when the collection is created. There are a couple of options that, if provided, are attached to the collection directly: model and comparator.
Pass null for models to create an empty Collection with options.

var tabs = new TabSet([tab1, tab2, tab3]);
var spaces = new Backbone.Collection([], {
  model: Space
});

collection.models
Raw access to the JavaScript array of models inside of the collection. Usually you'll want to use get, at, or the Underscore methods to access model objects, but occasionally a direct reference to the array is desired.

collection.toJSON([options])
Return an array containing the attributes hash of each model (via toJSON) in the collection. This can be used to serialize and persist the collection as a whole. The name of this method is a bit confusing, because it conforms to [JavaScript's JSON API][26].

var collection = new Backbone.Collection([
  {name: "Tim", age: 5},
  {name: "Ida", age: 26},
  {name: "Rob", age: 55}
]);

alert(JSON.stringify(collection));

collection.sync(method, collection, [options])
Uses Backbone.sync to persist the state of a collection to the server. Can be overridden for custom behavior.

Backbone proxies to Underscore.js to provide 46 iteration functions on Backbone.Collection. They aren't all documented here, but you can take a look at the Underscore documentation for the full details…

books.each(function(book) {
  book.publish();
});

var titles = books.map(function(book) {
  return book.get("title");
});

var publishedBooks = books.filter(function(book) {
  return book.get("published") === true;
});

var alphabetical = books.sortBy(function(book) {
  return book.author.get("name").toLowerCase();
});

collection.add(models, [options])
Add a model (or an array of models) to the collection, firing an "add" event for each model, and an "update" event afterwards. If a model property is defined, you may also pass raw attributes objects, and have them be vivified as instances of the model. Returns the added (or preexisting, if duplicate) models. Pass {at: index} to splice the model into the collection at the specified index. If you're adding models to the collection that are already in the collection, they'll be ignored, unless you pass {merge: true}, in which case their attributes will be merged into the corresponding models, firing any appropriate "change" events.

var ships = new Backbone.Collection;

ships.on("add", function(ship) {
  alert("Ahoy " + ship.get("name") + "!");
});

ships.add([
  {name: "Flying Dutchman"},
  {name: "Black Pearl"}
]);

Note that adding the same model (a model with the same id) to a collection more than once
is a no-op.

collection.remove(models, [options])
Remove a model (or an array of models) from the collection, and return them. Each model can be a Model instance, an id string or a JS object, any value acceptable as the id argument of collection.get. Fires a "remove" event for each model, and a single "update" event afterwards. The model's index before removal is available to listeners as options.index.

collection.reset([models], [options])
Adding and removing models one at a time is all well and good, but sometimes you have so many models to change that you'd rather just update the collection in bulk. Use reset to replace a collection with a new list of models (or attribute hashes), triggering a single "reset" event at the end. Returns the newly-set models. For convenience, within a "reset" event, the list of any previous models is available as options.previousModels.
Pass null for models to empty your Collection with options.

Here's an example using reset to bootstrap a collection during initial page load, in a Rails application:

<script>
  var accounts = new Backbone.Collection;
  accounts.reset(<%= @accounts.to_json %>);
</script>

Calling collection.reset() without passing any models as arguments will empty the entire collection.

collection.set(models, [options])
The set method performs a "smart" update of the collection with the passed list of models. If a model in the list isn't yet in the collection it will be added; if the model is already in the collection its attributes will be merged; and if the collection contains any models that aren't present in the list, they'll be removed. All of the appropriate "add", "remove", and "change" events are fired as this happens. Returns the touched models in the collection. If you'd like to customize the behavior, you can disable it with options: {add: false}, {remove: false}, or {merge: false}.

var vanHalen = new Backbone.Collection([eddie, alex, stone, roth]);

vanHalen.set([eddie, alex, stone, hagar]);

// Fires a "remove" event for roth, and an "add" event for "hagar".
// Updates any of stone, alex, and eddie's attributes that may have
// changed over the years.

collection.get(id)
Get a model from a collection, specified by an id, a cid, or by passing in a model.

var book = library.get(110);

collection.at(index)
Get a model from a collection, specified by index. Useful if your collection is sorted, and if your collection isn't sorted, at will still retrieve models in insertion order.

collection.push(model, [options])
Add a model at the end of a collection. Takes the same options as add.

collection.pop([options])
Remove and return the last model from a collection. Takes the same options as remove.

collection.unshift(model, [options])
Add a model at the beginning of a collection. Takes the same options as add.

collection.shift([options])
Remove and return the first model from a collection. Takes the same options as remove.

collection.slice(begin, end)
Return a shallow copy of this collection's models, using the same options as native [Array#slice][28].

collection.length
Like an array, a Collection maintains a length property, counting the number of models it contains.

collection.comparator
By default there is no comparator for a collection. If you define a comparator, it will be used to maintain the collection in sorted order. This means that as models are added, they are inserted at the correct index in collection.models. A comparator can be defined as a [sortBy][29] (pass a function that takes a single argument), as a [sort][30] (pass a comparator function that expects two arguments), or as a string indicating the attribute to sort by.

"sortBy" comparator functions take a model and return a numeric or string value by which the model should be ordered relative to others. "sort" comparator functions take two models, and return -1 if the first model should come before the second, 0 if they are of the same rank and 1 if the first model should come after. Note that Backbone depends on the arity of your comparator function to determine between the two styles, so be careful if your comparator function is bound.

Note how even though all of the chapters in this example are added backwards, they come out in the proper order:

var Chapter  = Backbone.Model;
var chapters = new Backbone.Collection;

chapters.comparator = 'page';

chapters.add(new Chapter({page: 9, title: "The End"}));
chapters.add(new Chapter({page: 5, title: "The Middle"}));
chapters.add(new Chapter({page: 1, title: "The Beginning"}));

alert(chapters.pluck('title'));

Collections with a comparator will not automatically re-sort if you later change model attributes, so you may wish to call sort after changing model attributes that would affect the order.

collection.sort([options])
Force a collection to re-sort itself. You don't need to call this under normal circumstances, as a collection with a comparator will sort itself whenever a model is added. To disable sorting when adding a model, pass {sort: false} to add. Calling sort triggers a "sort" event on the collection.

collection.pluck(attribute)
Pluck an attribute from each model in the collection. Equivalent to calling map and returning a single attribute from the iterator.

var stooges = new Backbone.Collection([
  {name: "Curly"},
  {name: "Larry"},
  {name: "Moe"}
]);

var names = stooges.pluck("name");

alert(JSON.stringify(names));

collection.where(attributes)
Return an array of all the models in a collection that match the passed attributes. Useful for simple cases of filter.

var friends = new Backbone.Collection([
  {name: "Athos",      job: "Musketeer"},
  {name: "Porthos",    job: "Musketeer"},
  {name: "Aramis",     job: "Musketeer"},
  {name: "d'Artagnan", job: "Guard"},
]);

var musketeers = friends.where({job: "Musketeer"});

alert(musketeers.length);

collection.findWhere(attributes)
Just like where, but directly returns only the first model in the collection that matches the passed attributes.

collection.url or collection.url()
Set the url property (or function) on a collection to reference its location on the server. Models within the collection will use url to construct URLs of their own.

var Notes = Backbone.Collection.extend({
  url: '/notes'
});

// Or, something more sophisticated:

var Notes = Backbone.Collection.extend({
  url: function() {
    return this.document.url() + '/notes';
  }
});

collection.parse(response, options)
parse is called by Backbone whenever a collection's models are returned by the server, in fetch. The function is passed the raw response object, and should return the array of model attributes to be added to the collection. The default implementation is a no-op, simply passing through the JSON response. Override this if you need to work with a preexisting API, or better namespace your responses.

var Tweets = Backbone.Collection.extend({
  // The Twitter Search API returns tweets under "results".
  parse: function(response) {
    return response.results;
  }
});

collection.clone()
Returns a new instance of the collection with an identical list of models.

collection.fetch([options])
Fetch the default set of models for this collection from the server, setting them on the collection when they arrive. The options hash takes success and error callbacks which will both be passed (collection, response, options) as arguments. When the model data returns from the server, it uses set to (intelligently) merge the fetched models, unless you pass {reset: true}, in which case the collection will be (efficiently) reset. Delegates to Backbone.sync under the covers for custom persistence strategies and returns a [jqXHR][27]. The server handler for fetch requests should return a JSON array of models.

Backbone.sync = function(method, model) {
  alert(method + ": " + model.url);
};

var accounts = new Backbone.Collection;
accounts.url = '/accounts';

accounts.fetch();

The behavior of fetch can be customized by using the available set options. For example, to fetch a collection, getting an "add" event for every new model, and a "change" event for every changed existing model, without removing anything: collection.fetch({remove: false})

jQuery.ajax options can also be passed directly as fetch options, so to fetch a specific page of a paginated collection: Documents.fetch({data: {page: 3}})

Note that fetch should not be used to populate collections on page load — all models needed at load time should already be bootstrapped in to place. fetch is intended for lazily-loading models for interfaces that are not needed immediately: for example, documents with collections of notes that may be toggled open and closed.

collection.create(attributes, [options])
Convenience to create a new instance of a model within a collection. Equivalent to instantiating a model with a hash of attributes, saving the model to the server, and adding the model to the set after being successfully created. Returns the new model. If client-side validation failed, the model will be unsaved, with validation errors. In order for this to work, you should set the model property of the collection. The create method can accept either an attributes hash or an existing, unsaved model object.

Creating a model will cause an immediate "add" event to be triggered on the collection, a "request" event as the new model is sent to the server, as well as a "sync" event, once the server has responded with the successful creation of the model. Pass {wait: true} if you'd like to wait for the server before adding the new model to the collection.

var Library = Backbone.Collection.extend({
  model: Book
});

var nypl = new Library;

var othello = nypl.create({
  title: "Othello",
  author: "William Shakespeare"
});

Backbone.Router

Web applications often provide linkable, bookmarkable, shareable URLs for important locations in the app. Until recently, hash fragments (#page) were used to provide these permalinks, but with the arrival of the History API, it's now possible to use standard URLs (/page). Backbone.Router provides methods for routing client-side pages, and connecting them to actions and events. For browsers which don't yet support the History API, the Router handles graceful fallback and transparent translation to the fragment version of the URL.

During page load, after your application has finished creating all of its routers, be sure to call Backbone.history.start() or Backbone.history.start({pushState: true}) to route the initial URL.

Backbone.Router.extend(properties, [classProperties])
Get started by creating a custom router class. Define actions that are triggered when certain URL fragments are matched, and provide a routes hash that pairs routes to actions. Note that you'll want to avoid using a leading slash in your route definitions:

var Workspace = Backbone.Router.extend({

  routes: {
    "help":                 "help",    // #help
    "search/:query":        "search",  // #search/kiwis
    "search/:query/p:page": "search"   // #search/kiwis/p7
  },

  help: function() {
    ...
  },

  search: function(query, page) {
    ...
  }

});

router.routes
The routes hash maps URLs with parameters to functions on your router (or just direct function definitions, if you prefer), similar to the View's events hash. Routes can contain parameter parts, :param, which match a single URL component between slashes; and splat parts *splat, which can match any number of URL components. Part of a route can be made optional by surrounding it in parentheses (/:optional).

For example, a route of "search/:query/p:page" will match a fragment of #search/obama/p2, passing "obama" and "2" to the action.

A route of "file/*path" will match #file/nested/folder/file.txt, passing "nested/folder/file.txt" to the action.

A route of "docs/:section(/:subsection)" will match #docs/faq and #docs/faq/installing, passing "faq" to the action in the first case, and passing "faq" and "installing" to the action in the second.

Trailing slashes are treated as part of the URL, and (correctly) treated as a unique route when accessed. docs and docs/ will fire different callbacks. If you can't avoid generating both types of URLs, you can define a "docs(/)" matcher to capture both cases.

When the visitor presses the back button, or enters a URL, and a particular route is matched, the name of the action will be fired as an event, so that other objects can listen to the router, and be notified. In the following example, visiting #help/uploading will fire a route:help event from the router.

routes: {
  "help/:page":         "help",
  "download/*path":     "download",
  "folder/:name":       "openFolder",
  "folder/:name-:mode": "openFolder"
}

router.on("route:help", function(page) {
  ...
});

new Router([options])
When creating a new router, you may pass its routes hash directly as an option, if you choose. All options will also be passed to your initialize function, if defined.

router.route(route, name, [callback])
Manually create a route for the router, The route argument may be a routing string or regular expression. Each matching capture from the route or regular expression will be passed as an argument to the callback. The name argument will be triggered as a "route:name" event whenever the route is matched. If the callback argument is omitted router[name] will be used instead. Routes added later may override previously declared routes.

initialize: function(options) {

  // Matches #page/10, passing "10"
  this.route("page/:number", "page", function(number){ ... });

  // Matches /117-a/b/c/open, passing "117-a/b/c" to this.open
  this.route(/^(.*?)/open$/, "open");

},

open: function(id) { ... }

router.navigate(fragment, [options])
Whenever you reach a point in your application that you'd like to save as a URL, call navigate in order to update the URL. If you also wish to call the route function, set the trigger option to true. To update the URL without creating an entry in the browser's history, set the replace option to true.

openPage: function(pageNumber) {
  this.document.pages.at(pageNumber).open();
  this.navigate("page/" + pageNumber);
}

# Or ...

app.navigate("help/troubleshooting", {trigger: true});

# Or ...

app.navigate("help/troubleshooting", {trigger: true, replace: true});

router.execute(callback, args, name)
This method is called internally within the router, whenever a route matches and its corresponding callback is about to be executed. Return false from execute to cancel the current transition. Override it to perform custom parsing or wrapping of your routes, for example, to parse query strings before handing them to your route callback, like so:

var Router = Backbone.Router.extend({
  execute: function(callback, args, name) {
    if (!loggedIn) {
      goToLogin();
      return false;
    }
    args.push(parseQueryString(args.pop()));
    if (callback) callback.apply(this, args);
  }
});

Backbone.history

History serves as a global router (per frame) to handle hashchange events or pushState, match the appropriate route, and trigger callbacks. You shouldn't ever have to create one of these yourself since Backbone.history already contains one.

pushState support exists on a purely opt-in basis in Backbone. Older browsers that don't support pushState will continue to use hash-based URL fragments, and if a hash URL is visited by a pushState-capable browser, it will be transparently upgraded to the true URL. Note that using real URLs requires your web server to be able to correctly render those pages, so back-end changes are required as well. For example, if you have a route of /documents/100, your web server must be able to serve that page, if the browser visits that URL directly. For full search-engine crawlability, it's best to have the server generate the complete HTML for the page ... but if it's a web application, just rendering the same content you would have for the root URL, and filling in the rest with Backbone Views and JavaScript works fine.

Backbone.history.start([options])
When all of your Routers have been created, and all of the routes are set up properly, call Backbone.history.start() to begin monitoring hashchange events, and dispatching routes. Subsequent calls to Backbone.history.start() will throw an error, and Backbone.History.started is a boolean value indicating whether it has already been called.

To indicate that you'd like to use HTML5 pushState support in your application, use Backbone.history.start({pushState: true}). If you'd like to use pushState, but have browsers that don't support it natively use full page refreshes instead, you can add {hashChange: false} to the options.

If your application is not being served from the root url / of your domain, be sure to tell History where the root really is, as an option: Backbone.history.start({pushState: true, root: "/public/search/"})

When called, if a route succeeds with a match for the current URL, Backbone.history.start() returns true. If no defined route matches the current URL, it returns false.

If the server has already rendered the entire page, and you don't want the initial route to trigger when starting History, pass silent: true.

Because hash-based history in Internet Explorer relies on an <iframe>, be sure to call start() only after the DOM is ready.

$(function(){
  new WorkspaceRouter();
  new HelpPaneRouter();
  Backbone.history.start({pushState: true});
});

Backbone.sync

Backbone.sync is the function that Backbone calls every time it attempts to read or save a model to the server. By default, it uses jQuery.ajax to make a RESTful JSON request and returns a [jqXHR][27]. You can override it in order to use a different persistence strategy, such as WebSockets, XML transport, or Local Storage.

The method signature of Backbone.sync is sync(method, model, [options])

• method – the CRUD method ("create", "read", "update", or "delete")
• model – the model to be saved (or collection to be read)
• options – success and error callbacks, and all other jQuery request options

With the default implementation, when Backbone.sync sends up a request to save a model, its attributes will be passed, serialized as JSON, and sent in the HTTP body with content-type application/json. When returning a JSON response, send down the attributes of the model that have been changed by the server, and need to be updated on the client. When responding to a "read" request from a collection (Collection#fetch), send down an array of model attribute objects.

Whenever a model or collection begins a sync with the server, a "request" event is emitted. If the request completes successfully you'll get a "sync" event, and an "error" event if not.

The sync function may be overridden globally as Backbone.sync, or at a finer-grained level, by adding a sync function to a Backbone collection or to an individual model.

The default sync handler maps CRUD to REST like so:

• **create → POST **/collection
• **read → GET **/collection[/id]
• **update → PUT **/collection/id
• **patch → PATCH **/collection/id
• **delete → DELETE **/collection/id

As an example, a Rails 4 handler responding to an "update" call from Backbone might look like this:

def update
  account = Account.find params[:id]
  account.update_attributes params.require(:account).permit(:name, :otherparam)
  render :json => account
end

One more tip for integrating Rails versions prior to 3.1 is to disable the default namespacing for to_json calls on models by setting ActiveRecord::Base.include_root_in_json = false

Backbone.ajax = function(request) { ... };
If you want to use a custom AJAX function, or your endpoint doesn't support the [jQuery.ajax][31] API and you need to tweak things, you can do so by setting Backbone.ajax.

Backbone.emulateHTTP = true
If you want to work with a legacy web server that doesn't support Backbone's default REST/HTTP approach, you may choose to turn on Backbone.emulateHTTP. Setting this option will fake PUT, PATCH and DELETE requests with a HTTP POST, setting the X-HTTP-Method-Override header with the true method. If emulateJSON is also on, the true method will be passed as an additional _method parameter.

Backbone.emulateHTTP = true;

model.save();  // POST to "/collection/id", with "_method=PUT" + header.

Backbone.emulateJSON = true
If you're working with a legacy web server that can't handle requests encoded as application/json, setting Backbone.emulateJSON = true; will cause the JSON to be serialized under a model parameter, and the request to be made with a application/x-www-form-urlencoded MIME type, as if from an HTML form.

Backbone.View

Backbone views are almost more convention than they are code — they don't determine anything about your HTML or CSS for you, and can be used with any JavaScript templating library. The general idea is to organize your interface into logical views, backed by models, each of which can be updated independently when the model changes, without having to redraw the page. Instead of digging into a JSON object, looking up an element in the DOM, and updating the HTML by hand, you can bind your view's render function to the model's "change" event — and now everywhere that model data is displayed in the UI, it is always immediately up to date.

Backbone.View.extend(properties, [classProperties])
Get started with views by creating a custom view class. You'll want to override the render function, specify your declarative events, and perhaps the tagName, className, or id of the View's root element.

var DocumentRow = Backbone.View.extend({

  tagName: "li",

  className: "document-row",

  events: {
    "click .icon":          "open",
    "click .button.edit":   "openEditDialog",
    "click .button.delete": "destroy"
  },

  initialize: function() {
    this.listenTo(this.model, "change", this.render);
  },

  render: function() {
    ...
  }

});

Properties like tagName, id, className, el, and events may also be defined as a function, if you want to wait to define them until runtime.

new View([options])
There are several special options that, if passed, will be attached directly to the view: model, collection, el, id, className, tagName, attributes and events. If the view defines an initialize function, it will be called when the view is first created. If you'd like to create a view that references an element already in the DOM, pass in the element as an option: new View({el: existingElement})

var doc = documents.first();

new DocumentRow({
  model: doc,
  id: "document-row-" + doc.id
});

view.el
All views have a DOM element at all times (the el property), whether they've already been inserted into the page or not. In this fashion, views can be rendered at any time, and inserted into the DOM all at once, in order to get high-performance UI rendering with as few reflows and repaints as possible.

this.el can be resolved from a DOM selector string or an Element; otherwise it will be created from the view's tagName, className, id and attributes properties. If none are set, this.el is an empty div, which is often just fine. An el reference may also be passed in to the view's constructor.

var ItemView = Backbone.View.extend({
  tagName: 'li'
});

var BodyView = Backbone.View.extend({
  el: 'body'
});

var item = new ItemView();
var body = new BodyView();

alert(item.el + ' ' + body.el);

view.$el
A cached jQuery object for the view's element. A handy reference instead of re-wrapping the DOM element all the time.

view.$el.show();

listView.$el.append(itemView.el);

view.setElement(element)
If you'd like to apply a Backbone view to a different DOM element, use setElement, which will also create the cached $el reference and move the view's delegated events from the old element to the new one.

view.attributes
A hash of attributes that will be set as HTML DOM element attributes on the view's el (id, class, data-properties, etc.), or a function that returns such a hash.

view.$(selector)
If jQuery is included on the page, each view has a $ function that runs queries scoped within the view's element. If you use this scoped jQuery function, you don't have to use model ids as part of your query to pull out specific elements in a list, and can rely much more on HTML class attributes. It's equivalent to running: view.$el.find(selector)

ui.Chapter = Backbone.View.extend({
  serialize : function() {
    return {
      title: this.$(".title").text(),
      start: this.$(".start-page").text(),
      end:   this.$(".end-page").text()
    };
  }
});

view.template([data])
While templating for a view isn't a function provided directly by Backbone, it's often a nice convention to define a template function on your views. In this way, when rendering your view, you have convenient access to instance data. For example, using Underscore templates:

var LibraryView = Backbone.View.extend({
  template: _.template(...)
});

view.render()
The default implementation of render is a no-op. Override this function with your code that renders the view template from model data, and updates this.el with the new HTML. A good convention is to return this at the end of render to enable chained calls.

var Bookmark = Backbone.View.extend({
  template: _.template(...),
  render: function() {
    this.$el.html(this.template(this.model.attributes));
    return this;
  }
});

Backbone is agnostic with respect to your preferred method of HTML templating. Your render function could even munge together an HTML string, or use document.createElement to generate a DOM tree. However, we suggest choosing a nice JavaScript templating library. [Mustache.js][32], [Haml-js][33], and [Eco][34] are all fine alternatives. Because [Underscore.js][13] is already on the page, [_.template][21] is available, and is an excellent choice if you prefer simple interpolated-JavaScript style templates.

Whatever templating strategy you end up with, it's nice if you never have to put strings of HTML in your JavaScript. At DocumentCloud, we use [Jammit][35] in order to package up JavaScript templates stored in /app/views as part of our main core.js asset package.

view.remove()
Removes a view from the DOM, and calls stopListening to remove any bound events that the view has listenTo'd.

delegateEvents([events])
Uses jQuery's on function to provide declarative callbacks for DOM events within a view. If an events hash is not passed directly, uses this.events as the source. Events are written in the format {"event selector": "callback"}. The callback may be either the name of a method on the view, or a direct function body. Omitting the selector causes the event to be bound to the view's root element (this.el). By default, delegateEvents is called within the View's constructor for you, so if you have a simple events hash, all of your DOM events will always already be connected, and you will never have to call this function yourself.

The events property may also be defined as a function that returns an events hash, to make it easier to programmatically define your events, as well as inherit them from parent views.

Using delegateEvents provides a number of advantages over manually using jQuery to bind events to child elements during render. All attached callbacks are bound to the view before being handed off to jQuery, so when the callbacks are invoked, this continues to refer to the view object. When delegateEvents is run again, perhaps with a different events hash, all callbacks are removed and delegated afresh — useful for views which need to behave differently when in different modes.

A view that displays a document in a search result might look something like this:

var DocumentView = Backbone.View.extend({

  events: {
    "dblclick"                : "open",
    "click .icon.doc"         : "select",
    "contextmenu .icon.doc"   : "showMenu",
    "click .show_notes"       : "toggleNotes",
    "click .title .lock"      : "editAccessLevel",
    "mouseover .title .date"  : "showTooltip"
  },

  render: function() {
    this.$el.html(this.template(this.model.attributes));
    return this;
  },

  open: function() {
    window.open(this.model.get("viewer_url"));
  },

  select: function() {
    this.model.set({selected: true});
  },

  ...

});

undelegateEvents()
Removes all of the view's delegated events. Useful if you want to disable or remove a view from the DOM temporarily.

Utility

var backbone = Backbone.noConflict();
Returns the Backbone object back to its original value. You can use the return value of Backbone.noConflict() to keep a local reference to Backbone. Useful for embedding Backbone on third-party websites, where you don't want to clobber the existing Backbone.

var localBackbone = Backbone.noConflict();
var model = localBackbone.Model.extend(...);

Backbone.$ = $;
If you have multiple copies of jQuery on the page, or simply want to tell Backbone to use a particular object as its DOM / Ajax library, this is the property for you.

Backbone.$ = require('jquery');

F.A.Q.

If your eye hasn't already been caught by the adaptability and elan on display in the above list of examples, we can get more specific: Backbone.js aims to provide the common foundation that data-rich web applications with ambitious interfaces require — while very deliberately avoiding painting you into a corner by making any decisions that you're better equipped to make yourself.

• The focus is on supplying you with helpful methods to manipulate and query your data, not on HTML widgets or reinventing the JavaScript object model.
• Backbone does not force you to use a single template engine. Views can bind to HTML constructed in [your][21] [favorite][36] [way][37].
• It's smaller. There are fewer kilobytes for your browser or phone to download, and less conceptual surface area. You can read and understand the source in an afternoon.
• It doesn't depend on stuffing application logic into your HTML. There's no embedded JavaScript, template logic, or binding hookup code in data- or ng- attributes, and no need to invent your own HTML tags.
• Synchronous events are used as the fundamental building block, not a difficult-to-reason-about run loop, or by constantly polling and traversing your data structures to hunt for changes. And if you want a specific event to be asynchronous and aggregated, [no problem][38].
• Backbone scales well, from [embedded widgets][39] to [massive apps][40].
• Backbone is a library, not a framework, and plays well with others. You can embed Backbone widgets in Dojo apps without trouble, or use Backbone models as the data backing for D3 visualizations (to pick two entirely random examples).
• "Two-way data-binding" is avoided. While it certainly makes for a nifty demo, and works for the most basic CRUD, it doesn't tend to be terribly useful in your real-world app. Sometimes you want to update on every keypress, sometimes on blur, sometimes when the panel is closed, and sometimes when the "save" button is clicked. In almost all cases, simply serializing the form to JSON is faster and easier. All that aside, if your heart is set, [go][41] [for it][42].
• There's no built-in performance penalty for choosing to structure your code with Backbone. And if you do want to optimize further, thin models and templates with flexible granularity make it easy to squeeze every last drop of potential performance out of, say, IE8.

It's common for folks just getting started to treat the examples listed on this page as some sort of gospel truth. In fact, Backbone.js is intended to be fairly agnostic about many common patterns in client-side code. For example...

References between Models and Views can be handled several ways. Some people like to have direct pointers, where views correspond 1:1 with models (model.view and view.model). Others prefer to have intermediate "controller" objects that orchestrate the creation and organization of views into a hierarchy. Others still prefer the evented approach, and always fire events instead of calling methods directly. All of these styles work well.

Batch operations on Models are common, but often best handled differently depending on your server-side setup. Some folks don't mind making individual Ajax requests. Others create explicit resources for RESTful batch operations: /notes/batch/destroy?ids=1,2,3,4. Others tunnel REST over JSON, with the creation of "changeset" requests:

  {
    "create":  [array of models to create]
    "update":  [array of models to update]
    "destroy": [array of model ids to destroy]
  }

Feel free to define your own events. Backbone.Events is designed so that you can mix it in to any JavaScript object or prototype. Since you can use any string as an event, it's often handy to bind and trigger your own custom events: model.on("selected:true") or model.on("editing")

Render the UI as you see fit. Backbone is agnostic as to whether you use [Underscore templates][21], [Mustache.js][43], direct DOM manipulation, server-side rendered snippets of HTML, or [jQuery UI][44] in your render function. Sometimes you'll create a view for each model ... sometimes you'll have a view that renders thousands of models at once, in a tight loop. Both can be appropriate in the same app, depending on the quantity of data involved, and the complexity of the UI.

It's common to nest collections inside of models with Backbone. For example, consider a Mailbox model that contains many Message models. One nice pattern for handling this is have a this.messages collection for each mailbox, enabling the lazy-loading of messages, when the mailbox is first opened ... perhaps with MessageList views listening for "add" and "remove" events.

var Mailbox = Backbone.Model.extend({

  initialize: function() {
    this.messages = new Messages;
    this.messages.url = '/mailbox/' + this.id + '/messages';
    this.messages.on("reset", this.updateCounts);
  },

  ...

});

var inbox = new Mailbox;

// And then, when the Inbox is opened:

inbox.messages.fetch({reset: true});

If you're looking for something more opinionated, there are a number of Backbone plugins that add sophisticated associations among models, [available on the wiki][45].

Backbone doesn't include direct support for nested models and collections or "has many" associations because there are a number of good patterns for modeling structured data on the client side, and Backbone should provide the foundation for implementing any of them. You may want to…

• Mirror an SQL database's structure, or the structure of a NoSQL database.
• Use models with arrays of "foreign key" ids, and join to top level collections (a-la tables).
• For associations that are numerous, use a range of ids instead of an explicit list.
• Avoid ids, and use direct references, creating a partial object graph representing your data set.
• Lazily load joined models from the server, or lazily deserialize nested models from JSON documents.

When your app first loads, it's common to have a set of initial models that you know you're going to need, in order to render the page. Instead of firing an extra AJAX request to fetch them, a nicer pattern is to have their data already bootstrapped into the page. You can then use reset to populate your collections with the initial data. At DocumentCloud, in the [ERB][46] template for the workspace, we do something along these lines:

<script>
  var accounts = new Backbone.Collection;
  accounts.reset(<%= @accounts.to_json %>);
  var projects = new Backbone.Collection;
  projects.reset(<%= @projects.to_json(:collaborators => true) %>);
</script>

You have to [escape][47] </ within the JSON string, to prevent javascript injection attacks.

Many JavaScript libraries are meant to be insular and self-enclosed, where you interact with them by calling their public API, but never peek inside at the guts. Backbone.js is not that kind of library.

Because it serves as a foundation for your application, you're meant to extend and enhance it in the ways you see fit — the entire source code is [annotated][3] to make this easier for you. You'll find that there's very little there apart from core functions, and most of those can be overridden or augmented should you find the need. If you catch yourself adding methods to Backbone.Model.prototype, or creating your own base subclass, don't worry — that's how things are supposed to work.

Different implementations of the [Model-View-Controller][48] pattern tend to disagree about the definition of a controller. If it helps any, in Backbone, the View class can also be thought of as a kind of controller, dispatching events that originate from the UI, with the HTML template serving as the true view. We call it a View because it represents a logical chunk of UI, responsible for the contents of a single DOM element.

Comparing the overall structure of Backbone to a server-side MVC framework like Rails, the pieces line up like so:

• Backbone.Model – Like a Rails model minus the class methods. Wraps a row of data in business logic.
• Backbone.Collection – A group of models on the client-side, with sorting/filtering/aggregation logic.
• Backbone.Router – Rails routes.rb + Rails controller actions. Maps URLs to functions.
• Backbone.View – A logical, re-usable piece of UI. Often, but not always, associated with a model.
• Client-side Templates – Rails .html.erb views, rendering a chunk of HTML.

Perhaps the single most common JavaScript "gotcha" is the fact that when you pass a function as a callback, its value for this is lost. When dealing with events and callbacks in Backbone, you'll often find it useful to rely on listenTo or the optional context argument that many of Underscore and Backbone's methods use to specify the this that will be used when the callback is later invoked. (See [.each][49], [.map][50], and object.on, to name a few). View events are automatically bound to the view's context for you. You may also find it helpful to use [.bind][51] and [.bindAll][52] from Underscore.js.

var MessageList = Backbone.View.extend({

  initialize: function() {
    var messages = this.collection;
    messages.on("reset", this.render, this);
    messages.on("add", this.addMessage, this);
    messages.on("remove", this.removeMessage, this);

    messsages.each(this.addMessage, this);
  }

});

// Later, in the app...

Inbox.messages.add(newMessage);

Backbone.js was originally extracted from [a Rails application][53]; getting your client-side (Backbone) Models to sync correctly with your server-side (Rails) Models is painless, but there are still a few things to be aware of.

By default, Rails versions prior to 3.1 add an extra layer of wrapping around the JSON representation of models. You can disable this wrapping by setting:

ActiveRecord::Base.include_root_in_json = false

... in your configuration. Otherwise, override parse to pull model attributes out of the wrapper. Similarly, Backbone PUTs and POSTs direct JSON representations of models, where by default Rails expects namespaced attributes. You can have your controllers filter attributes directly from params, or you can override toJSON in Backbone to add the extra wrapping Rails expects.

Change Log

— Jun. 4, 2015 — [Diff][205] — [Docs][206]

• Collection#add now avoids trying to parse a model instance when passed parse: true.

• Bug fix in Collection#remove. The removed models are now actually returned.

• Model#fetch no longer parses the response when passing patch: false.

• Bug fix for iframe-based History when used with JSDOM.

• Bug fix where Collection#invoke was not taking additional arguments.

• When using on with an event map, you can now pass the context as the second argument. This was previously an undocumented behavior from 1.1.2 removed in 1.2.0. — May 13, 2015 — [Diff][207] — [Docs][208]

• Added new hooks to Views to allow them to work without jQuery. See the [wiki page][209] for more info.

• As a neat side effect, Backbone.History no longer uses jQuery's event methods for pushState and hashChange listeners. We're native all the way.

• Also on the subject of jQuery, if you're using Backbone with CommonJS (node, browserify, webpack) Backbone will automatically try to load jQuery for you.

• Views now always delegate their events in setElement. You can no longer modify the events hash or your view's el property in initialize.

• Added an "update" event that triggers after any amount of models are added or removed from a collection. Handy to re-render lists of things without debouncing.

• Collection#at can take a negative index.

• Added modelId to Collection for generating unique ids on polymorphic collections. Handy for cases when your model ids would otherwise collide.

• Added an overridable _isModel for more advanced control of what's considered a model by your Collection.

• The success callback passed to Model#destroy is always called asynchronously now.

• Router#execute passes back the route name as its third argument.

• Cancel the current Router transition by returning false in Router#execute. Great for checking logged-in status or other prerequisites.

• Added getSearch and getPath methods to Backbone.History as cross-browser and overridable ways of slicing up the URL.

• Added delegate and undelegate as finer-grained versions of delegateEvents and undelegateEvents. Useful for plugin authors to use a consistent events interface in Backbone.

• A collection will only fire a "sort" event if its order was actually updated, not on every set.

• Any passed options.attrs are now respected when saving a model with patch: true.

• Collection#clone now sets the model and comparator functions of the cloned collection to the new one.

• Adding models to your Collection when specifying an at position now sends the actual position of your model in the add event, not just the one you've passed in.

• Collection#remove will now only return a list of models that have actually been removed from the collection.

• Fixed loading Backbone.js in strict ES6 module loaders. — Feb. 20, 2014 — [Diff][210] — [Docs][211]

• Backbone no longer tries to require jQuery in Node/CommonJS environments, for better compatibility with folks using Browserify. If you'd like to have Backbone use jQuery from Node, assign it like so: Backbone.$ = require('jquery');

• Bugfix for route parameters with newlines in them. — Feb. 13, 2014 — [Diff][212] — [Docs][213]

• Backbone now registers itself for AMD (Require.js), Bower and Component, as well as being a CommonJS module and a regular (Java)Script. Whew.

• Added an execute hook to the Router, which allows you to hook in and custom-parse route arguments, like query strings, for example.

• Performance fine-tuning for Backbone Events.

• Better matching for Unicode in routes, in old browsers.

• Backbone Routers now handle query params in route fragments, passing them into the handler as the last argument. Routes specified as strings should no longer include the query string ('foo?:query' should be 'foo'). — Oct. 10, 2013 — [Diff][214] — [Docs][215]

• Made the return values of Collection's set, add, remove, and reset more useful. Instead of returning this, they now return the changed (added, removed or updated) model or list of models.

• Backbone Views no longer automatically attach options passed to the constructor as this.options and Backbone Models no longer attach url and urlRoot options, but you can do it yourself if you prefer.

• All "invalid" events now pass consistent arguments. First the model in question, then the error object, then options.

• You are no longer permitted to change the id of your model during parse. Use idAttribute instead.

• On the other hand, parse is now an excellent place to extract and vivify incoming nested JSON into associated submodels.

• Many tweaks, optimizations and bugfixes relating to Backbone 1.0, including URL overrides, mutation of options, bulk ordering, trailing slashes, edge-case listener leaks, nested model parsing... — March 20, 2013 — [Diff][216] — [Docs][217]

• Renamed Collection's "update" to set, for parallelism with the similar model.set(), and contrast with reset. It's now the default updating mechanism after a fetch. If you'd like to continue using "reset", pass {reset: true}.

• Your route handlers will now receive their URL parameters pre-decoded.

• Added listenToOnce as the analogue of once.

• Added the findWhere method to Collections, similar to where.

• Added the keys, values, pairs, invert, pick, and omit Underscore.js methods to Backbone Models.

• The routes in a Router's route map may now be function literals, instead of references to methods, if you like.

• url and urlRoot properties may now be passed as options when instantiating a new Model. — Jan. 15, 2013 — [Diff][218] — [Docs][219]

• A "route" event is triggered on the router in addition to being fired on Backbone.history.

• Model validation is now only enforced by default in Model#save and no longer enforced by default upon construction or in Model#set, unless the {validate:true} option is passed.

• View#make has been removed. You'll need to use $ directly to construct DOM elements now.

• Passing {silent:true} on change will no longer delay individual "change:attr" events, instead they are silenced entirely.

• The Model#change method has been removed, as delayed attribute changes are no longer available.

• Bug fix on change where attribute comparison uses !== instead of _.isEqual.

• Bug fix where an empty response from the server on save would not call the success function.

• parse now receives options as its second argument.

• Model validation now fires invalid event instead of error. — Dec. 13, 2012 — [Diff][220] — [Docs][221]

• Added listenTo and stopListening to Events. They can be used as inversion-of-control flavors of on and off, for convenient unbinding of all events an object is currently listening to. view.remove() automatically calls view.stopListening().

• When using add on a collection, passing {merge: true} will now cause duplicate models to have their attributes merged in to the existing models, instead of being ignored.

• Added update (which is also available as an option to fetch) for "smart" updating of sets of models.

• HTTP PATCH support in save by passing {patch: true}.

• The Backbone object now extends Events so that you can use it as a global event bus, if you like.

• Added a "request" event to Backbone.sync, which triggers whenever a request begins to be made to the server. The natural complement to the "sync" event.

• Router URLs now support optional parts via parentheses, without having to use a regex.

• Backbone events now supports once, similar to Node's once, or jQuery's one.

• Backbone events now support jQuery-style event maps obj.on({click: action}).

• While listening to a reset event, the list of previous models is now available in options.previousModels, for convenience.

• Validation now occurs even during "silent" changes. This change means that the isValid method has been removed. Failed validations also trigger an error, even if an error callback is specified in the options.

• Consolidated "sync" and "error" events within Backbone.sync. They are now triggered regardless of the existence of success or error callbacks.

• For mixed-mode APIs, Backbone.sync now accepts emulateHTTP and emulateJSON as inline options.

• Collections now also proxy Underscore method name aliases (collect, inject, foldl, foldr, head, tail, take, and so on...)

• Removed getByCid from Collections. collection.get now supports lookup by both id and cid.

• After fetching a model or a collection, all defined parse functions will now be run. So fetching a collection and getting back new models could cause both the collection to parse the list, and then each model to be parsed in turn, if you have both functions defined.

• Bugfix for normalizing leading and trailing slashes in the Router definitions. Their presence (or absence) should not affect behavior.

• When declaring a View, options, el, tagName, id and className may now be defined as functions, if you want their values to be determined at runtime.

• Added a Backbone.ajax hook for more convenient overriding of the default use of $.ajax. If AJAX is too passé, set it to your preferred method for server communication.

• Collection#sort now triggers a sort event, instead of a reset event.

• Calling destroy on a Model will now return false if the model isNew.

• To set what library Backbone uses for DOM manipulation and Ajax calls, use Backbone.$ = ... instead of setDomLibrary.

• Removed the Backbone.wrapError helper method. Overriding sync should work better for those particular use cases.

• To improve the performance of add, options.index will no longer be set in the add event callback. collection.indexOf(model) can be used to retrieve the index of a model as necessary.

• For semantic and cross browser reasons, routes will now ignore search parameters. Routes like search?query=…&page=3 should become search/…/3.

• Model#set no longer accepts another model as an argument. This leads to subtle problems and is easily replaced with model.set(other.attributes). — March 21, 2012 — [Diff][222] — [Docs][223]

• Instead of throwing an error when adding duplicate models to a collection, Backbone will now silently skip them instead.

• Added push, pop, unshift, and shift to collections.

• A model's changed hash is now exposed for easy reading of the changed attribute delta, since the model's last "change" event.

• Added where to collections for simple filtering.

• You can now use a single off call to remove all callbacks bound to a specific object.

• Bug fixes for nested individual change events, some of which may be "silent".

• Bug fixes for URL encoding in location.hash fragments.

• Bug fix for client-side validation in advance of a save call with {wait: true}.

• Updated / refreshed the example [Todo List][5] app. — Feb. 2, 2012 — [Diff][224] — [Docs][225]

• Reverted to 0.5.3-esque behavior for validating models. Silent changes no longer trigger validation (making it easier to work with forms). Added an isValid function that you can use to check if a model is currently in a valid state.

• If you have multiple versions of jQuery on the page, you can now tell Backbone which one to use with Backbone.setDomLibrary.

• Fixes regressions in 0.9.0 for routing with "root", saving with both "wait" and "validate", and the order of nested "change" events. — Jan. 30, 2012 — [Diff][226] — [Docs][227]

• Creating and destroying models with create and destroy are now optimistic by default. Pass {wait: true} as an option if you'd like them to wait for a successful server response to proceed.

• Two new properties on views: $el — a cached jQuery (or Zepto) reference to the view's element, and setElement, which should be used instead of manually setting a view's el. It will both set view.el and view.$el correctly, as well as re-delegating events on the new DOM element.

• You can now bind and trigger multiple spaced-delimited events at once. For example: model.on("change:name change:age", ...)

• When you don't know the key in advance, you may now call model.set(key, value) as well as save.

• Multiple models with the same id are no longer allowed in a single collection.

• Added a "sync" event, which triggers whenever a model's state has been successfully synced with the server (create, save, destroy).

• bind and unbind have been renamed to on and off for clarity, following jQuery's lead. The old names are also still supported.

• A Backbone collection's comparator function may now behave either like a [sortBy][29] (pass a function that takes a single argument), or like a [sort][30] (pass a comparator function that expects two arguments). The comparator function is also now bound by default to the collection — so you can refer to this within it.

• A view's events hash may now also contain direct function values as well as the string names of existing view methods.

• Validation has gotten an overhaul — a model's validate function will now be run even for silent changes, and you can no longer create a model in an initially invalid state.

• Added shuffle and initial to collections, proxied from Underscore.

• Model#urlRoot may now be defined as a function as well as a value.

• View#attributes may now be defined as a function as well as a value.

• Calling fetch on a collection will now cause all fetched JSON to be run through the collection's model's parse function, if one is defined.

• You may now tell a router to navigate(fragment, {replace: true}), which will either use history.replaceState or location.hash.replace, in order to change the URL without adding a history entry.

• Within a collection's add and remove events, the index of the model being added or removed is now available as options.index.

• Added an undelegateEvents to views, allowing you to manually remove all configured event delegations.

• Although you shouldn't be writing your routes with them in any case — leading slashes (/) are now stripped from routes.

• Calling clone on a model now only passes the attributes for duplication, not a reference to the model itself.

• Calling clear on a model now removes the id attribute.

— August 9, 2011 — [Diff][228] — [Docs][229]
A View's events property may now be defined as a function, as well as an object literal, making it easier to programmatically define and inherit events. groupBy is now proxied from Underscore as a method on Collections. If the server has already rendered everything on page load, pass Backbone.history.start({silent: true}) to prevent the initial route from triggering. Bugfix for pushState with encoded URLs.

— July 26, 2011 — [Diff][230] — [Docs][231]
The bind function, can now take an optional third argument, to specify the this of the callback function. Multiple models with the same id are now allowed in a collection. Fixed a bug where calling .fetch(jQueryOptions) could cause an incorrect URL to be serialized. Fixed a brief extra route fire before redirect, when degrading from pushState.

— July 5, 2011 — [Diff][232] — [Docs][233]
Cleanups from the 0.5.0 release, to wit: improved transparent upgrades from hash-based URLs to pushState, and vice-versa. Fixed inconsistency with non-modified attributes being passed to Model#initialize. Reverted a 0.5.0 change that would strip leading hashbangs from routes. Added contains as an alias for includes.

— July 1, 2011 — [Diff][234] — [Docs][235]
A large number of tiny tweaks and micro bugfixes, best viewed by looking at [the commit diff][234]. HTML5 pushState support, enabled by opting-in with: Backbone.history.start({pushState: true}). Controller was renamed to Router, for clarity. Collection#refresh was renamed to Collection#reset to emphasize its ability to both reset the collection with new models, as well as empty out the collection when used with no parameters. saveLocation was replaced with navigate. RESTful persistence methods (save, fetch, etc.) now return the jQuery deferred object for further success/error chaining and general convenience. Improved XSS escaping for Model#escape. Added a urlRoot option to allow specifying RESTful urls without the use of a collection. An error is thrown if Backbone.history.start is called multiple times. Collection#create now validates before initializing the new model. view.el can now be a jQuery string lookup. Backbone Views can now also take an attributes parameter. Model#defaults can now be a function as well as a literal attributes object.

— Dec 1, 2010 — [Diff][236] — [Docs][237]
Backbone.js now supports [Zepto][17], alongside jQuery, as a framework for DOM manipulation and Ajax support. Implemented Model#escape, to efficiently handle attributes intended for HTML interpolation. When trying to persist a model, failed requests will now trigger an "error" event. The ubiquitous options argument is now passed as the final argument to all "change" events.

— Nov 23, 2010 — [Diff][238] — [Docs][239]
Bugfix for IE7 + iframe-based "hashchange" events. sync may now be overridden on a per-model, or per-collection basis. Fixed recursion error when calling save with no changed attributes, within a "change" event.

— Nov 15, 2010 — [Diff][240] — [Docs][241]
All "add" and "remove" events are now sent through the model, so that views can listen for them without having to know about the collection. Added a remove method to Backbone.View. toJSON is no longer called at all for 'read' and 'delete' requests. Backbone routes are now able to load empty URL fragments.

— Nov 9, 2010 — [Diff][242] — [Docs][243]
Backbone now has Controllers and History, for doing client-side routing based on URL fragments. Added emulateHTTP to provide support for legacy servers that don't do PUT and DELETE. Added emulateJSON for servers that can't accept application/json encoded requests. Added Model#clear, which removes all attributes from a model. All Backbone classes may now be seamlessly inherited by CoffeeScript classes.

— Oct 25, 2010 — [Diff][244] — [Docs][245]
Instead of requiring server responses to be namespaced under a model key, now you can define your own parse method to convert responses into attributes for Models and Collections. The old handleEvents function is now named delegateEvents, and is automatically called as part of the View's constructor. Added a toJSON function to Collections. Added Underscore's chain to Collections.

— Oct 19, 2010 — [Diff][246] — [Docs][247]
Added a Model#fetch method for refreshing the attributes of single model from the server. An error callback may now be passed to set and save as an option, which will be invoked if validation fails, overriding the "error" event. You can now tell backbone to use the _method hack instead of HTTP methods by setting Backbone.emulateHTTP = true. Existing Model and Collection data is no longer sent up unnecessarily with GET and DELETE requests. Added a rake lint task. Backbone is now published as an [NPM][248] module.

— Oct 14, 2010 — [Diff][249] — [Docs][250]
Added a convention for initialize functions to be called upon instance construction, if defined. Documentation tweaks.

— Oct 13, 2010 — [Docs][251]
Initial Backbone release.