# Technical Documentation: jashkenas/backbone

> ℹ️ **Provenance:** Hybrid Fusion: `jashkenas/backbone` + DevDocs `backbone` · [CodeWiki Reference](https://codewiki.google/github.com/jashkenas/backbone) · Recency: Active (< 180 days)

## 1. Repository Overview (jashkenas/backbone)

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Backbone supplies structure to JavaScript-heavy 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 application over a RESTful JSON interface.

For Docs, License, Tests, pre-packed downloads, and everything else, really, see:
https://backbonejs.org

To suggest a feature or report a bug:
https://github.com/jashkenas/backbone/issues

For questions on working with Backbone or general discussions:
[security policy](SECURITY.md),
https://stackoverflow.com/questions/tagged/backbone.js,
https://matrix.to/#/#jashkenas_backbone:gitter.im or
https://groups.google.com/g/backbonejs

Backbone is an open-sourced component of DocumentCloud:
https://github.com/documentcloud

Testing powered by SauceLabs:
https://saucelabs.com

Many thanks to our contributors:
https://github.com/jashkenas/backbone/graphs/contributors

Special thanks to Robert Kieffer for the original philosophy behind Backbone.
https://github.com/broofa

This project adheres to a [code of conduct](CODE_OF_CONDUCT.md). By participating, you are expected to uphold this code.

## 2. Official Technical Reference (DevDocs: Backbone.js)

# DevDocs Technical Reference: backbone

> Source: DevDocs.io (backbone) | Formatted for AI Agents & Developers | Zero-Egress Edge Persistence


## Section: index

# Backbone.js

## 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)` 

 

 on`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", ...);
```

 

 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:author": authorPane.update,
  "change:title change:subtitle": titleView.update,
  "destroy": bookView.remove
});
```

 

 To supply a **context** value for `this` when the callback is invoked, pass the optional last argument: `model.on('change', this.render, this)` or `model.on({change: this.render}, this)`. 

 

 off`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. 

 

 trigger`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. 

 

 once`object.once(event, callback, [context])` 
 Just like [on](#Events-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 

 

 listenTo`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);
```

 

 stopListening`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](#Events-listenTo) 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);
```

 

 listenToOnce`object.listenToOnce(other, event, callback)` 
 Just like [listenTo](#Events-listenTo), but causes the bound callback to fire only once before being removed. 

 

 Catalog of Events 
 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, removed or changed in a collection.
 - 
**"reset"** (collection, options) — when the collection's entire contents have been [reset](#Collection-reset).
 - 
**"sort"** (collection, options) — when the collection has been re-sorted.
 - 
**"change"** (model, options) — when a model's attributes have changed.
 - 
**"changeId"** (model, previousId, options) — when the model's id has been updated.
 - 
**"change:[attribute]"** (model, value, options) — when a specific attribute has been updated.
 - 
**"destroy"** (model, collection, options) — when a model is [destroyed](#Model-destroy).
 - 
**"request"** (model_or_collection, xhr, options) — when a model or collection has started a request to the server.
 - 
**"sync"** (model_or_collection, response, options) — when a model or collection has been successfully synced with the server.
 - 
**"error"** (model_or_collection, xhr, options) — when a model's or collection's request to the server has failed.
 - 
**"invalid"** (model, error, options) — when a model's [validation](#Model-validate) 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.
 - 
**"notfound"** () — Fired by history when *no* route could be matched.
 - 
**"all"** — this special event fires for *any* triggered event, passing the event name as the first argument followed by all trigger arguments.
 
 

 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();
```

 

 extend`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);
    ...
  }
});
```

 

 preinitialize`new Model([attributes], [options])` 
 For use with models as ES classes. If you define a **preinitialize** method, it will be invoked when the Model is first created, before any instantiation logic is run for the Model. 

 

```
class Country extends Backbone.Model {
    preinitialize({countryCode}) {
      this.name = COUNTRY_NAMES[countryCode];
    }

    initialize() { ... }
}
```

 

 constructor / initialize`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](#Model-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](#Model-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](#Model-parse) before being [set](#Model-set) on the model. 

 

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

 

 set`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");
```

 

 escape`model.escape(attribute)` 
 Similar to [get](#Model-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](https://en.wikipedia.org/wiki/Cross-site_scripting) attacks. 

 

```
var hacker = new Backbone.Model({
  name: "alert('xss')"
});

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

 

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

 

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

 

 unset`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. 

 

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

 

 id`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. `model.id` should not be manipulated directly, it should be modified only via `model.set('id', …)`. Models can be retrieved by id from collections, and the id is used to generate model URLs by default. 

 

 idAttribute`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`. If you set `idAttribute`, you may also want to override [`cidPrefix`](#Model-cidPrefix). 

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

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

 

 cid`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. 

 

 cidPrefix`model.cidPrefix` 
 If your model has an `id` that is anything other than an integer or a UUID, there is the possibility that it might collide with its `cid`. To prevent this, you can override the prefix that `cid`s start with. 

```
// If both lengths are 2, refresh the page before running this example.
var clashy = new Backbone.Collection([
  {id: 'c2'},
  {id: 'c1'},
]);
alert('clashy length: ' + clashy.length);

var ClashFree = Backbone.Model.extend({cidPrefix: 'm'});
var clashless = new Backbone.Collection([
  {id: 'c3'},
  {id: 'c2'},
], {model: ClashFree});
alert('clashless length: ' + clashless.length);
```

 

 attributes`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](#Model-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](#Events) accepts space separated lists of events, attribute names should not include spaces. 

 

 changed`model.changed` 
 The **changed** property is the internal hash containing all the attributes that have changed since its last [set](#Model-set). Please do not update **changed** directly since its state is internally maintained by [set](#Model-set). A copy of **changed** can be acquired from [changedAttributes](#Model-changedAttributes). 

 

 defaults`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. 

 

 If you set a value for the model’s [`idAttribute`](#Model-idAttribute), you should define the defaults as a function that returns a different, universally unique id on every invocation. Not doing so would likely prevent an instance of `Backbone.Collection` from correctly identifying model hashes and is almost certainly a mistake, unless you never add instances of the model class to a collection. 

 

 toJSON`model.toJSON([options])` 
 Return a shallow copy of the model's [attributes](#Model-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**](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#toJSON_behavior) works. 

 

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

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

alert(JSON.stringify(artist));
```

 

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

 

 fetch`model.fetch([options])` 
 Merges the model's state with attributes fetched from the server by delegating to [Backbone.sync](#Sync). Returns a [jqXHR](http://api.jquery.com/jQuery.ajax/#jqXHR). 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);
```

 

 save`model.save([attributes], [options])` 
 Save a model to your database (or alternative persistence layer), by delegating to [Backbone.sync](#Sync). Returns a [jqXHR](http://api.jquery.com/jQuery.ajax/#jqXHR) if validation is successful and `false` otherwise. The **attributes** hash (as in [set](#Model-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](#Model-validate) method, and validation fails, the model will not be saved. If the model [isNew](#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(){ ... }});
```

 

 destroy`model.destroy([options])` 
 Destroys the model on the server by delegating an HTTP `DELETE` request to [Backbone.sync](#Sync). Returns a [jqXHR](http://api.jquery.com/jQuery.ajax/#jqXHR) object, or `false` if the model [isNew](#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) {
  ...
}});
```

 

 Underscore Methods (9) 
 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… 

 
 - [keys](http://underscorejs.org/#keys)
 - [values](http://underscorejs.org/#values)
 - [pairs](http://underscorejs.org/#pairs)
 - [invert](http://underscorejs.org/#invert)
 - [pick](http://underscorejs.org/#pick)
 - [omit](http://underscorejs.org/#omit)
 - [chain](http://underscorejs.org/#chain)
 - [isEmpty](http://underscorejs.org/#isEmpty)
 
 

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

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

 

 validate`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. 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. 

 

 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 **validate** returns an error, `save` does not continue, the model attributes are not modified on the server, an `"invalid"` event is triggered, and the `validationError` property is set 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. 

 

 validationError`model.validationError` 
 The value returned by [validate](#Model-validate) during the last failed validation. 

 

 isValid`model.isValid(options)` 
 Run [validate](#Model-validate) to check the model state. 

 

 The `validate` method receives the model attributes as well as any options passed to **isValid**, if `validate` returns an error an `"invalid"` event is triggered, and the error is set on the model in the `validationError` property. 

 

```
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);
}
```

 

 url`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](#Collection-url) to generate the URL, so make sure that you have it defined, or a [urlRoot](#Model-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](#Collection) with a `url` of `"/documents/7/notes"`, would have this URL: `"/documents/7/notes/101"` 

 

 urlRoot`model.urlRoot or model.urlRoot()` 
 Specify a `urlRoot` if you're using a model *outside* of a collection, to enable the default [url](#Model-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());
```

 

 parse`model.parse(response, options)` 
 **parse** is called whenever a model's data is returned by the server, in [fetch](#Model-fetch), and [save](#Model-save). The function is passed the raw `response` object, and should return the attributes hash to be [set](#Model-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
```

 

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

 

 isNew`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. 

 

 hasChanged`model.hasChanged([attribute])` 
 Has the model changed since its last [set](#Model-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")) {
    ...
  }
});
```

 

 changedAttributes`model.changedAttributes([attributes])` 
 Retrieve a hash of only the model's attributes that have changed since the last [set](#Model-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. 

 

 previous`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"});
```

 

 previousAttributes`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](#Collection-Underscore-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", ...)` 

 

 extend`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. 

 

 model`collection.model([attrs], [options])` 
 Override this property to specify the model class that the collection contains. If defined, you can pass raw attributes objects (and arrays) and options to [add](#Collection-add), [create](#Collection-create), and [reset](#Collection-reset), and the attributes will be converted into a model of the proper type using the provided options, if any. 

 

```
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);
    }
  }

});
```

 

 modelId`collection.modelId(attrs, idAttribute)` 
 Override this method to return the value the collection will use to identify a model given its attributes. Useful for combining models from multiple tables with different [`idAttribute`](#Model-idAttribute) values into a single collection. 

 

 By default returns the value of the given [`idAttribute`](#Model-idAttribute) within the `attrs`, or failing that, `id`. If your collection uses a [model factory](#Collection-model) and the id ranges of those models might collide, you must override this method. 

 

```
var Library = Backbone.Collection.extend({
  modelId: function(attrs) {
    return attrs.type + attrs.id;
  }
});

var library = new Library([
  {type: 'dvd', id: 1},
  {type: 'vhs', id: 1}
]);

var dvdId = library.get('dvd1').id;
var vhsId = library.get('vhs1').id;
alert('dvd: ' + dvdId + ', vhs: ' + vhsId);
```

 

 preinitialize`new Backbone.Collection([models], [options])` 
 For use with collections as ES classes. If you define a **preinitialize** method, it will be invoked when the Collection is first created and before any instantiation logic is run for the Collection. 

 

```
class Library extends Backbone.Collection {
  preinitialize() {
    this.on("add", function() {
      console.log("Add model event got fired!");
    });
  }
}
```

 

 constructor / initialize`new Backbone.Collection([models], [options])` 
 When creating a Collection, you may choose to pass in the initial array of **models**. The collection's [comparator](#Collection-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(null, {
  model: Space
});
```

 

 If `{parse: true}` is passed as an **option**, the **attributes** will first be converted by [parse](#Collection-parse) before being [set](#Collection-set) on the collection. 

 

 models`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. 

 

 toJSON`collection.toJSON([options])` 
 Return an array containing the attributes hash of each model (via [toJSON](#Model-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](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#toJSON_behavior). 

 

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

alert(JSON.stringify(collection));
```

 

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

 

 Underscore Methods (46) 
 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… 

 

 Most methods can take an object or string to support model-attribute-style predicates or a function that receives the model instance as an argument. 

 
 - [forEach (each)](http://underscorejs.org/#each)
 - [map (collect)](http://underscorejs.org/#map)
 - [reduce (foldl, inject)](http://underscorejs.org/#reduce)
 - [reduceRight (foldr)](http://underscorejs.org/#reduceRight)
 - [find (detect)](http://underscorejs.org/#find)
 - [findIndex](http://underscorejs.org/#findIndex)
 - [findLastIndex](http://underscorejs.org/#findLastIndex)
 - [filter (select)](http://underscorejs.org/#filter)
 - [reject](http://underscorejs.org/#reject)
 - [every (all)](http://underscorejs.org/#every)
 - [some (any)](http://underscorejs.org/#some)
 - [contains (includes)](http://underscorejs.org/#contains)
 - [invoke](http://underscorejs.org/#invoke)
 - [max](http://underscorejs.org/#max)
 - [min](http://underscorejs.org/#min)
 - [sortBy](http://underscorejs.org/#sortBy)
 - [groupBy](http://underscorejs.org/#groupBy)
 - [shuffle](http://underscorejs.org/#shuffle)
 - [toArray](http://underscorejs.org/#toArray)
 - [size](http://underscorejs.org/#size)
 - [first (head, take)](http://underscorejs.org/#first)
 - [initial](http://underscorejs.org/#initial)
 - [rest (tail, drop)](http://underscorejs.org/#rest)
 - [last](http://underscorejs.org/#last)
 - [without](http://underscorejs.org/#without)
 - [indexOf](http://underscorejs.org/#indexOf)
 - [lastIndexOf](http://underscorejs.org/#lastIndexOf)
 - [isEmpty](http://underscorejs.org/#isEmpty)
 - [chain](http://underscorejs.org/#chain)
 - [difference](http://underscorejs.org/#difference)
 - [sample](http://underscorejs.org/#sample)
 - [partition](http://underscorejs.org/#partition)
 - [countBy](http://underscorejs.org/#countBy)
 - [indexBy](http://underscorejs.org/#indexBy)
 
 

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

var titles = books.map("title");

var publishedBooks = books.filter({published: true});

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

var randomThree = books.sample(3);
```

 

 add`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. This is a variant of [set](#Collection-set) with the same options and return value, but it always adds and never removes. 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. 

 

 remove`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`](#Collection-get). Fires a `"remove"` event for each model, and a single `"update"` event afterwards, unless `{silent: true}` is passed. The model's index before removal is available to listeners as `options.index`. 

 

 reset`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 on completion, and *without* triggering any add or remove events on any models. 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: 

 

```

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

```

 

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

 

 set`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, with a single `"update"` event at the end. Returns the touched models in the collection. If you'd like to customize this behavior, you can change it with options: `{add: false}`, `{remove: false}`, or `{merge: false}`. 

 

 If a [model](#Collection-model) property is defined, you may also pass raw attributes objects and options, and have them be vivified as instances of the model using the provided options. If you set a [comparator](#Collection-comparator), the collection will automatically sort itself and trigger a `"sort"` event, unless you pass `{sort: false}` or use the `{at: index}` option. Pass `{at: index}` to splice the model(s) into the collection at the specified `index`. 

 

```
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.
```

 

 get`collection.get(id)` 
 Get a model from a collection, specified by an [id](#Model-id), a [cid](#Model-cid), or by passing in a **model**. 

 

```
var book = library.get(110);
```

 

 at`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. When passed a negative index, it will retrieve the model from the back of the collection. 

 

 push`collection.push(model, [options])` 
 Like [add](#Collection-add), but always adds a model at the end of the collection and never sorts. 

 

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

 

 unshift`collection.unshift(model, [options])` 
 Like [add](#Collection-add), but always adds a model at the beginning of the collection and never sorts. 

 

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

 

 slice`collection.slice(begin, end)` 
 Return a shallow copy of this collection's models, using the same options as native [Array#slice](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/slice). 

 

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

 

 comparator`collection.comparator` 
 By default there is no **comparator** for a collection. If you define a comparator, it will be used to sort the collection any time a model is added. A comparator can be defined as a [sortBy](http://underscorejs.org/#sortBy) (pass a function that takes a single argument), as a [sort](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/sort) (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. 

 

 sort`collection.sort([options])` 
 Force a collection to re-sort itself. Note that a collection with a [comparator](#Collection-comparator) will sort itself automatically 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. 

 

 pluck`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));
```

 

 where`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);
```

 

 findWhere`collection.findWhere(attributes)` 
 Just like [where](#Collection-where), but directly returns only the first model in the collection that matches the passed **attributes**. If no model matches returns `undefined`. 

 

 url`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';
  }
});
```

 

 parse`collection.parse(response, options)` 
 **parse** is called by Backbone whenever a collection's models are returned by the server, in [fetch](#Collection-fetch). The function is passed the raw `response` object, and should return the array of model attributes to be [added](#Collection-add) 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;
  }
});
```

 

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

 

 fetch`collection.fetch([options])` 
 Fetch the default set of models for this collection from the server, [setting](#Collection-set) 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](#Collection-set) to (intelligently) merge the fetched models, unless you pass `{reset: true}`, in which case the collection will be (efficiently) [reset](#Collection-reset). Delegates to [Backbone.sync](#Sync) under the covers for custom persistence strategies and returns a [jqXHR](http://api.jquery.com/jQuery.ajax/#jqXHR). 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](#Collection-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](#FAQ-bootstrap) 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. 

 

 create`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](#Collection-model) property of the collection. The **create** method can accept either an attributes hash and options to be passed down during model instantiation 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"
});
```

 

 mixin`Backbone.Collection.mixin(properties)` 
 `mixin` provides a way to enhance the base **Backbone.Collection** and any collections which extend it. This can be used to add generic methods (e.g. additional [**Underscore Methods**](#Collection-Underscore-Methods)). 

 

```
Backbone.Collection.mixin({
  sum: function(models, iteratee) {
    return _.reduce(models, function(s, m) {
      return s + iteratee(m);
    }, 0);
  }
});

var cart = new Backbone.Collection([
  {price: 16, name: 'monopoly'},
  {price: 5, name: 'deck of cards'},
  {price: 20, name: 'chess'}
]);

var cost = cart.sum('price');
```

 

## 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. 

 

 extend`Backbone.Router.extend(properties, [classProperties])` 
 Get started by creating a custom router class. Define action functions that are triggered when certain URL fragments are matched, and provide a [routes](#Router-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) {
    ...
  }

});
```

 

 routes`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](#View)'s [events hash](#View-delegateEvents). 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 as positional arguments. 

 

 A route of `"file/*path"` will match `#file/folder/file.txt`, passing `"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. 

 

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

 

 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](#Events), 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) {
  ...
});
```

 

 preinitialize`new Backbone.Router([options])` 
 For use with routers as ES classes. If you define a **preinitialize** method, it will be invoked when the Router is first created and before any instantiation logic is run for the Router. 

 

```
class Router extends Backbone.Router {
  preinitialize() {
    // Override execute method
    this.execute = function(callback, args, name) {
      if (!loggedIn) {
        goToLogin();
        return false;
      }
      args.push(parseQueryString(args.pop()));
      if (callback) callback.apply(this, args);
    }
  }
}
```

 

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

 

 route`router.route(route, name, [callback])` 
 Manually create a route for the router, The `route` argument may be a [routing string](#Router-routes) 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) { ... }
```

 

 navigate`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});
```

 

 execute`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. It forwards the `"route"` and `"route[name]"` events of the matching router, or `"notfound"` when no route in any router matches the current URL. 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. 

 

 start`Backbone.history.start([options])` 
 When all of your [Routers](#Router) 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/"})`. 

 

 The value provided for `root` will be normalized to include a leading and trailing slash. When navigating to a route the default behavior is to exclude the trailing slash from the URL (e.g., `/public/search?query=...`). If you prefer to include the trailing slash (e.g., `/public/search/?query=...`) use `Backbone.history.start({trailingSlash: true})`. URLs will always contain a leading slash. When root is `/` URLs will look like `/?query=...` regardless of the value of `trailingSlash`. 

 

 When called, if a route succeeds with a match for the current URL, `Backbone.history.start()` returns `true` and the `"route"` and `"route[name]"` events are triggered. If no defined route matches the current URL, it returns `false` and `"notfound"` is triggered instead. 

 

 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 ``, 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](http://api.jquery.com/jQuery.ajax/#jqXHR). 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](#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]
  permitted = params.require(:account).permit(:name, :otherparam)
  account.update_attributes permitted
  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` 

 

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

 

 emulateHTTP`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.
```

 

 emulateJSON`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. 

 

 extend`Backbone.View.extend(properties, [classProperties])` 
 Get started with views by creating a custom view class. You'll want to override the [render](#View-render) function, specify your declarative [events](#View-delegateEvents), 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. 

 

 preinitialize`new View([options])` 
 For use with views as ES classes. If you define a **preinitialize** method, it will be invoked when the view is first created, before any instantiation logic is run. 

 

```
class Document extends Backbone.View {
  preinitialize({autoRender}) {
    this.autoRender = autoRender;
  }

  initialize() {
    if (this.autoRender) {
      this.listenTo(this.model, "change", this.render);
    }
  }
}
```

 

 constructor / initialize`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
});
```

 

 el`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`](#View-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);
```

 

 $el`view.$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);
```

 

 setElement`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. 

 

 attributes`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. 

 

 $ (jQuery)`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()
    };
  }
});
```

 

 template`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(...)
});
```

 

 render`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](https://github.com/janl/mustache.js), [Haml-js](https://github.com/creationix/haml-js), and [Eco](https://github.com/sstephenson/eco) are all fine alternatives. Because [Underscore.js](http://underscorejs.org/) is already on the page, [_.template](http://underscorejs.org/#template) 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](https://documentcloud.github.com/jammit/) in order to package up JavaScript templates stored in `/app/views` as part of our main `core.js` asset package. 

 

 remove`view.remove()` 
 Removes a view and its `el` from the DOM, and calls [stopListening](#Events-stopListening) to remove any bound events that the view has [listenTo](#Events-listenTo)'d. 

 

 events`view.events or view.events()` 
 The **events** hash (or method) can be used to specify a set of DOM events that will be bound to methods on your View through [delegateEvents](#View-delegateEvents). 

 

 Backbone will automatically attach the event listeners at instantiation time, right before invoking [initialize](#View-constructor). 

 

```
var ENTER_KEY = 13;
var InputView = Backbone.View.extend({

  tagName: 'input',

  events: {
    "keydown" : "keyAction",
  },

  render: function() { ... },

  keyAction: function(e) {
    if (e.which === ENTER_KEY) {
      this.collection.add({text: this.$el.val()});
    }
  }
});
```

 

 delegateEvents`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](#View-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 single-event version of **delegateEvents** is available as `delegate`. In fact, **delegateEvents** is simply a multi-event wrapper around `delegate`. A counterpart to `undelegateEvents` is available as `undelegate`. 

 

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

 

## Utility

 

 Backbone.noConflict`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.$`Backbone.$ = $;` 
 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');
```

 

 debugInfo`debugInfo();` 
 In the unfortunate event that you need to submit a [bug report](https://github.com/jashkenas/backbone/issues/new?template=Bugs.yml), this function makes it easier to provide detailed information about your setup. It prints a JSON object with version information about Backbone and its dependencies through `console.debug`. It also returns this object in case you want to inspect it in code. 

 

 `debugInfo` comes in a separate module that ships with the [edge version](#downloads) and releases later than 1.5.0. It is available in UMD format under the same prefix as `backbone.js`, but with `debug-info.js` as the file name. It is also experimentally available in ES module format under `backbone/modules/`. 

 

```
<!-- browser embeds -->
  
  
  
    Backbone.debugInfo();
  

  // CommonJS
  require('backbone/debug-info.js')();

  // ESM
  import debugInfo from 'backbone/modules/debug-info.js';
  debugInfo();
```

  

    &copy; 2010&ndash;2024 Jeremy Ashkenas, DocumentCloud
Licensed under the MIT License.

    [https://backbonejs.org/](https://backbonejs.org/)

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