backbone.js

backbone.js
§
```
Backbone.js 1.6.0
```

(c) 2010-2024 Jeremy Ashkenas and DocumentCloud
Backbone may be freely distributed under the MIT license.
For all details and documentation:
http://backbonejs.org

(function(factory) {

Establish the root object, window (self) in the browser, or global on the server. We use self instead of window for WebWorker support.

  var root = typeof self == 'object' && self.self === self && self ||
            typeof global == 'object' && global.global === global && global;

Set up Backbone appropriately for the environment. Start with AMD.

  if (typeof define === 'function' && define.amd) {
    define(['underscore', 'jquery', 'exports'], function(_, $, exports) {

§

Export global even in AMD case in case this script is loaded with others that may still expect a global Backbone.
```
      root.Backbone = factory(root, exports, _, $);
    });
```

Next for Node.js or CommonJS. jQuery may not be needed as a module.

  } else if (typeof exports !== 'undefined') {
    var _ = require('underscore'), $;
    try { $ = require('jquery'); } catch (e) {}
    factory(root, exports, _, $);

Finally, as a browser global.

  } else {
    root.Backbone = factory(root, {}, root._, root.jQuery || root.Zepto || root.ender || root.$);
  }

})(function(root, Backbone, _, $) {

§

Initial Setup
§
§

Save the previous value of the Backbone variable, so that it can be restored later on, if noConflict is used.
```
  var previousBackbone = root.Backbone;
```
§

Create a local reference to a common array method we’ll want to use later.
```
  var slice = Array.prototype.slice;
```
§

Current version of the library. Keep in sync with package.json.
```
  Backbone.VERSION = '1.6.0';
```
§

For Backbone’s purposes, jQuery, Zepto, Ender, or My Library (kidding) owns the $ variable.
```
  Backbone.$ = $;
```
§

Runs Backbone.js in noConflict mode, returning the Backbone variable to its previous owner. Returns a reference to this Backbone object.
```
  Backbone.noConflict = function() {
    root.Backbone = previousBackbone;
    return this;
  };
```
§

Turn on emulateHTTP to support legacy HTTP servers. Setting this option will fake "PATCH", "PUT" and "DELETE" requests via the _method parameter and set a X-Http-Method-Override header.
```
  Backbone.emulateHTTP = false;
```
§

Turn on emulateJSON to support legacy servers that can’t deal with direct application/json requests … this will encode the body as application/x-www-form-urlencoded instead and will send the model in a form param named model.
```
  Backbone.emulateJSON = false;
```
§

Backbone.Events
§
§

A module that can be mixed in to any object in order to provide it with a custom event channel. You may bind a callback to an event with on or remove with off; trigger-ing an event fires all callbacks in succession.
```
var object = {};
_.extend(object, Backbone.Events);
object.on('expand', function(){ alert('expanded'); });
object.trigger('expand');
```
```
  var Events = Backbone.Events = {};
```
§

Regular expression used to split event strings.
```
  var eventSplitter = /\s+/;
```
§

A private global variable to share between listeners and listenees.
```
  var _listening;
```
§

Iterates over the standard event, callback (as well as the fancy multiple space-separated events "change blur", callback and jQuery-style event maps {event: callback}).
```
  var eventsApi = function(iteratee, events, name, callback, opts) {
    var i = 0, names;
    if (name && typeof name === 'object') {
```

Handle event maps.

      if (callback !== void 0 && 'context' in opts && opts.context === void 0) opts.context = callback;
      for (names = _.keys(name); i < names.length ; i++) {
        events = eventsApi(iteratee, events, names[i], name[names[i]], opts);
      }
    } else if (name && eventSplitter.test(name)) {

Handle space-separated event names by delegating them individually.

      for (names = name.split(eventSplitter); i < names.length; i++) {
        events = iteratee(events, names[i], callback, opts);
      }
    } else {

Finally, standard events.

      events = iteratee(events, name, callback, opts);
    }
    return events;
  };

Bind an event to a callback function. Passing "all" will bind the callback to all events fired.

  Events.on = function(name, callback, context) {
    this._events = eventsApi(onApi, this._events || {}, name, callback, {
      context: context,
      ctx: this,
      listening: _listening
    });

    if (_listening) {
      var listeners = this._listeners || (this._listeners = {});
      listeners[_listening.id] = _listening;

§

Allow the listening to use a counter, instead of tracking callbacks for library interop
```
      _listening.interop = false;
    }

    return this;
  };
```

Inversion-of-control versions of on. Tell this object to listen to an event in another object… keeping track of what it’s listening to for easier unbinding later.

  Events.listenTo = function(obj, name, callback) {
    if (!obj) return this;
    var id = obj._listenId || (obj._listenId = _.uniqueId('l'));
    var listeningTo = this._listeningTo || (this._listeningTo = {});
    var listening = _listening = listeningTo[id];

This object is not listening to any other events on obj yet. Setup the necessary references to track the listening callbacks.

    if (!listening) {
      this._listenId || (this._listenId = _.uniqueId('l'));
      listening = _listening = listeningTo[id] = new Listening(this, obj);
    }

Bind callbacks on obj.

    var error = tryCatchOn(obj, name, callback, this);
    _listening = void 0;

    if (error) throw error;

If the target obj is not Backbone.Events, track events manually.

    if (listening.interop) listening.on(name, callback);

    return this;
  };

The reducing API that adds a callback to the events object.

  var onApi = function(events, name, callback, options) {
    if (callback) {
      var handlers = events[name] || (events[name] = []);
      var context = options.context, ctx = options.ctx, listening = options.listening;
      if (listening) listening.count++;

      handlers.push({callback: callback, context: context, ctx: context || ctx, listening: listening});
    }
    return events;
  };

An try-catch guarded #on function, to prevent poisoning the global _listening variable.

  var tryCatchOn = function(obj, name, callback, context) {
    try {
      obj.on(name, callback, context);
    } catch (e) {
      return e;
    }
  };

Remove one or many callbacks. If context is null, removes all callbacks with that function. If callback is null, removes all callbacks for the event. If name is null, removes all bound callbacks for all events.

  Events.off = function(name, callback, context) {
    if (!this._events) return this;
    this._events = eventsApi(offApi, this._events, name, callback, {
      context: context,
      listeners: this._listeners
    });

    return this;
  };

Tell this object to stop listening to either specific events … or to every object it’s currently listening to.

  Events.stopListening = function(obj, name, callback) {
    var listeningTo = this._listeningTo;
    if (!listeningTo) return this;

    var ids = obj ? [obj._listenId] : _.keys(listeningTo);
    for (var i = 0; i < ids.length; i++) {
      var listening = listeningTo[ids[i]];

If listening doesn’t exist, this object is not currently listening to obj. Break out early.

      if (!listening) break;

      listening.obj.off(name, callback, this);
      if (listening.interop) listening.off(name, callback);
    }
    if (_.isEmpty(listeningTo)) this._listeningTo = void 0;

    return this;
  };

The reducing API that removes a callback from the events object.

  var offApi = function(events, name, callback, options) {
    if (!events) return;

    var context = options.context, listeners = options.listeners;
    var i = 0, names;

Delete all event listeners and “drop” events.

    if (!name && !context && !callback) {
      for (names = _.keys(listeners); i < names.length; i++) {
        listeners[names[i]].cleanup();
      }
      return;
    }

    names = name ? [name] : _.keys(events);
    for (; i < names.length; i++) {
      name = names[i];
      var handlers = events[name];

§

Bail out if there are no events stored.
```
      if (!handlers) break;
```

Find any remaining events.

      var remaining = [];
      for (var j = 0; j < handlers.length; j++) {
        var handler = handlers[j];
        if (
          callback && callback !== handler.callback &&
            callback !== handler.callback._callback ||
              context && context !== handler.context
        ) {
          remaining.push(handler);
        } else {
          var listening = handler.listening;
          if (listening) listening.off(name, callback);
        }
      }

Replace events if there are any remaining. Otherwise, clean up.

      if (remaining.length) {
        events[name] = remaining;
      } else {
        delete events[name];
      }
    }

    return events;
  };

§

Bind an event to only be triggered a single time. After the first time the callback is invoked, its listener will be removed. If multiple events are passed in using the space-separated syntax, the handler will fire once for each event, not once for a combination of all events.
```
  Events.once = function(name, callback, context) {
```

Map the event into a {event: once} object.

    var events = eventsApi(onceMap, {}, name, callback, this.off.bind(this));
    if (typeof name === 'string' && context == null) callback = void 0;
    return this.on(events, callback, context);
  };

Inversion-of-control versions of once.

  Events.listenToOnce = function(obj, name, callback) {

Map the event into a {event: once} object.

    var events = eventsApi(onceMap, {}, name, callback, this.stopListening.bind(this, obj));
    return this.listenTo(obj, events);
  };

Reduces the event callbacks into a map of {event: onceWrapper}. offer unbinds the onceWrapper after it has been called.

  var onceMap = function(map, name, callback, offer) {
    if (callback) {
      var once = map[name] = _.once(function() {
        offer(name, once);
        callback.apply(this, arguments);
      });
      once._callback = callback;
    }
    return map;
  };

Trigger one or many events, firing all bound callbacks. Callbacks are passed the same arguments as trigger is, apart from the event name (unless you’re listening on "all", which will cause your callback to receive the true name of the event as the first argument).

  Events.trigger = function(name) {
    if (!this._events) return this;

    var length = Math.max(0, arguments.length - 1);
    var args = Array(length);
    for (var i = 0; i < length; i++) args[i] = arguments[i + 1];

    eventsApi(triggerApi, this._events, name, void 0, args);
    return this;
  };

Handles triggering the appropriate event callbacks.

  var triggerApi = function(objEvents, name, callback, args) {
    if (objEvents) {
      var events = objEvents[name];
      var allEvents = objEvents.all;
      if (events && allEvents) allEvents = allEvents.slice();
      if (events) triggerEvents(events, args);
      if (allEvents) triggerEvents(allEvents, [name].concat(args));
    }
    return objEvents;
  };

A difficult-to-believe, but optimized internal dispatch function for triggering events. Tries to keep the usual cases speedy (most internal Backbone events have 3 arguments).

  var triggerEvents = function(events, args) {
    var ev, i = -1, l = events.length, a1 = args[0], a2 = args[1], a3 = args[2];
    switch (args.length) {
      case 0: while (++i < l) (ev = events[i]).callback.call(ev.ctx); return;
      case 1: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1); return;
      case 2: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1, a2); return;
      case 3: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1, a2, a3); return;
      default: while (++i < l) (ev = events[i]).callback.apply(ev.ctx, args); return;
    }
  };

A listening class that tracks and cleans up memory bindings when all callbacks have been offed.

  var Listening = function(listener, obj) {
    this.id = listener._listenId;
    this.listener = listener;
    this.obj = obj;
    this.interop = true;
    this.count = 0;
    this._events = void 0;
  };

  Listening.prototype.on = Events.on;

Offs a callback (or several). Uses an optimized counter if the listenee uses Backbone.Events. Otherwise, falls back to manual tracking to support events library interop.

  Listening.prototype.off = function(name, callback) {
    var cleanup;
    if (this.interop) {
      this._events = eventsApi(offApi, this._events, name, callback, {
        context: void 0,
        listeners: void 0
      });
      cleanup = !this._events;
    } else {
      this.count--;
      cleanup = this.count === 0;
    }
    if (cleanup) this.cleanup();
  };

Cleans up memory bindings between the listener and the listenee.

  Listening.prototype.cleanup = function() {
    delete this.listener._listeningTo[this.obj._listenId];
    if (!this.interop) delete this.obj._listeners[this.id];
  };

Aliases for backwards compatibility.

  Events.bind   = Events.on;
  Events.unbind = Events.off;

§

Allow the Backbone object to serve as a global event bus, for folks who want global “pubsub” in a convenient place.
```
  _.extend(Backbone, Events);
```
§

Backbone.Model
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§

Backbone Models are the basic data object in the framework – frequently representing a row in a table in a database on your server. A discrete chunk of data and a bunch of useful, related methods for performing computations and transformations on that data.

Create a new model with the specified attributes. A client id (cid) is automatically generated and assigned for you.

  var Model = Backbone.Model = function(attributes, options) {
    var attrs = attributes || {};
    options || (options = {});
    this.preinitialize.apply(this, arguments);
    this.cid = _.uniqueId(this.cidPrefix);
    this.attributes = {};
    if (options.collection) this.collection = options.collection;
    if (options.parse) attrs = this.parse(attrs, options) || {};
    var defaults = _.result(this, 'defaults');

Just _.defaults would work fine, but the additional _.extends is in there for historical reasons. See #3843.

    attrs = _.defaults(_.extend({}, defaults, attrs), defaults);

    this.set(attrs, options);
    this.changed = {};
    this.initialize.apply(this, arguments);
  };

§

Attach all inheritable methods to the Model prototype.
```
  _.extend(Model.prototype, Events, {
```
§

A hash of attributes whose current and previous value differ.
```
    changed: null,
```
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The value returned during the last failed validation.
```
    validationError: null,
```
§

The default name for the JSON id attribute is "id". MongoDB and CouchDB users may want to set this to "_id".
```
    idAttribute: 'id',
```
§

The prefix is used to create the client id which is used to identify models locally. You may want to override this if you’re experiencing name clashes with model ids.
```
    cidPrefix: 'c',
```
§

preinitialize is an empty function by default. You can override it with a function or object. preinitialize will run before any instantiation logic is run in the Model.
```
    preinitialize: function(){},
```
§

Initialize is an empty function by default. Override it with your own initialization logic.
```
    initialize: function(){},
```

Return a copy of the model’s attributes object.

    toJSON: function(options) {
      return _.clone(this.attributes);
    },

§

Proxy Backbone.sync by default – but override this if you need custom syncing semantics for this particular model.
```
    sync: function() {
      return Backbone.sync.apply(this, arguments);
    },
```

Get the value of an attribute.

    get: function(attr) {
      return this.attributes[attr];
    },

Get the HTML-escaped value of an attribute.

    escape: function(attr) {
      return _.escape(this.get(attr));
    },

Returns true if the attribute contains a value that is not null or undefined.

    has: function(attr) {
      return this.get(attr) != null;
    },

Special-cased proxy to underscore’s _.matches method.

    matches: function(attrs) {
      return !!_.iteratee(attrs, this)(this.attributes);
    },

§

Set a hash of model attributes on the object, firing "change". This is the core primitive operation of a model, updating the data and notifying anyone who needs to know about the change in state. The heart of the beast.
```
    set: function(key, val, options) {
      if (key == null) return this;
```

Handle both "key", value and {key: value} -style arguments.

      var attrs;
      if (typeof key === 'object') {
        attrs = key;
        options = val;
      } else {
        (attrs = {})[key] = val;
      }

      options || (options = {});

Run validation.

      if (!this._validate(attrs, options)) return false;

Extract attributes and options.

      var unset      = options.unset;
      var silent     = options.silent;
      var changes    = [];
      var changing   = this._changing;
      this._changing = true;

      if (!changing) {
        this._previousAttributes = _.clone(this.attributes);
        this.changed = {};
      }

      var current = this.attributes;
      var changed = this.changed;
      var prev    = this._previousAttributes;

For each set attribute, update or delete the current value.

      for (var attr in attrs) {
        val = attrs[attr];
        if (!_.isEqual(current[attr], val)) changes.push(attr);
        if (!_.isEqual(prev[attr], val)) {
          changed[attr] = val;
        } else {
          delete changed[attr];
        }
        unset ? delete current[attr] : current[attr] = val;
      }

Update the id.

      if (this.idAttribute in attrs) {
        var prevId = this.id;
        this.id = this.get(this.idAttribute);
        this.trigger('changeId', this, prevId, options);
      }

Trigger all relevant attribute changes.

      if (!silent) {
        if (changes.length) this._pending = options;
        for (var i = 0; i < changes.length; i++) {
          this.trigger('change:' + changes[i], this, current[changes[i]], options);
        }
      }

You might be wondering why there’s a while loop here. Changes can be recursively nested within "change" events.

      if (changing) return this;
      if (!silent) {
        while (this._pending) {
          options = this._pending;
          this._pending = false;
          this.trigger('change', this, options);
        }
      }
      this._pending = false;
      this._changing = false;
      return this;
    },

Remove an attribute from the model, firing "change". unset is a noop if the attribute doesn’t exist.

    unset: function(attr, options) {
      return this.set(attr, void 0, _.extend({}, options, {unset: true}));
    },

Clear all attributes on the model, firing "change".

    clear: function(options) {
      var attrs = {};
      for (var key in this.attributes) attrs[key] = void 0;
      return this.set(attrs, _.extend({}, options, {unset: true}));
    },

Determine if the model has changed since the last "change" event. If you specify an attribute name, determine if that attribute has changed.

    hasChanged: function(attr) {
      if (attr == null) return !_.isEmpty(this.changed);
      return _.has(this.changed, attr);
    },

Return an object containing all the attributes that have changed, or false if there are no changed attributes. Useful for determining what parts of a view need to be updated and/or what attributes need to be persisted to the server. Unset attributes will be set to undefined. You can also pass an attributes object to diff against the model, determining if there would be a change.

    changedAttributes: function(diff) {
      if (!diff) return this.hasChanged() ? _.clone(this.changed) : false;
      var old = this._changing ? this._previousAttributes : this.attributes;
      var changed = {};
      var hasChanged;
      for (var attr in diff) {
        var val = diff[attr];
        if (_.isEqual(old[attr], val)) continue;
        changed[attr] = val;
        hasChanged = true;
      }
      return hasChanged ? changed : false;
    },

Get the previous value of an attribute, recorded at the time the last "change" event was fired.

    previous: function(attr) {
      if (attr == null || !this._previousAttributes) return null;
      return this._previousAttributes[attr];
    },

Get all of the attributes of the model at the time of the previous "change" event.

    previousAttributes: function() {
      return _.clone(this._previousAttributes);
    },

Fetch the model from the server, merging the response with the model’s local attributes. Any changed attributes will trigger a “change” event.

    fetch: function(options) {
      options = _.extend({parse: true}, options);
      var model = this;
      var success = options.success;
      options.success = function(resp) {
        var serverAttrs = options.parse ? model.parse(resp, options) : resp;
        if (!model.set(serverAttrs, options)) return false;
        if (success) success.call(options.context, model, resp, options);
        model.trigger('sync', model, resp, options);
      };
      wrapError(this, options);
      return this.sync('read', this, options);
    },

§

Set a hash of model attributes, and sync the model to the server. If the server returns an attributes hash that differs, the model’s state will be set again.
```
    save: function(key, val, options) {
```

Handle both "key", value and {key: value} -style arguments.

      var attrs;
      if (key == null || typeof key === 'object') {
        attrs = key;
        options = val;
      } else {
        (attrs = {})[key] = val;
      }

      options = _.extend({validate: true, parse: true}, options);
      var wait = options.wait;

If we’re not waiting and attributes exist, save acts as set(attr).save(null, opts) with validation. Otherwise, check if the model will be valid when the attributes, if any, are set.

      if (attrs && !wait) {
        if (!this.set(attrs, options)) return false;
      } else if (!this._validate(attrs, options)) {
        return false;
      }

After a successful server-side save, the client is (optionally) updated with the server-side state.

      var model = this;
      var success = options.success;
      var attributes = this.attributes;
      options.success = function(resp) {

Ensure attributes are restored during synchronous saves.

        model.attributes = attributes;
        var serverAttrs = options.parse ? model.parse(resp, options) : resp;
        if (wait) serverAttrs = _.extend({}, attrs, serverAttrs);
        if (serverAttrs && !model.set(serverAttrs, options)) return false;
        if (success) success.call(options.context, model, resp, options);
        model.trigger('sync', model, resp, options);
      };
      wrapError(this, options);

Set temporary attributes if {wait: true} to properly find new ids.

      if (attrs && wait) this.attributes = _.extend({}, attributes, attrs);

      var method = this.isNew() ? 'create' : options.patch ? 'patch' : 'update';
      if (method === 'patch' && !options.attrs) options.attrs = attrs;
      var xhr = this.sync(method, this, options);

Restore attributes.

      this.attributes = attributes;

      return xhr;
    },

Destroy this model on the server if it was already persisted. Optimistically removes the model from its collection, if it has one. If wait: true is passed, waits for the server to respond before removal.

    destroy: function(options) {
      options = options ? _.clone(options) : {};
      var model = this;
      var success = options.success;
      var wait = options.wait;

      var destroy = function() {
        model.stopListening();
        model.trigger('destroy', model, model.collection, options);
      };

      options.success = function(resp) {
        if (wait) destroy();
        if (success) success.call(options.context, model, resp, options);
        if (!model.isNew()) model.trigger('sync', model, resp, options);
      };

      var xhr = false;
      if (this.isNew()) {
        _.defer(options.success);
      } else {
        wrapError(this, options);
        xhr = this.sync('delete', this, options);
      }
      if (!wait) destroy();
      return xhr;
    },

Default URL for the model’s representation on the server – if you’re using Backbone’s restful methods, override this to change the endpoint that will be called.

    url: function() {
      var base =
        _.result(this, 'urlRoot') ||
        _.result(this.collection, 'url') ||
        urlError();
      if (this.isNew()) return base;
      var id = this.get(this.idAttribute);
      return base.replace(/[^\/]$/, '$&/') + encodeURIComponent(id);
    },

§

parse converts a response into the hash of attributes to be set on the model. The default implementation is just to pass the response along.
```
    parse: function(resp, options) {
      return resp;
    },
```

Create a new model with identical attributes to this one.

    clone: function() {
      return new this.constructor(this.attributes);
    },

A model is new if it has never been saved to the server, and lacks an id.

    isNew: function() {
      return !this.has(this.idAttribute);
    },

Check if the model is currently in a valid state.

    isValid: function(options) {
      return this._validate({}, _.extend({}, options, {validate: true}));
    },

Run validation against the next complete set of model attributes, returning true if all is well. Otherwise, fire an "invalid" event.

    _validate: function(attrs, options) {
      if (!options.validate || !this.validate) return true;
      attrs = _.extend({}, this.attributes, attrs);
      var error = this.validationError = this.validate(attrs, options) || null;
      if (!error) return true;
      this.trigger('invalid', this, error, _.extend(options, {validationError: error}));
      return false;
    }

  });

§

Backbone.Collection
§
§

If models tend to represent a single row of data, a Backbone Collection is more analogous to a table full of data … or a small slice or page of that table, or a collection of rows that belong together for a particular reason – all of the messages in this particular folder, all of the documents belonging to this particular author, and so on. Collections maintain indexes of their models, both in order, and for lookup by id.

Create a new Collection, perhaps to contain a specific type of model. If a comparator is specified, the Collection will maintain its models in sort order, as they’re added and removed.

  var Collection = Backbone.Collection = function(models, options) {
    options || (options = {});
    this.preinitialize.apply(this, arguments);
    if (options.model) this.model = options.model;
    if (options.comparator !== void 0) this.comparator = options.comparator;
    this._reset();
    this.initialize.apply(this, arguments);
    if (models) this.reset(models, _.extend({silent: true}, options));
  };

Default options for Collection#set.

  var setOptions = {add: true, remove: true, merge: true};
  var addOptions = {add: true, remove: false};

Splices insert into array at index at.

  var splice = function(array, insert, at) {
    at = Math.min(Math.max(at, 0), array.length);
    var tail = Array(array.length - at);
    var length = insert.length;
    var i;
    for (i = 0; i < tail.length; i++) tail[i] = array[i + at];
    for (i = 0; i < length; i++) array[i + at] = insert[i];
    for (i = 0; i < tail.length; i++) array[i + length + at] = tail[i];
  };

§

Define the Collection’s inheritable methods.
```
  _.extend(Collection.prototype, Events, {
```
§

The default model for a collection is just a Backbone.Model. This should be overridden in most cases.
```
    model: Model,
```
§

preinitialize is an empty function by default. You can override it with a function or object. preinitialize will run before any instantiation logic is run in the Collection.
```
    preinitialize: function(){},
```
§

Initialize is an empty function by default. Override it with your own initialization logic.
```
    initialize: function(){},
```

The JSON representation of a Collection is an array of the models’ attributes.

    toJSON: function(options) {
      return this.map(function(model) { return model.toJSON(options); });
    },

Proxy Backbone.sync by default.

    sync: function() {
      return Backbone.sync.apply(this, arguments);
    },

§

Add a model, or list of models to the set. models may be Backbone Models or raw JavaScript objects to be converted to Models, or any combination of the two.
```
    add: function(models, options) {
      return this.set(models, _.extend({merge: false}, options, addOptions));
    },
```

Remove a model, or a list of models from the set.

    remove: function(models, options) {
      options = _.extend({}, options);
      var singular = !_.isArray(models);
      models = singular ? [models] : models.slice();
      var removed = this._removeModels(models, options);
      if (!options.silent && removed.length) {
        options.changes = {added: [], merged: [], removed: removed};
        this.trigger('update', this, options);
      }
      return singular ? removed[0] : removed;
    },

Update a collection by set-ing a new list of models, adding new ones, removing models that are no longer present, and merging models that already exist in the collection, as necessary. Similar to Model#set, the core operation for updating the data contained by the collection.

    set: function(models, options) {
      if (models == null) return;

      options = _.extend({}, setOptions, options);
      if (options.parse && !this._isModel(models)) {
        models = this.parse(models, options) || [];
      }

      var singular = !_.isArray(models);
      models = singular ? [models] : models.slice();

      var at = options.at;
      if (at != null) at = +at;
      if (at > this.length) at = this.length;
      if (at < 0) at += this.length + 1;

      var set = [];
      var toAdd = [];
      var toMerge = [];
      var toRemove = [];
      var modelMap = {};

      var add = options.add;
      var merge = options.merge;
      var remove = options.remove;

      var sort = false;
      var sortable = this.comparator && at == null && options.sort !== false;
      var sortAttr = _.isString(this.comparator) ? this.comparator : null;

Turn bare objects into model references, and prevent invalid models from being added.

      var model, i;
      for (i = 0; i < models.length; i++) {
        model = models[i];

If a duplicate is found, prevent it from being added and optionally merge it into the existing model.

        var existing = this.get(model);
        if (existing) {
          if (merge && model !== existing) {
            var attrs = this._isModel(model) ? model.attributes : model;
            if (options.parse) attrs = existing.parse(attrs, options);
            existing.set(attrs, options);
            toMerge.push(existing);
            if (sortable && !sort) sort = existing.hasChanged(sortAttr);
          }
          if (!modelMap[existing.cid]) {
            modelMap[existing.cid] = true;
            set.push(existing);
          }
          models[i] = existing;

If this is a new, valid model, push it to the toAdd list.

        } else if (add) {
          model = models[i] = this._prepareModel(model, options);
          if (model) {
            toAdd.push(model);
            this._addReference(model, options);
            modelMap[model.cid] = true;
            set.push(model);
          }
        }
      }

Remove stale models.

      if (remove) {
        for (i = 0; i < this.length; i++) {
          model = this.models[i];
          if (!modelMap[model.cid]) toRemove.push(model);
        }
        if (toRemove.length) this._removeModels(toRemove, options);
      }

See if sorting is needed, update length and splice in new models.

      var orderChanged = false;
      var replace = !sortable && add && remove;
      if (set.length && replace) {
        orderChanged = this.length !== set.length || _.some(this.models, function(m, index) {
          return m !== set[index];
        });
        this.models.length = 0;
        splice(this.models, set, 0);
        this.length = this.models.length;
      } else if (toAdd.length) {
        if (sortable) sort = true;
        splice(this.models, toAdd, at == null ? this.length : at);
        this.length = this.models.length;
      }

Silently sort the collection if appropriate.

      if (sort) this.sort({silent: true});

Unless silenced, it’s time to fire all appropriate add/sort/update events.

      if (!options.silent) {
        for (i = 0; i < toAdd.length; i++) {
          if (at != null) options.index = at + i;
          model = toAdd[i];
          model.trigger('add', model, this, options);
        }
        if (sort || orderChanged) this.trigger('sort', this, options);
        if (toAdd.length || toRemove.length || toMerge.length) {
          options.changes = {
            added: toAdd,
            removed: toRemove,
            merged: toMerge
          };
          this.trigger('update', this, options);
        }
      }

Return the added (or merged) model (or models).

      return singular ? models[0] : models;
    },

When you have more items than you want to add or remove individually, you can reset the entire set with a new list of models, without firing any granular add or remove events. Fires reset when finished. Useful for bulk operations and optimizations.

    reset: function(models, options) {
      options = options ? _.clone(options) : {};
      for (var i = 0; i < this.models.length; i++) {
        this._removeReference(this.models[i], options);
      }
      options.previousModels = this.models;
      this._reset();
      models = this.add(models, _.extend({silent: true}, options));
      if (!options.silent) this.trigger('reset', this, options);
      return models;
    },

Add a model to the end of the collection.

    push: function(model, options) {
      return this.add(model, _.extend({at: this.length}, options));
    },

Remove a model from the end of the collection.

    pop: function(options) {
      var model = this.at(this.length - 1);
      return this.remove(model, options);
    },

Add a model to the beginning of the collection.

    unshift: function(model, options) {
      return this.add(model, _.extend({at: 0}, options));
    },

Remove a model from the beginning of the collection.

    shift: function(options) {
      var model = this.at(0);
      return this.remove(model, options);
    },

Slice out a sub-array of models from the collection.

    slice: function() {
      return slice.apply(this.models, arguments);
    },

Get a model from the set by id, cid, model object with id or cid properties, or an attributes object that is transformed through modelId.

    get: function(obj) {
      if (obj == null) return void 0;
      return this._byId[obj] ||
        this._byId[this.modelId(this._isModel(obj) ? obj.attributes : obj, obj.idAttribute)] ||
        obj.cid && this._byId[obj.cid];
    },

Returns true if the model is in the collection.

    has: function(obj) {
      return this.get(obj) != null;
    },

Get the model at the given index.

    at: function(index) {
      if (index < 0) index += this.length;
      return this.models[index];
    },

Return models with matching attributes. Useful for simple cases of filter.

    where: function(attrs, first) {
      return this[first ? 'find' : 'filter'](attrs);
    },

Return the first model with matching attributes. Useful for simple cases of find.

    findWhere: function(attrs) {
      return this.where(attrs, true);
    },

Force the collection to re-sort itself. You don’t need to call this under normal circumstances, as the set will maintain sort order as each item is added.

    sort: function(options) {
      var comparator = this.comparator;
      if (!comparator) throw new Error('Cannot sort a set without a comparator');
      options || (options = {});

      var length = comparator.length;
      if (_.isFunction(comparator)) comparator = comparator.bind(this);

Run sort based on type of comparator.

      if (length === 1 || _.isString(comparator)) {
        this.models = this.sortBy(comparator);
      } else {
        this.models.sort(comparator);
      }
      if (!options.silent) this.trigger('sort', this, options);
      return this;
    },

Pluck an attribute from each model in the collection.

    pluck: function(attr) {
      return this.map(attr + '');
    },

Fetch the default set of models for this collection, resetting the collection when they arrive. If reset: true is passed, the response data will be passed through the reset method instead of set.

    fetch: function(options) {
      options = _.extend({parse: true}, options);
      var success = options.success;
      var collection = this;
      options.success = function(resp) {
        var method = options.reset ? 'reset' : 'set';
        collection[method](resp, options);
        if (success) success.call(options.context, collection, resp, options);
        collection.trigger('sync', collection, resp, options);
      };
      wrapError(this, options);
      return this.sync('read', this, options);
    },

Create a new instance of a model in this collection. Add the model to the collection immediately, unless wait: true is passed, in which case we wait for the server to agree.

    create: function(model, options) {
      options = options ? _.clone(options) : {};
      var wait = options.wait;
      model = this._prepareModel(model, options);
      if (!model) return false;
      if (!wait) this.add(model, options);
      var collection = this;
      var success = options.success;
      options.success = function(m, resp, callbackOpts) {
        if (wait) {
          m.off('error', collection._forwardPristineError, collection);
          collection.add(m, callbackOpts);
        }
        if (success) success.call(callbackOpts.context, m, resp, callbackOpts);
      };

§

In case of wait:true, our collection is not listening to any of the model’s events yet, so it will not forward the error event. In this special case, we need to listen for it separately and handle the event just once. (The reason we don’t need to do this for the sync event is in the success handler above: we add the model first, which causes the collection to listen, and then invoke the callback that triggers the event.)
```
      if (wait) {
        model.once('error', this._forwardPristineError, this);
      }
      model.save(null, options);
      return model;
    },
```
§

parse converts a response into a list of models to be added to the collection. The default implementation is just to pass it through.
```
    parse: function(resp, options) {
      return resp;
    },
```

Create a new collection with an identical list of models as this one.

    clone: function() {
      return new this.constructor(this.models, {
        model: this.model,
        comparator: this.comparator
      });
    },

Define how to uniquely identify models in the collection.

    modelId: function(attrs, idAttribute) {
      return attrs[idAttribute || this.model.prototype.idAttribute || 'id'];
    },

Get an iterator of all models in this collection.

    values: function() {
      return new CollectionIterator(this, ITERATOR_VALUES);
    },

Get an iterator of all model IDs in this collection.

    keys: function() {
      return new CollectionIterator(this, ITERATOR_KEYS);
    },

Get an iterator of all [ID, model] tuples in this collection.

    entries: function() {
      return new CollectionIterator(this, ITERATOR_KEYSVALUES);
    },

Private method to reset all internal state. Called when the collection is first initialized or reset.

    _reset: function() {
      this.length = 0;
      this.models = [];
      this._byId  = {};
    },

Prepare a hash of attributes (or other model) to be added to this collection.

    _prepareModel: function(attrs, options) {
      if (this._isModel(attrs)) {
        if (!attrs.collection) attrs.collection = this;
        return attrs;
      }
      options = options ? _.clone(options) : {};
      options.collection = this;

      var model;
      if (this.model.prototype) {
        model = new this.model(attrs, options);
      } else {

ES class methods didn’t have prototype

        model = this.model(attrs, options);
      }

      if (!model.validationError) return model;
      this.trigger('invalid', this, model.validationError, options);
      return false;
    },

Internal method called by both remove and set.

    _removeModels: function(models, options) {
      var removed = [];
      for (var i = 0; i < models.length; i++) {
        var model = this.get(models[i]);
        if (!model) continue;

        var index = this.indexOf(model);
        this.models.splice(index, 1);
        this.length--;

Remove references before triggering ‘remove’ event to prevent an infinite loop. #3693

        delete this._byId[model.cid];
        var id = this.modelId(model.attributes, model.idAttribute);
        if (id != null) delete this._byId[id];

        if (!options.silent) {
          options.index = index;
          model.trigger('remove', model, this, options);
        }

        removed.push(model);
        this._removeReference(model, options);
      }
      if (models.length > 0 && !options.silent) delete options.index;
      return removed;
    },

§

Method for checking whether an object should be considered a model for the purposes of adding to the collection.
```
    _isModel: function(model) {
      return model instanceof Model;
    },
```

Internal method to create a model’s ties to a collection.

    _addReference: function(model, options) {
      this._byId[model.cid] = model;
      var id = this.modelId(model.attributes, model.idAttribute);
      if (id != null) this._byId[id] = model;
      model.on('all', this._onModelEvent, this);
    },

Internal method to sever a model’s ties to a collection.

    _removeReference: function(model, options) {
      delete this._byId[model.cid];
      var id = this.modelId(model.attributes, model.idAttribute);
      if (id != null) delete this._byId[id];
      if (this === model.collection) delete model.collection;
      model.off('all', this._onModelEvent, this);
    },

Internal method called every time a model in the set fires an event. Sets need to update their indexes when models change ids. All other events simply proxy through. “add” and “remove” events that originate in other collections are ignored.

    _onModelEvent: function(event, model, collection, options) {
      if (model) {
        if ((event === 'add' || event === 'remove') && collection !== this) return;
        if (event === 'destroy') this.remove(model, options);
        if (event === 'changeId') {
          var prevId = this.modelId(model.previousAttributes(), model.idAttribute);
          var id = this.modelId(model.attributes, model.idAttribute);
          if (prevId != null) delete this._byId[prevId];
          if (id != null) this._byId[id] = model;
        }
      }
      this.trigger.apply(this, arguments);
    },

§

Internal callback method used in create. It serves as a stand-in for the _onModelEvent method, which is not yet bound during the wait period of the create call. We still want to forward any 'error' event at the end of the wait period, hence a customized callback.
```
    _forwardPristineError: function(model, collection, options) {
```

Prevent double forward if the model was already in the collection before the call to create.

      if (this.has(model)) return;
      this._onModelEvent('error', model, collection, options);
    }
  });

Defining an @@iterator method implements JavaScript’s Iterable protocol. In modern ES2015 browsers, this value is found at Symbol.iterator.

  /* global Symbol */
  var $$iterator = typeof Symbol === 'function' && Symbol.iterator;
  if ($$iterator) {
    Collection.prototype[$$iterator] = Collection.prototype.values;
  }

§

CollectionIterator
§
§

A CollectionIterator implements JavaScript’s Iterator protocol, allowing the use of for of loops in modern browsers and interoperation between Backbone.Collection and other JavaScript functions and third-party libraries which can operate on Iterables.
```
  var CollectionIterator = function(collection, kind) {
    this._collection = collection;
    this._kind = kind;
    this._index = 0;
  };
```
§

This “enum” defines the three possible kinds of values which can be emitted by a CollectionIterator that correspond to the values(), keys() and entries() methods on Collection, respectively.
```
  var ITERATOR_VALUES = 1;
  var ITERATOR_KEYS = 2;
  var ITERATOR_KEYSVALUES = 3;
```

All Iterators should themselves be Iterable.

  if ($$iterator) {
    CollectionIterator.prototype[$$iterator] = function() {
      return this;
    };
  }

  CollectionIterator.prototype.next = function() {
    if (this._collection) {

Only continue iterating if the iterated collection is long enough.

      if (this._index < this._collection.length) {
        var model = this._collection.at(this._index);
        this._index++;

Construct a value depending on what kind of values should be iterated.

        var value;
        if (this._kind === ITERATOR_VALUES) {
          value = model;
        } else {
          var id = this._collection.modelId(model.attributes, model.idAttribute);
          if (this._kind === ITERATOR_KEYS) {
            value = id;
          } else { // ITERATOR_KEYSVALUES
            value = [id, model];
          }
        }
        return {value: value, done: false};
      }

§

Once exhausted, remove the reference to the collection so future calls to the next method always return done.
```
      this._collection = void 0;
    }

    return {value: void 0, done: true};
  };
```
§

Backbone.View
§
§

Backbone Views are almost more convention than they are actual code. A View is simply a JavaScript object that represents a logical chunk of UI in the DOM. This might be a single item, an entire list, a sidebar or panel, or even the surrounding frame which wraps your whole app. Defining a chunk of UI as a View allows you to define your DOM events declaratively, without having to worry about render order … and makes it easy for the view to react to specific changes in the state of your models.

Creating a Backbone.View creates its initial element outside of the DOM, if an existing element is not provided…

  var View = Backbone.View = function(options) {
    this.cid = _.uniqueId('view');
    this.preinitialize.apply(this, arguments);
    _.extend(this, _.pick(options, viewOptions));
    this._ensureElement();
    this.initialize.apply(this, arguments);
  };

Cached regex to split keys for delegate.

  var delegateEventSplitter = /^(\S+)\s*(.*)$/;

List of view options to be set as properties.

  var viewOptions = ['model', 'collection', 'el', 'id', 'attributes', 'className', 'tagName', 'events'];

§

Set up all inheritable Backbone.View properties and methods.
```
  _.extend(View.prototype, Events, {
```
§

The default tagName of a View’s element is "div".
```
    tagName: 'div',
```
§

jQuery delegate for element lookup, scoped to DOM elements within the current view. This should be preferred to global lookups where possible.
```
    $: function(selector) {
      return this.$el.find(selector);
    },
```
§

preinitialize is an empty function by default. You can override it with a function or object. preinitialize will run before any instantiation logic is run in the View
```
    preinitialize: function(){},
```
§

Initialize is an empty function by default. Override it with your own initialization logic.
```
    initialize: function(){},
```
§

render is the core function that your view should override, in order to populate its element (this.el), with the appropriate HTML. The convention is for render to always return this.
```
    render: function() {
      return this;
    },
```

Remove this view by taking the element out of the DOM, and removing any applicable Backbone.Events listeners.

    remove: function() {
      this._removeElement();
      this.stopListening();
      return this;
    },

§

Remove this view’s element from the document and all event listeners attached to it. Exposed for subclasses using an alternative DOM manipulation API.
```
    _removeElement: function() {
      this.$el.remove();
    },
```

Change the view’s element (this.el property) and re-delegate the view’s events on the new element.

    setElement: function(element) {
      this.undelegateEvents();
      this._setElement(element);
      this.delegateEvents();
      return this;
    },

§

Creates the this.el and this.$el references for this view using the given el. el can be a CSS selector or an HTML string, a jQuery context or an element. Subclasses can override this to utilize an alternative DOM manipulation API and are only required to set the this.el property.
```
    _setElement: function(el) {
      this.$el = el instanceof Backbone.$ ? el : Backbone.$(el);
      this.el = this.$el[0];
    },
```

Set callbacks, where this.events is a hash of

{“event selector”: “callback”}

{
  'mousedown .title':  'edit',
  'click .button':     'save',
  'click .open':       function(e) { ... }
}

pairs. Callbacks will be bound to the view, with this set properly. Uses event delegation for efficiency. Omitting the selector binds the event to this.el.

    delegateEvents: function(events) {
      events || (events = _.result(this, 'events'));
      if (!events) return this;
      this.undelegateEvents();
      for (var key in events) {
        var method = events[key];
        if (!_.isFunction(method)) method = this[method];
        if (!method) continue;
        var match = key.match(delegateEventSplitter);
        this.delegate(match[1], match[2], method.bind(this));
      }
      return this;
    },

§

Add a single event listener to the view’s element (or a child element using selector). This only works for delegate-able events: not focus, blur, and not change, submit, and reset in Internet Explorer.
```
    delegate: function(eventName, selector, listener) {
      this.$el.on(eventName + '.delegateEvents' + this.cid, selector, listener);
      return this;
    },
```
§

Clears all callbacks previously bound to the view by delegateEvents. You usually don’t need to use this, but may wish to if you have multiple Backbone views attached to the same DOM element.
```
    undelegateEvents: function() {
      if (this.$el) this.$el.off('.delegateEvents' + this.cid);
      return this;
    },
```

A finer-grained undelegateEvents for removing a single delegated event. selector and listener are both optional.

    undelegate: function(eventName, selector, listener) {
      this.$el.off(eventName + '.delegateEvents' + this.cid, selector, listener);
      return this;
    },

§

Produces a DOM element to be assigned to your view. Exposed for subclasses using an alternative DOM manipulation API.
```
    _createElement: function(tagName) {
      return document.createElement(tagName);
    },
```

Ensure that the View has a DOM element to render into. If this.el is a string, pass it through $(), take the first matching element, and re-assign it to el. Otherwise, create an element from the id, className and tagName properties.

    _ensureElement: function() {
      if (!this.el) {
        var attrs = _.extend({}, _.result(this, 'attributes'));
        if (this.id) attrs.id = _.result(this, 'id');
        if (this.className) attrs['class'] = _.result(this, 'className');
        this.setElement(this._createElement(_.result(this, 'tagName')));
        this._setAttributes(attrs);
      } else {
        this.setElement(_.result(this, 'el'));
      }
    },

§

Set attributes from a hash on this view’s element. Exposed for subclasses using an alternative DOM manipulation API.
```
    _setAttributes: function(attributes) {
      this.$el.attr(attributes);
    }

  });
```

Proxy Backbone class methods to Underscore functions, wrapping the model’s attributes object or collection’s models array behind the scenes.

collection.filter(function(model) { return model.get(‘age’) > 10 }); collection.each(this.addView);

Function#apply can be slow so we use the method’s arg count, if we know it.

  var addMethod = function(base, length, method, attribute) {
    switch (length) {
      case 1: return function() {
        return base[method](this[attribute]);
      };
      case 2: return function(value) {
        return base[method](this[attribute], value);
      };
      case 3: return function(iteratee, context) {
        return base[method](this[attribute], cb(iteratee, this), context);
      };
      case 4: return function(iteratee, defaultVal, context) {
        return base[method](this[attribute], cb(iteratee, this), defaultVal, context);
      };
      default: return function() {
        var args = slice.call(arguments);
        args.unshift(this[attribute]);
        return base[method].apply(base, args);
      };
    }
  };

  var addUnderscoreMethods = function(Class, base, methods, attribute) {
    _.each(methods, function(length, method) {
      if (base[method]) Class.prototype[method] = addMethod(base, length, method, attribute);
    });
  };

Support collection.sortBy('attr') and collection.findWhere({id: 1}).

  var cb = function(iteratee, instance) {
    if (_.isFunction(iteratee)) return iteratee;
    if (_.isObject(iteratee) && !instance._isModel(iteratee)) return modelMatcher(iteratee);
    if (_.isString(iteratee)) return function(model) { return model.get(iteratee); };
    return iteratee;
  };
  var modelMatcher = function(attrs) {
    var matcher = _.matches(attrs);
    return function(model) {
      return matcher(model.attributes);
    };
  };

Underscore methods that we want to implement on the Collection. 90% of the core usefulness of Backbone Collections is actually implemented right here:

  var collectionMethods = {forEach: 3, each: 3, map: 3, collect: 3, reduce: 0,
    foldl: 0, inject: 0, reduceRight: 0, foldr: 0, find: 3, detect: 3, filter: 3,
    select: 3, reject: 3, every: 3, all: 3, some: 3, any: 3, include: 3, includes: 3,
    contains: 3, invoke: 0, max: 3, min: 3, toArray: 1, size: 1, first: 3,
    head: 3, take: 3, initial: 3, rest: 3, tail: 3, drop: 3, last: 3,
    without: 0, difference: 0, indexOf: 3, shuffle: 1, lastIndexOf: 3,
    isEmpty: 1, chain: 1, sample: 3, partition: 3, groupBy: 3, countBy: 3,
    sortBy: 3, indexBy: 3, findIndex: 3, findLastIndex: 3};

Underscore methods that we want to implement on the Model, mapped to the number of arguments they take.

  var modelMethods = {keys: 1, values: 1, pairs: 1, invert: 1, pick: 0,
    omit: 0, chain: 1, isEmpty: 1};

Mix in each Underscore method as a proxy to Collection#models.

  _.each([
    [Collection, collectionMethods, 'models'],
    [Model, modelMethods, 'attributes']
  ], function(config) {
    var Base = config[0],
        methods = config[1],
        attribute = config[2];

    Base.mixin = function(obj) {
      var mappings = _.reduce(_.functions(obj), function(memo, name) {
        memo[name] = 0;
        return memo;
      }, {});
      addUnderscoreMethods(Base, obj, mappings, attribute);
    };

    addUnderscoreMethods(Base, _, methods, attribute);
  });

§

Backbone.sync
§
§

Override this function to change the manner in which Backbone persists models to the server. You will be passed the type of request, and the model in question. By default, makes a RESTful Ajax request to the model’s url(). Some possible customizations could be:
- Use setTimeout to batch rapid-fire updates into a single request.
- Send up the models as XML instead of JSON.
- Persist models via WebSockets instead of Ajax.
Turn on Backbone.emulateHTTP in order to send PUT and DELETE requests as POST, with a _method parameter containing the true HTTP method, as well as all requests with the body as application/x-www-form-urlencoded instead of application/json with the model in a param named model. Useful when interfacing with server-side languages like PHP that make it difficult to read the body of PUT requests.
```
  Backbone.sync = function(method, model, options) {
    var type = methodMap[method];
```

Default options, unless specified.

    _.defaults(options || (options = {}), {
      emulateHTTP: Backbone.emulateHTTP,
      emulateJSON: Backbone.emulateJSON
    });

Default JSON-request options.

    var params = {type: type, dataType: 'json'};

Ensure that we have a URL.

    if (!options.url) {
      params.url = _.result(model, 'url') || urlError();
    }

Ensure that we have the appropriate request data.

    if (options.data == null && model && (method === 'create' || method === 'update' || method === 'patch')) {
      params.contentType = 'application/json';
      params.data = JSON.stringify(options.attrs || model.toJSON(options));
    }

For older servers, emulate JSON by encoding the request into an HTML-form.

    if (options.emulateJSON) {
      params.contentType = 'application/x-www-form-urlencoded';
      params.data = params.data ? {model: params.data} : {};
    }

For older servers, emulate HTTP by mimicking the HTTP method with _method And an X-HTTP-Method-Override header.

    if (options.emulateHTTP && (type === 'PUT' || type === 'DELETE' || type === 'PATCH')) {
      params.type = 'POST';
      if (options.emulateJSON) params.data._method = type;
      var beforeSend = options.beforeSend;
      options.beforeSend = function(xhr) {
        xhr.setRequestHeader('X-HTTP-Method-Override', type);
        if (beforeSend) return beforeSend.apply(this, arguments);
      };
    }

Don’t process data on a non-GET request.

    if (params.type !== 'GET' && !options.emulateJSON) {
      params.processData = false;
    }

Pass along textStatus and errorThrown from jQuery.

    var error = options.error;
    options.error = function(xhr, textStatus, errorThrown) {
      options.textStatus = textStatus;
      options.errorThrown = errorThrown;
      if (error) error.call(options.context, xhr, textStatus, errorThrown);
    };

Make the request, allowing the user to override any Ajax options.

    var xhr = options.xhr = Backbone.ajax(_.extend(params, options));
    model.trigger('request', model, xhr, options);
    return xhr;
  };

Map from CRUD to HTTP for our default Backbone.sync implementation.

  var methodMap = {
    'create': 'POST',
    'update': 'PUT',
    'patch': 'PATCH',
    'delete': 'DELETE',
    'read': 'GET'
  };

§

Set the default implementation of Backbone.ajax to proxy through to $. Override this if you’d like to use a different library.
```
  Backbone.ajax = function() {
    return Backbone.$.ajax.apply(Backbone.$, arguments);
  };
```
§

Backbone.Router
§

Routers map faux-URLs to actions, and fire events when routes are matched. Creating a new one sets its routes hash, if not set statically.

  var Router = Backbone.Router = function(options) {
    options || (options = {});
    this.preinitialize.apply(this, arguments);
    if (options.routes) this.routes = options.routes;
    this._bindRoutes();
    this.initialize.apply(this, arguments);
  };

Cached regular expressions for matching named param parts and splatted parts of route strings.

  var optionalParam = /\((.*?)\)/g;
  var namedParam    = /(\(\?)?:\w+/g;
  var splatParam    = /\*\w+/g;
  var escapeRegExp  = /[\-{}\[\]+?.,\\\^$|#\s]/g;

§

Set up all inheritable Backbone.Router properties and methods.
```
  _.extend(Router.prototype, Events, {
```
§

preinitialize is an empty function by default. You can override it with a function or object. preinitialize will run before any instantiation logic is run in the Router.
```
    preinitialize: function(){},
```
§

Initialize is an empty function by default. Override it with your own initialization logic.
```
    initialize: function(){},
```

Manually bind a single named route to a callback. For example:

this.route('search/:query/p:num', 'search', function(query, num) {
  ...
});

    route: function(route, name, callback) {
      if (!_.isRegExp(route)) route = this._routeToRegExp(route);
      if (_.isFunction(name)) {
        callback = name;
        name = '';
      }
      if (!callback) callback = this[name];
      var router = this;
      Backbone.history.route(route, function(fragment) {
        var args = router._extractParameters(route, fragment);
        if (router.execute(callback, args, name) !== false) {
          router.trigger.apply(router, ['route:' + name].concat(args));
          router.trigger('route', name, args);
          Backbone.history.trigger('route', router, name, args);
        }
      });
      return this;
    },

§

Execute a route handler with the provided parameters. This is an excellent place to do pre-route setup or post-route cleanup.
```
    execute: function(callback, args, name) {
      if (callback) callback.apply(this, args);
    },
```

Simple proxy to Backbone.history to save a fragment into the history.

    navigate: function(fragment, options) {
      Backbone.history.navigate(fragment, options);
      return this;
    },

Bind all defined routes to Backbone.history. We have to reverse the order of the routes here to support behavior where the most general routes can be defined at the bottom of the route map.

    _bindRoutes: function() {
      if (!this.routes) return;
      this.routes = _.result(this, 'routes');
      var route, routes = _.keys(this.routes);
      while ((route = routes.pop()) != null) {
        this.route(route, this.routes[route]);
      }
    },

Convert a route string into a regular expression, suitable for matching against the current location hash.

    _routeToRegExp: function(route) {
      route = route.replace(escapeRegExp, '\\$&')
      .replace(optionalParam, '(?:$1)?')
      .replace(namedParam, function(match, optional) {
        return optional ? match : '([^/?]+)';
      })
      .replace(splatParam, '([^?]*?)');
      return new RegExp('^' + route + '(?:\\?([\\s\\S]*))?$');
    },

§

Given a route, and a URL fragment that it matches, return the array of extracted decoded parameters. Empty or unmatched parameters will be treated as null to normalize cross-browser behavior.
```
    _extractParameters: function(route, fragment) {
      var params = route.exec(fragment).slice(1);
      return _.map(params, function(param, i) {
```

Don’t decode the search params.

        if (i === params.length - 1) return param || null;
        return param ? decodeURIComponent(param) : null;
      });
    }

  });

§

Backbone.History
§
§

Handles cross-browser history management, based on either pushState and real URLs, or onhashchange and URL fragments. If the browser supports neither (old IE, natch), falls back to polling.
```
  var History = Backbone.History = function() {
    this.handlers = [];
    this.checkUrl = this.checkUrl.bind(this);
```

Ensure that History can be used outside of the browser.

    if (typeof window !== 'undefined') {
      this.location = window.location;
      this.history = window.history;
    }
  };

§

Cached regex for stripping a leading hash/slash and trailing space.
```
  var routeStripper = /^[#\/]|\s+$/g;
```
§

Cached regex for stripping leading and trailing slashes.
```
  var rootStripper = /^\/+|\/+$/g;
```
§

Cached regex for stripping urls of hash.
```
  var pathStripper = /#.*$/;
```
§

Has the history handling already been started?
```
  History.started = false;
```
§

Set up all inheritable Backbone.History properties and methods.
```
  _.extend(History.prototype, Events, {
```
§

The default interval to poll for hash changes, if necessary, is twenty times a second.
```
    interval: 50,
```

Are we at the app root?

    atRoot: function() {
      var path = this.location.pathname.replace(/[^\/]$/, '$&/');
      return path === this.root && !this.getSearch();
    },

Does the pathname match the root?

    matchRoot: function() {
      var path = this.decodeFragment(this.location.pathname);
      var rootPath = path.slice(0, this.root.length - 1) + '/';
      return rootPath === this.root;
    },

§

Unicode characters in location.pathname are percent encoded so they’re decoded for comparison. %25 should not be decoded since it may be part of an encoded parameter.
```
    decodeFragment: function(fragment) {
      return decodeURI(fragment.replace(/%25/g, '%2525'));
    },
```

In IE6, the hash fragment and search params are incorrect if the fragment contains ?.

    getSearch: function() {
      var match = this.location.href.replace(/#.*/, '').match(/\?.+/);
      return match ? match[0] : '';
    },

Gets the true hash value. Cannot use location.hash directly due to bug in Firefox where location.hash will always be decoded.

    getHash: function(window) {
      var match = (window || this).location.href.match(/#(.*)$/);
      return match ? match[1] : '';
    },

Get the pathname and search params, without the root.

    getPath: function() {
      var path = this.decodeFragment(
        this.location.pathname + this.getSearch()
      ).slice(this.root.length - 1);
      return path.charAt(0) === '/' ? path.slice(1) : path;
    },

Get the cross-browser normalized URL fragment from the path or hash.

    getFragment: function(fragment) {
      if (fragment == null) {
        if (this._usePushState || !this._wantsHashChange) {
          fragment = this.getPath();
        } else {
          fragment = this.getHash();
        }
      }
      return fragment.replace(routeStripper, '');
    },

Start the hash change handling, returning true if the current URL matches an existing route, and false otherwise.

    start: function(options) {
      if (History.started) throw new Error('Backbone.history has already been started');
      History.started = true;

Figure out the initial configuration. Do we need an iframe? Is pushState desired … is it available?

      this.options          = _.extend({root: '/'}, this.options, options);
      this.root             = this.options.root;
      this._trailingSlash   = this.options.trailingSlash;
      this._wantsHashChange = this.options.hashChange !== false;
      this._hasHashChange   = 'onhashchange' in window && (document.documentMode === void 0 || document.documentMode > 7);
      this._useHashChange   = this._wantsHashChange && this._hasHashChange;
      this._wantsPushState  = !!this.options.pushState;
      this._hasPushState    = !!(this.history && this.history.pushState);
      this._usePushState    = this._wantsPushState && this._hasPushState;
      this.fragment         = this.getFragment();

Normalize root to always include a leading and trailing slash.

      this.root = ('/' + this.root + '/').replace(rootStripper, '/');

§

Transition from hashChange to pushState or vice versa if both are requested.
```
      if (this._wantsHashChange && this._wantsPushState) {
```

If we’ve started off with a route from a pushState-enabled browser, but we’re currently in a browser that doesn’t support it…

        if (!this._hasPushState && !this.atRoot()) {
          var rootPath = this.root.slice(0, -1) || '/';
          this.location.replace(rootPath + '#' + this.getPath());

§

Return immediately as browser will do redirect to new url
```
          return true;
```

Or if we’ve started out with a hash-based route, but we’re currently in a browser where it could be pushState-based instead…

        } else if (this._hasPushState && this.atRoot()) {
          this.navigate(this.getHash(), {replace: true});
        }

      }

Proxy an iframe to handle location events if the browser doesn’t support the hashchange event, HTML5 history, or the user wants hashChange but not pushState.

      if (!this._hasHashChange && this._wantsHashChange && !this._usePushState) {
        this.iframe = document.createElement('iframe');
        this.iframe.src = 'javascript:0';
        this.iframe.style.display = 'none';
        this.iframe.tabIndex = -1;
        var body = document.body;

Using appendChild will throw on IE < 9 if the document is not ready.

        var iWindow = body.insertBefore(this.iframe, body.firstChild).contentWindow;
        iWindow.document.open();
        iWindow.document.close();
        iWindow.location.hash = '#' + this.fragment;
      }

Add a cross-platform addEventListener shim for older browsers.

      var addEventListener = window.addEventListener || function(eventName, listener) {
        return attachEvent('on' + eventName, listener);
      };

Depending on whether we’re using pushState or hashes, and whether ‘onhashchange’ is supported, determine how we check the URL state.

      if (this._usePushState) {
        addEventListener('popstate', this.checkUrl, false);
      } else if (this._useHashChange && !this.iframe) {
        addEventListener('hashchange', this.checkUrl, false);
      } else if (this._wantsHashChange) {
        this._checkUrlInterval = setInterval(this.checkUrl, this.interval);
      }

      if (!this.options.silent) return this.loadUrl();
    },

§

Disable Backbone.history, perhaps temporarily. Not useful in a real app, but possibly useful for unit testing Routers.
```
    stop: function() {
```

Add a cross-platform removeEventListener shim for older browsers.

      var removeEventListener = window.removeEventListener || function(eventName, listener) {
        return detachEvent('on' + eventName, listener);
      };

Remove window listeners.

      if (this._usePushState) {
        removeEventListener('popstate', this.checkUrl, false);
      } else if (this._useHashChange && !this.iframe) {
        removeEventListener('hashchange', this.checkUrl, false);
      }

Clean up the iframe if necessary.

      if (this.iframe) {
        document.body.removeChild(this.iframe);
        this.iframe = null;
      }

Some environments will throw when clearing an undefined interval.

      if (this._checkUrlInterval) clearInterval(this._checkUrlInterval);
      History.started = false;
    },

Add a route to be tested when the fragment changes. Routes added later may override previous routes.

    route: function(route, callback) {
      this.handlers.unshift({route: route, callback: callback});
    },

§

Checks the current URL to see if it has changed, and if it has, calls loadUrl, normalizing across the hidden iframe.
```
    checkUrl: function(e) {
      var current = this.getFragment();
```

If the user pressed the back button, the iframe’s hash will have changed and we should use that for comparison.

      if (current === this.fragment && this.iframe) {
        current = this.getHash(this.iframe.contentWindow);
      }

      if (current === this.fragment) {
        if (!this.matchRoot()) return this.notfound();
        return false;
      }
      if (this.iframe) this.navigate(current);
      this.loadUrl();
    },

§

Attempt to load the current URL fragment. If a route succeeds with a match, returns true. If no defined routes matches the fragment, returns false.
```
    loadUrl: function(fragment) {
```

If the root doesn’t match, no routes can match either.

      if (!this.matchRoot()) return this.notfound();
      fragment = this.fragment = this.getFragment(fragment);
      return _.some(this.handlers, function(handler) {
        if (handler.route.test(fragment)) {
          handler.callback(fragment);
          return true;
        }
      }) || this.notfound();
    },

§

When no route could be matched, this method is called internally to trigger the 'notfound' event. It returns false so that it can be used in tail position.
```
    notfound: function() {
      this.trigger('notfound');
      return false;
    },
```
§

Save a fragment into the hash history, or replace the URL state if the ‘replace’ option is passed. You are responsible for properly URL-encoding the fragment in advance.

The options object can contain trigger: true if you wish to have the route callback be fired (not usually desirable), or replace: true, if you wish to modify the current URL without adding an entry to the history.
```
    navigate: function(fragment, options) {
      if (!History.started) return false;
      if (!options || options === true) options = {trigger: !!options};
```

Normalize the fragment.

      fragment = this.getFragment(fragment || '');

Strip trailing slash on the root unless _trailingSlash is true

      var rootPath = this.root;
      if (!this._trailingSlash && (fragment === '' || fragment.charAt(0) === '?')) {
        rootPath = rootPath.slice(0, -1) || '/';
      }
      var url = rootPath + fragment;

Strip the fragment of the query and hash for matching.

      fragment = fragment.replace(pathStripper, '');

Decode for matching.

      var decodedFragment = this.decodeFragment(fragment);

      if (this.fragment === decodedFragment) return;
      this.fragment = decodedFragment;

If pushState is available, we use it to set the fragment as a real URL.

      if (this._usePushState) {
        this.history[options.replace ? 'replaceState' : 'pushState']({}, document.title, url);

If hash changes haven’t been explicitly disabled, update the hash fragment to store history.

      } else if (this._wantsHashChange) {
        this._updateHash(this.location, fragment, options.replace);
        if (this.iframe && fragment !== this.getHash(this.iframe.contentWindow)) {
          var iWindow = this.iframe.contentWindow;

Opening and closing the iframe tricks IE7 and earlier to push a history entry on hash-tag change. When replace is true, we don’t want this.

          if (!options.replace) {
            iWindow.document.open();
            iWindow.document.close();
          }

          this._updateHash(iWindow.location, fragment, options.replace);
        }

If you’ve told us that you explicitly don’t want fallback hashchange- based history, then navigate becomes a page refresh.

      } else {
        return this.location.assign(url);
      }
      if (options.trigger) return this.loadUrl(fragment);
    },

Update the hash location, either replacing the current entry, or adding a new one to the browser history.

    _updateHash: function(location, fragment, replace) {
      if (replace) {
        var href = location.href.replace(/(javascript:|#).*$/, '');
        location.replace(href + '#' + fragment);
      } else {

Some browsers require that hash contains a leading #.

        location.hash = '#' + fragment;
      }
    }

  });

§

Create the default Backbone.history.
```
  Backbone.history = new History;
```
§

Helpers
§
§

Helper function to correctly set up the prototype chain for subclasses. Similar to goog.inherits, but uses a hash of prototype properties and class properties to be extended.
```
  var extend = function(protoProps, staticProps) {
    var parent = this;
    var child;
```

The constructor function for the new subclass is either defined by you (the “constructor” property in your extend definition), or defaulted by us to simply call the parent constructor.

    if (protoProps && _.has(protoProps, 'constructor')) {
      child = protoProps.constructor;
    } else {
      child = function(){ return parent.apply(this, arguments); };
    }

§

Add static properties to the constructor function, if supplied.
```
    _.extend(child, parent, staticProps);
```
§

Set the prototype chain to inherit from parent, without calling parent‘s constructor function and add the prototype properties.
```
    child.prototype = _.create(parent.prototype, protoProps);
    child.prototype.constructor = child;
```
§

Set a convenience property in case the parent’s prototype is needed later.
```
    child.__super__ = parent.prototype;

    return child;
  };
```

Set up inheritance for the model, collection, router, view and history.

  Model.extend = Collection.extend = Router.extend = View.extend = History.extend = extend;

Throw an error when a URL is needed, and none is supplied.

  var urlError = function() {
    throw new Error('A "url" property or function must be specified');
  };

Wrap an optional error callback with a fallback error event.

  var wrapError = function(model, options) {
    var error = options.error;
    options.error = function(resp) {
      if (error) error.call(options.context, model, resp, options);
      model.trigger('error', model, resp, options);
    };
  };

§

Provide useful information when things go wrong. This method is not meant to be used directly; it merely provides the necessary introspection for the external debugInfo function.
```
  Backbone._debug = function() {
    return {root: root, _: _};
  };

  return Backbone;
});
```

backbone.js

Initial Setup

Backbone.Events

Backbone.Model

Backbone.Collection

CollectionIterator

Backbone.View

Backbone.sync

Backbone.Router

Backbone.History

Helpers