Collections are a convenient way to group models together. An example usage of them is when you want to show a list of elements in your page. You can create a simple collection using Backbone.Collection (I am using the models I defined on Introduction to Backbone.js):

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// Create some people
var juan = new Person({'weight': 80});
var carlos = new Person({'weight': 70});
var alicia = new Person({'weight': 75});

var persons = new Backbone.Collection([
  juan,
  carlos,
  alicia
]);

And there are some cool things you can do with them:

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// Print weight of all persons
persons.each(function(person) {
  console.log(person.get('weight'));
});

// personWith80 = juan
var personWith80 = persons.find(function(person) {
  return person.get('weight') === 80;
});

// Returns an array of models with all the models that matched the criteria
var thinPeople = persons.filter(function(person) {
  return person.get('weight') < 80;
});

// Print how many models are there currently in persons
console.log(persons.size());

There a bunch of other methods that can be used with collections. You can see the whole list in backbone documentation.

We can also extend the default Collection to create our custom collections, for example:

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var Persons = Backbone.Collection.extend({
  // We can specify a model so the collection can instantiate new objects for us
  'model': Person,

  // By adding a comparator we make our collection an ordered list. Every time
  // we add a new element it will be added in the correct order
  'comparator': function(person) {
    return person.get('weight');
  }
});

// Persons creates the models for us
var friends = new Persons([{'weight': 68}, {'weight': 60}, {'weight': 65}]);

var fatPerson = new Person({'weight': 130});
// Add some more models to our collection
friends.add([fatPerson, {'weight': 55}]);

// Prints 50, 60, 65, 68, 130 because of the comparator
friends.each(function(person) {
  console.log(person.get('weight'));
});

If you know the ids or cids of your models you can use it to remove elements from your collection:

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// Create a new collection
var thinPerson = new Person({'weight': 50});
var fatPerson = new Person({'weight': 130});
var fid = fatPerson.cid;
var friends = new Persons([fatPerson, thinPerson]);

// remove a model based on the cid (client id)
friends.remove([fid]);

// Prints 1
console.log(friends.length);

Events

As with models we can add listeners so we can have our view updated:

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var Persons = Backbone.Collection.extend({
    // We can specify a model so the collection can instantiate new objects for us
    'model': Person,

    // Add event listeners on initialization
    'initialize': function() {
      this.on('add', function(person) {
        console.log('This person\'s weight is ' + person.get('weight'));
      });

      this.on('remove', function(person) {
        console.log('Person with cid ' + person.cid + ' is no longer part of the collection');
      });

      this.on('change', function(person) {
        console.log('Weight changed to ' + person.get('weight'));
      });
    }
  });

  var friends = new Persons([{'weight': 68}]);

  var adrian = new Person({'weight': 100});

  friends.add(adrian); // This person's weight is 100
  adrian.set({'weight': 150}); // Weight changed to 150
  friends.remove(adrian); // Person with cid c5 is no longer part of the collection
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