wherecollection.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);
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