Property is a way to define value in a model or in a collection and they convert to an observable so everything that could be done on an observable could be applied to a property. Let's see an example.

Note: Property have advantages over an observable which are described here

Here is the implementation using an observable

var features = blocks.observable([]).extend('filter', function (value) {
  return value.type == 'feature';

Here is the equivalent when using Property

var App = blocks.Application();

var Project = App.Model({
  features: App.Property({

  }).extend('filter', function (value) {
    return value.type == 'feature';