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    <p>In javascript the only way to do some processing when getting or setting one property is using <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty" rel="nofollow noreferrer">Object.defineProperty</a>:</p> <pre><code>Object.defineProperty(person, "b", { get : function() { return person.firstName + ' ' + person.surname; }, set : function(newValue) { var names = newValue.split(' '); person.firsname = names[0]; person.surname = names[1]; }, enumerable : true, configurable : true }); </code></pre> <p>But this have some disadvantages:</p> <ul> <li>Isn't cross browser</li> <li>Doesn't have binding, in other words, if <code>firstname</code> or <code>surname</code> changes, the dependent property <code>fullname</code> isn't changed.</li> <li>Calling <code>person.name</code> when <code>person</code> is undefined, make an error to be throwed</li> <li>Isn't possible to trigger observers, no without additional code and aware of the depency hierachy: <code>firstname</code> depends from <code>fullname</code>, and it can be dependency of others properties arghhh!</li> </ul> <p>Due to this Ember has the concept of "property", called <a href="http://emberjs.com/guides/object-model/computed-properties/" rel="nofollow noreferrer">computed property</a>.</p> <p>It can be declared in 2 ways:</p> <pre><code>foo: Ember.computed(function({ ... }).property(dependent keys); </code></pre> <p>or when using (the default) <code>Ember.ENV.EXTEND_PROTOTYPES = true</code>:</p> <pre><code>foo: function() { ... }.property(dependent keys); </code></pre> <p>The <code>property(dependent keys)</code>, is needed because it tell to ember what is the properies that when changed, will make the property be updated.</p> <pre><code>fullname: function(key, value) { // setter if (value !== undefined) { var names = value.split(' '); this.set('firstname', names[0]); this.set('surname', names[1]); } // always return the complete result, so nexts calls to this.get('fullname') will return the cached value return this.get('firstname') + ' ' + this.get('surname'); }.property('firstname', 'surname') </code></pre> <p>Using this, you have the advantage of:</p> <ul> <li>when changing <code>firstname</code> or <code>surname</code> to a new value, <code>fullname</code> is changed. </li> <li>The <code>beforeObserves</code> are triggered before changing the value, and the <code>observes</code> are triggered after the value change.</li> <li>Any template referencing some property is updated</li> <li>More then one call to person.get('firstname'), will return a cached value, saving processing. You can disable this using <code>.property(..).volatile()</code></li> <li>Avoid null or undefined errors, when accessing null objects like: <code>controller.get('person.dog.name')</code> returns <code>undefined</code>, if person or dog is undefined.</li> </ul> <p>I hope it helps</p>
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    1. POWhat does .property() do? in function(){}.property()
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    1. COOk thank you! I'm beginning to understand this better. So how come in the example in the original question is the dependent key `'model.isCompleted'` and not just `'isCompleted'`?
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    2. COThe confusion is because the name is nearly the same, but refer to two different properties. `isCompleted` is dependent of `model.isCompleted`, and if we use `isCompleted: function() { ... }.property('isCompleted')`, will refer to the property itself.
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