Lately I've found the need to override the Object getter and setters to build "Dictionaries" with custom logic. For example, if you need to validate the property value before it is setted on the Object you need to use Proxy.
The following jsfiddle shows the intent ( this only works on Firefox Gecko 18 ):
The Proxy API is only available on ECMAScript v6 "Harmony" so if you want to use this with node you have to run node with the flag --harmonyproxies.
If you need to use this pattern but you don't have ECMAScript 6 support there is no polyfill, but you can use things like Ember.ObjectProxy or some Observable library to accomplish this.