Decorator

A decorator can modify or encapsulate other providers. A service decorator intercepts the creation of a service, allowing it to override or modify the behaviour of the service. The object returned by the decorator may be the original service, or a new service object which replaces or wraps and delegates to the original service.

There is one exception and that a constant cannot be decorated.
snippet
var app = angular.module('app', []);

app.value('movieTitle', 'The Matrix');

app.config(function ($provide) {
$provide.decorator('movieTitle', function ($delegate) {
return $delegate + ' - starring Keanu Reeves';
});
});
Related Tutorial
4. Value