What are "decorators" and how are they used?

A good use case of $provide.decorator is when you need to do minor "tweak" on some third-party/upstream service, on which your module depends, while leaving the service intact (because you are not the owner/maintainer of the service). Here is a demonstration on plunkr.

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Decorators allow us to separate out cross-cutting concerns and allow services to preserve the single-responsibility-principle without worrying about "infrastructure" code.

Practical uses of decorators:

Caching: if we have a service which makes potentially expensive HTTP calls, we can wrap the service in a caching decorator which checks local storage before making the external call.

Debugging/Tracing: have a switch depending on your development/production configuration which decorates your services with debugging or tracing wrappers.

Throttling : wrap frequently triggered calls in a debouncing wrapper. Allows us to easily interact with rate-limited services, for example.

In all these cases, we limit the code in the service to its main responsibility.

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decorator can intercept service instance created by factory, service, value, provider, and gives the options to change some instance(service) that is otherwise not configurable / with options.

It can also provide mock up instances for testing purpose, for example $http.

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In simple word we can say that it’s like an extension method. For Ex. We have a class and it has two methods and at run time we want to add more method in it then we use Decorator.

We cannot use $provide.decorator with constants because we cannot change the constants they are heaving read only property.

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In short decorators can be described as follows :-

A decorator function intercepts the creation of a service, allowing it to override or modify the behavior of the service.

It uses the $provide service by angular and modify or replaces the implementation of another service

$provide.decorator('service to decorate',['$delegate', function($delegate) {
  // $delegate - The original service instance, 
  //             which can be replaced, monkey patched, 
  //             configured, decorated or delegated to. 
  //             ie here what is there in the 'service to decorate'

  //   This function will be invoked, 
  //   when the service needs to be provided 
  //   and should return the decorated service instance.
  return $delegate;
}]);

Example:

$provide.decorator('$log', ['$delegate', function($delegate) {
  // This will change implementation of log.war to log.error
  $delegate.warn = $delegate.error; 
  return $delegate;
}]);

Applications

In addition to @JBland answer.

Application wide locale settings :- You can find an example here

Changiging default behaviour of and existing implementation of a service by angular service :- You can find an eample here

Switching behavior of a function in different environments.

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