What is the type friendly injection?

Question

In the AngularJS documentation, there is an explanation of the differences between a factory, a service, a value, a constant and a provider .

At the end, we have a comparison table: One of the rows is "type friendly injection". I could not understand what it is.

What does that mean? Additionally, what does it mean that, in order that a value will have this "type friendly injection", is at the cost of "eager initialization by using new operator directly"?

Answer 1

In AngularJS, you can inject dependencies in multiple ways:

• in the directive link function by position
• in the directive definition by name
• in the controller function by name
• in the factory function by name
• in the service function by type

Type friendly injection allows you to implicity invoke a constructor function by reference:

myApp.service('Pattern', ["Infinity", RegExp]); 

rather than by explicity using the new keyword:

myApp.factory('Pattern',   ["Infinity", function(Infinity)    {   return new RegExp(Infinity);   }   ]); 

OR

function goInfinity(Infinity)   {   return new RegExp(Infinity);   }   goInfinity.$inject = ["Infinity"]; myApp.factory('Pattern', goInfinity); 

The Service recipe produces a service just like the Value or Factory recipes, but it does so by invoking a constructor with the new operator. The constructor can take zero or more arguments, which represent dependencies needed by the instance of this type.

Eager initialization means that a constant recipe must return a constructor in order to use the aforementioned syntax:

function RegExpConstant()    {   return new RegExp(Infinity);   }   myApp.constant('Pattern', RegExpConstant) 

rather than returning a function, object, or literal value.

The nomenclature comes from Java:

A service is a well-known set of interfaces. A service provider is a specific implementation of a service. A factory is an object that returns an object reference to another object

References

• Dependency Injection in Angular 2

• The main goals of Angular 2 and how they will be achieved

• Vojta Jina: Dependency Injection - NG-Conf

• AngularJS: Developer Guide - Providers, Service Recipe

• AngularJS: The Bad Parts

• Dependency Injection: Syntax Sugar Over Function Composition

• ServiceFinder (JAX-WS RI)

Answer 2

My interpretation of "type friendly injection":

Factories and Providers inject whatever is returned by the factory function and $get function respectively, which could be of any type and could potentially change dynamically at runtime. Whereas Service, Constant and Value injections are of a fixed type that Angular is aware of as it is well defined during the definition of the recipe. Hence they are type friendly injections.