Dependency Injection

Dependency injection can be a difficult concept to fully grasp. However, once you understand the basics, the AngularJS implementation becomes quite clear. Dependency injection is a well-known design pattern in many server-side languages but has not been used much in JavaScript before now.

The idea of AngularJS dependency injection is to define and dynamically inject a dependency object into another object, making available all the functionality provided by the dependency object. AngularJS provides dependency injection through the use of providers and an injector service.

A provider is essentially a definition of how to create an instance of an object with all the necessary functionality. Providers should be defined as part of an AngularJS module. A module registers the provider with the injector server. Only one instance of a provider’s object is ever created in the AngularJS application.

The injector service is responsible for keeping track of instances of provider objects. An injector service instance is created for each module that registers a provider. When a dependency request is made for a provider object, the injector service first checks whether an instance already exists in the injector cache. If so, that instance is used. If no instance is found in the cache, then a new instance is created using the provider definition, stored in the cache, and then returned.