AngularJS Dependency Injection

Today I am talking about Dependency Injection (DI) and how to work in AngularJs. Let's start with definition of Dependency Injection.

Definition of DI:
Dependency Injection is a software design pattern in which components are given their dependencies instead of hard coding them within the component. This replace a component from finding the dependency and makes dependencies configurable. This helps in making components reusable, maintainable and testable. 

How to DI work with AngularJs:
AngularJS provides a great Dependency Injection mechanism. It provides following list of core components which can be injected into each other as dependencies.

1. value
2. factory
3. service
4. provider
5. constant

Value
value is simple JavaScript object/var and it is used to pass values to controller during config phase.

//define a module
var mainApp = angular.module("mainApp", []);
//create a value object as "defaultValue" and pass it a value.
mainApp.value("defaultValue", 5);
//inject the value in the controller using its name "defaultValue"
mainApp.controller('MathsFunController', function($scope,  defaultValue) {
$scope.number = defaultValue;
});

Factory
factory is a function which is used to return value. It creates value on demand whenever a service or controller requires. It normally uses a factory function to calculate and return the value.

//define a module
var mainApp = angular.module("mainApp", []);
//create a factory "MultiplyService" which provides a method multiply to return multiplication of two numbers
mainApp.factory('MultiplyService', function() {
   var factory = {};   
   factory.multiply = function(a, b) {
      return a * b
   }
   return factory;
}); 
//inject the factory "MultiplyService" in a service to utilize the multiply method of factory.
mainApp.service('MathService', function(MultiplyService){
   this.square = function(a) {
      return MultiplyService.multiply(a,a);
   }
});

Service
service is a singleton JavaScript object containing a set of functions to perform certain tasks. Services are defined using service() functions and then injected into controllers.

//define a module
var mainApp = angular.module("mainApp", []);
//create a service which defines a method square to return square of a number.
mainApp.service('MathService', function(MultiplyService){
   this.square = function(a) {
      return MultiplyService.multiply(a,a); 
   }
});
//inject the service "MathService" into the controller
mainApp.controller('MathsFunController', function($scope, MathService, defaultValue) {
   $scope.number = defaultValue;
   $scope.result = MathService.square($scope.number);   
   $scope.square = function() {
      $scope.result = MathService.square($scope.number);
   }
});

Provider
provider is used by AngularJS internally to create services, factory etc. during config phase. following script can be used to create MultiplyService that I have created earlier. Provider is a special factory method with a method get() which is used to return the value/service/factory.

//define a module
var mainApp = angular.module("mainApp", []);
//create a service using provider which defines a method square to return square of a number.
mainApp.config(function($provide) {
   $provide.provider('MultiplyService', function() {
      this.$get = function() {
         var factory = {};           
         factory.multiply = function(a, b) {
            return a * b; 
         }
         return factory;
      };
   });
});

Constant
constants are used to pass values at config phase considering the fact that value can not be used to be passed during config phase.

mainApp.constant("configParam", "constant value");

Hope AngularJs lover clear some idea of  AngularJs Dependency Injection work with different component and how to inject it.

And I want also discuss regarding $inject Property but will cover on next blog. So please keep reading and give your suggestions and comments will be highly appreciated.