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AngularJS Dependency Injection
AngularJS provides a supreme Dependency Injection mechanism. It facilitates you to divide your application into multiple different types of components which can be injected into each other as dependencies.

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

AngularJS provides the following core components which can be injected into each other as dependencies.

  • value
  • factory
  • service
  • provider
  • constant
These objects and components can be injected into each other using AngularJS Dependency Injection.
Value
In AngularJS, value is a simple object. It can be a number, string or JavaScript object. It is used to pass values in factories, services or controllers during run and config phase.
//define a module
var mainApp = angular.module("mainApp", []);
 
//create a value object as "defaultInput" and pass it a data.
mainApp.value("defaultInput", 3);
.........
 
//inject the value in the controller using its name "defaultInput"
mainApp.controller('CalcController', function($scope, CalcService, defaultInput) {
   $scope.number = defaultInput;
   $scope.result = CalcService.square($scope.number);
   
   $scope.square = function() {
      $scope.result = CalcService.square($scope.number);
   }
});
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 "MathService" which provides a method multiply to return multiplication of two numbers
mainApp.factory('MathService', function() {
   var factory = {};
   
   factory.multiply = function(a, b) {
      return a * b
   }
   return factory;
}); 
 
//inject the factory "MathService" in a service to utilize the multiply method of factory.
mainApp.service('CalcService', function(MathService){
   this.square = function(a) {
      return MathService.multiply(a,a);
   }
});
service
service is a singleton javascript object containing a set of functions to perform certain tasks. Services are created by 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('CalcService', function(MathService){
   this.square = function(a) {
      return MathService.multiply(a,a); 
   }
});
 
//inject the service "CalcService" into the controller
mainApp.controller('CalcController', function($scope, CalcService, defaultInput) {
   $scope.number = defaultInput;
   $scope.result = CalcService.square($scope.number);
   
   $scope.square = function() {
      $scope.result = CalcService.square($scope.number);
   }
});
provider
AngularJS is provider internally to create services, factory etc. during config phase (phase during which AngularJS bootstraps itself). Below mention script can be used to create MathService that we've 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) {
   $provide.provider('MathService', function() {
      this.$get = function() {
         var factory = {};  
         
         factory.multiply = function(a, b) {
            return a * b; 
         }
         return factory;
      };
   });
});
constant
The Dependency Injection 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");
Dependency Injection Example
<!DOCTYPE html>
<html>
<head>
    <title>AngularJS Dependency Injection</title>
    <script type="text/javascript" src="js/angular.min.js"></script>
    <script type="text/javascript" src="js/angular-route.js"></script>
</head>
 
<body>
 
     <h2>AngularJS Sample Application</h2>
      <div ng-app = "mainApp" ng-controller = "CalcController">
         <p>Enter a number: <input type = "number" ng-model = "number" /></p>
         <button ng-click = "square()">X<sup>2</sup></button>
         <p>Result: {{result}}</p>
      </div>
      
      <script type="text/javascript">
         var mainApp = angular.module("mainApp", []);
         
         mainApp.config(function($provide) {
            $provide.provider('MathService', function() {
               this.$get = function() {
                  var factory = {};
                  
                  factory.multiply = function(a, b) {
                     return a * b;
                  }
                  return factory;
               };
            });
         });
 
         mainApp.value("defaultInput", 3);
         
         mainApp.factory('MathService', function() {
            var factory = {};
            
            factory.multiply = function(a, b) {
               return a * b;
            }
            return factory;
         });
         
         mainApp.service('CalcService', function(MathService){
            this.square = function(a) {
               return MathService.multiply(a,a);
            }
         });
         
         mainApp.controller('CalcController', function($scope, CalcService, defaultInput) {
            $scope.number = defaultInput;
            $scope.result = CalcService.square($scope.number);
 
            $scope.square = function() {
               $scope.result = CalcService.square($scope.number);
            }
         });
 
      </script>
      
   </body>
</html>
Output :