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AngularJS | Dependency Injection

AngularJS Dependency Injection


Hello folks! welcome back to a new edition of our tutorial on AngularJS. In this tutorial, we're going to be studying about AngularJS Dependency Injection.

Dependency Injection is an software design pattern in which components are given their dependencies instead of coding them within the component. It relieve a component from finding dependency and make dependencies configurable. This design pattern also helps to make components reusable, maintainable and testable.

AngularJS provides a supreme Dependency Injection mechanism. It gives the following core objects and components which can be injected into each other as dependencies.

  • Value
  • Factory
  • Service
  • Provider
  • Constant


Value

Value is a simple JavaScript object, needed to pass values to the controller during config phase (the config phase is when AngularJS bootstraps itself).

//define a module
var mainApp = angular.module("mainApp", []);

//create a value object as "defaultInput" and pass it a data.
mainApp.value("defaultInput", 5);
...

//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

It is a function which returns value. It create a value on demand each time a service or a controller requires it. It generally makes use of 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 that holds a set of functions to carry out certain tasks. Service is defined using the service() function 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

Provider is used internally by AngularJS for creating services, factory, and so on. during config phase. Following script below can be used to create MathService that we created earlier. Provider is a special factory method with get() method which is used in returning 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

Constants are used to pass values at config phase considering the fact that values can't be used during the config phase.

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

Example

The following example below illustrates the use of all the above mentioned directives -

Create a file testAngularJS.html.

<html>
   <head>
      <title>AngularJS Dependency Injection</title>
   </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 src = "https://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js">
      </script>
      
      <script>
         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", 5);
         
         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

Open the file testAngularJS.html in any web browser to see the result -

AngularJS Sample Application

Enter a number:  

Result: 25



Alright guys! This is where we are going to be rounding up for this tutorial. In our next tutorial, we are going to be studying about AngularJS Custom Directives.

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