AngularJS Service vs Factory

AngularJS internal services

AngularJS internally provides many services that we can use in our application. $http is one example (Note: All angularjs internal services starts with $ sign). There are other useful services such as $route, $window, $location etc. These services can be used within any Controller by just declaring them as dependencies. For example:

module.controller('FooController', function($http){
	//...
});

module.controller('BarController', function($window){
	//...
});

AngularJS custom services

We can define our own custom services in angular js app and use them wherever required. There are several ways to declare angularjs service within application. Following are two simple ways:

var module = angular.module('myapp', []);

module.service('userService', function(){
	this.users = ['John', 'James', 'Jake'];
});

or we can use factory method

module.factory('userService', function(){
	
	var fac = {};
	
	fac.users = ['John', 'James', 'Jake']; 
	
	return fac;

});

Both of the ways of defining a service function/object are valid. We will shortly see the difference between factory() and service() method. For now just keep in mind that both these apis defines a singleton service object that can be used within any controller, filter, directive etc.

AngularJS Service vs Factory

AngularJS services as already seen earlier are singleton objects. These objects are application wide. Thus a service object once created can be used within any other services or controllers etc. We saw there are two ways (actually four, but for sake of simplicity lets focus on 2 ways that are widely used) of defining an angularjs service. Using module.factory and module.service.

module.service( 'serviceName', function );

module.factory( 'factoryName', function );

When declaring serviceName as an injectable argument you will be provided with an instance of the function. In other words new FunctionYouPassedToService(). This object instance becomes the service object that AngularJS registers and injects later to other services / controllers if required. When declaring factoryName as an injectable argument you will be provided with the value that is returned by invoking the function reference passed to module.factory. In below example we define MyService in two different ways. Note how in .service we create service methods using this.methodname. In .factory we created a factory object and assigned the methods to it. AngularJS .service

module.service('MyService', function() {
	this.method1 = function() {
			//..
		}

	this.method2 = function() {
			//..
		}
});

AngularJS .factory

module.factory('MyService', function() {
	
	var factory = {}; 

	factory.method1 = function() {
			//..
		}

	factory.method2 = function() {
			//..
		}

	return factory;
});

Example 1

Injecting dependencies in services Angularjs provides out of the box support for dependency management. In general the wikipedia definition of dependency injection is: Dependency injection is a software design pattern that allows the removal of hard-coded dependencies and makes it possible to change them, whether at run-time or compile-time. … We already saw in previous tutorials how to use angularjs dependency management and inject dependencies in controllers. We injected $scope object in our controller class. Dependency injection mainly reduces the tight coupling of code and create modular code that is more maintainable and testable. AngularJS services are the objects that can be injected in any other Angular construct (like controller, filter, directive etc). You can define a service which does certain tasks and inject it wherever you want. In that way you are sure your tested service code works without any glitch. Like it is possible to inject service object into other angular constructs, you can also inject other objects into service object. One service might be dependence on another. Let us consider an example where we use dependency injection between different services and controller. For this demo let us create a small calculator app that does two things: squares and cubes. We will create following entities in AngularJS: MathService – A simple custom angular service that has 4 methods: add, subtract, multiply and divide. We will only use multiply in our example. CalculatorService – A simple custom angular service that has 2 methods: square and cube. This service has dependency on MathService and it uses MathService.multiply method to do its work. CalculatorController – This is a simple controller that handler user interactions. For UI we have one textbox to take a number from user and two buttons; one to square another to multiply. Below is the code: The HTML

<div ng-app="app">
    <div ng-controller="CalculatorController">
        Enter a number:
        <input type="number" ng-model="number" />
        <button ng-click="doSquare()">X<sup>2</sup></button>
        <button ng-click="doCube()">X<sup>3</sup></button>
        
        <div>Answer: {{answer}}</div>
    </div>
</div>

The JavaScript

var app = angular.module('app', []);

app.service('MathService', function() {
    this.add = function(a, b) { return a + b };
    
    this.subtract = function(a, b) { return a - b };
    
    this.multiply = function(a, b) { return a * b };
    
    this.divide = function(a, b) { return a / b };
});

app.service('CalculatorService', function(MathService){
    
    this.square = function(a) { return MathService.multiply(a,a); };
    this.cube = function(a) { return MathService.multiply(a, MathService.multiply(a,a)); };

});

app.controller('CalculatorController', function($scope, CalculatorService) {

    $scope.doSquare = function() {
        $scope.answer = CalculatorService.square($scope.number);
    }

    $scope.doCube = function() {
        $scope.answer = CalculatorService.cube($scope.number);
    }
});