AngularJS Services Part 3: Service

Posted by Ilija Eftimov on February 28, 2015

So, Service is basically the same as Factory, it just has one key difference. Service treats the function as a constructor, meaning, the service will call new on the function and return the resulting object as a service object. Oh, if you haven’t yet read about Factory, check out AngularJS Services Part 2: Factory.

Using Service

Declaring a Service is easy. Lets look at an example.

(function(){
  angular.module('app', [])
      .service("GreetingService", GreetingService);

  function GreetingService(){
    this.greet = function(){
      console.log("Hello");
    }
  }
})();

As you can see, we create a named function GreetingService and we create a service out of it. One slight difference is that we are treating the function as a constructor. This means that, like I said before, the Service will call new GreetingService() when instantiating the service singleton.

In case you are wondering, in the example, we can use the GreetingService prototype too.

(function(){
  angular.module('app', [])
      .service("GreetingService", GreetingService);

  function GreetingService(){ }

  GreetingService.prototype.greet = function(){
    console.log("Hello");
  }
})();

These examples are basically the same. One advantage of Prototypes is that it allows us to use prototype-based inheritance.

Service in the wild

I will use the same example from my last post about Factory.

(function(){
	angular.module("app", [])

	.service('TemperatureService', TemperatureService);

	function TemperatureService($http){
		this.API_URL = "http://api.openweathermap.org/data/2.5/weather?q=London,uk";

		this.get = function(){
			return $http.get(this.API_URL)
				.then(function(results){
					return results.data.main.temp;
				})
				.catch(function(err){
					console.log(err);
				});
		}
	}

})();

We use OpenWeatherMap’s API and we’ll fetch London’s (UK) current temperature using a service.

Angular’s built-in $http service is injected into our service so we can make HTTP calls to a external API.

The rest, is simple - we issue an HTTP call, use promises to handle successful or errored requests and we return back the promise itself.

What would injecting the service in a controller look like?

(function(){
	angular.module('app')
		.controller('TemperaturesController', TemperaturesController);

	function TemperaturesController(TemperatureService){

		TemperatureService.get().then(function(temperature){
		  // Use the temperature in whichever way you want...
			console.log(temperature);
		});

	}
})();

Easy! The service is injected in the controller, get() is called on the service and the returned promise from the service is handled. Then, when we get the temperature in the promise we can do whatever we want with it.

As you can see, services are pretty simple compared to Provier or Factory. Although simple, we can use protype-based inheritance. What does that mean?

Inheritance via prototype chains

If you are unfamiliar with this topic, I recommend you read MDN’s article on this topic before continuing with the example.

So, since Service allows us to use prototypes, let’s see how we can leverage this.

(function(){
	"use strict";

	angular.module("app", [])
			.service('GreetingService', GreetingService);

	function Greeting {
		this.hello = function(){
			console.log("Hello!");
		}
	}

	function GreetingService {
		Greeting.call(this);

		this.bye = function(){
			console.log('Bye!');
		}
	}

	GreetingService.prototype = Object.create(Greeting.prototype);


})();

You can see in the code above how we can use inheritance to our own benefit here. I am guessing you already know that GreetingService will have both, the hello() and bye() functions. Also, what’s cool about this approach is that we can make Greet a service as well. These are some nice ways you can use the Service service in AngularJS. Although it provides a lot of flexibility, can be a bit confusing.

In the next post we will look at the last two types of services: Constants and Values.


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