[AngularJS] Services, Factories, and Providers -- Service vs Factory

Creating a Service:

Before actual create an angular service, first create a constructor in Javascript:

    //constructor function
    function DroidService() {
        this.name = '';
    }

    DroidService.prototype.speak = function () {
        return "Hi I am " + this.name;
    };

Then you we want to use this constutor function, you need to new an instance:

    var droid = new DroidService();
    droid.name = 'r2-d2';
    console.log(droid.speak());

What need to understand here is that, you when do new opration, under the hook, Javascript does tow thing for you:

    function DroidService() {
        // var this = {}    
        this.name = '';
       // return this;
    }

First is var a new this object, then return this object.

Angular service is a constructor function.

    //constructor function
    function DroidService() {
        this.name = '';
    }

    DroidService.prototype.speak = function () {
        return "Hi I am " + this.name;
    };

    angular.module('app', [])
        .service('droid', DroidService)
        .controller('DroidController', DroidController);

    function DroidController(droid) {
        var droidCtrl = this;
        droid.name = 'r2-d2';
        droidCtrl.message = droid.speak();

    }

When you create a service in angular, it helps to 'new' the constructor (service), then inject this instance whenever you want to use it.

Creating a Factory:

You can create an function which return an object:

    function droidFactory() {
        function speakingPrivately() {
            return "Hi I am " + this.name;
        }
        
        return {
            name: '',
            speak: speakingPrivately
        };
    }

This is called reaveling modular partten, because you can choose which function or props to be public or private by include those into return object. 

Then you just need to invoke the function, you can get the object.

    var droid = droidFactory();
    droid.name = 'c3-po';
    console.log(droid.speak());

Angular Factory is a function which return an object. (No constructor fucntion, no new opreation):

    //revealing module pattern
    function droidFactory() {
        function speakingPrivately() {
            return "Hi I am " + this.name;
        }

        return {
            name: '',
            speak: speakingPrivately
        };
    }


    angular.module('app', [])
        .factory('droid', droidFactory)
        .controller('DroidController', DroidController);

    function DroidController(droid) {
        var droidCtrl = this;
        droid.name = 'c3-po';
        droidCtrl.message = droid.speak();
    }

When you create a factory, Angular will help to invoke the function so when you inject into controller, you will get the object back.