Demystification of Angular's services.

service-demystification-test.js

var expect = chai.expect;

describe('services', function(){
  var goat, monkey, monkey2, Donkey, tiger1, tiger2, lion;

  beforeEach(function(){
    //load the module
    module('app');

    //configure providers
    angular.module('app').config(function(TigerProvider){
      TigerProvider.setScariness('high');
    });

    inject(function($injector) {
      monkey = $injector.get('monkey');
      monkey2 = $injector.get('monkey');
      goat = $injector.get('goat');
      Donkey = $injector.get('Donkey');
      tiger1 = $injector.get('Tiger');
      tiger2 = $injector.get('Tiger');
      lion = $injector.get('Lion');
    });
  });
  
  describe('#service', function(){
    it('creates an instance of the constructor function', function(){
      expect(monkey.name).to.equal('joe');
    });

    it('returns the same instance every time', function(){
      expect(monkey.rand).to.equal(monkey2.rand);
    });
  });

  describe('#factory', function(){
    it('returns the result of invoking the function', function(){
      expect(goat.name).to.equal('goat singleton');
    });

    describe('having that function return a constructor function', function(){
      it('lets you create your own instances', function(){
        var donkey = new Donkey('sam');
        expect(donkey.name).to.equal('sam');
      });
    });
  });

  describe('#provider', function(){
    describe('providers that are functions', function(){
      it('creates a new instance of your function, and calls $get() on it, returning whatever $get returns', function(){
        expect(tiger1.name).to.equal('tiger singleton');
      });

      it('returns the same instance every time', function(){
        expect(tiger1.rand).to.equal(tiger2.rand);
      });

      it('calls $get only once', function(){
        expect(tiger1.instanceRand).to.equal(tiger2.instanceRand);
      });
    });

    describe('providers that are objects', function(){
      it("invokes the object's $get method to get the instance", function(){
        expect(lion.name).to.equal('lion singleton');
      });
    });

    describe('configuring a provider', function(){
      it('lets you set things up before any injection takes place', function(){
        expect(tiger1.scariness).to.equal('high');
      });
    });
  });

});

service-demystification.js

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

//define a "class", will be instanciated
app.service('monkey', function(){
  this.name = "joe";
  this.rand = Math.random();
});

//can return an object (singleton)
app.factory('goat', function(){
  return {
    name: "goat singleton"
  };
});

//can return a constructor function to use yourself
app.factory('Donkey', function(){
  return function Donkey(name){
    this.name = name;
  };
});

//.factory is just a shortcut for $provide.provider(name, {$get: function()...})
//provider function gets new()'d then returns result of $get()
// new Tiger().$get()
app.provider('Tiger', function(){
  var scariness = 'low';
  var rand = Math.random();
  
  this.setScariness = function(s){
    scariness = s;
  };

  this.$get = function(){
    return {
      name: "tiger singleton",
      rand: rand,
      instanceRand: Math.random(),
      scariness: scariness
    };
  };
});

app.provider('Lion', {
  $get: function(){
    return {
      name: "lion singleton"
    };
  }
});

here's my karma.conf.js for running this:

files = [
  MOCHA,
  MOCHA_ADAPTER,
  'angular.js',
  'angular-mocks.js',
  'node_modules/chai/chai.js',
  'services/services.js',
  'services/services-test.js'
]