As I understand it, when inside a factory I return an object that gets injected into a controller. When inside a service I am dealing with the object using this and not returning anything.

I was under the assumption that a service was always a singleton, and that a new factory object gets injected in every controller. However, as it turns out, a factory object is a singleton too?

Example code to demonstrate:

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

factories.factory('User', function () {
  return {
    first: 'John',
    last: 'Doe'
  };
});

app.controller('ACtrl', function($scope, User) {
  $scope.user = User;
});

app.controller('BCtrl', function($scope, User) {
  $scope.user = User;
});

When changing user.first in ACtrl it turns out that user.first in BCtrl is also changed, e.g. User is a singleton?

My assumption was that a new instance was injected in a controller with a factory?