0_card.js

app.factory('card', function(payment) {
  card = function(attrs) {
    this.errors = {};

    return this;
  };

  card.prototype.isValid = function() {
    if (!payment.validateCardNumber(this.number)) {
      this.errors.number = 'invalid';
    }
  };

  return card;
});

0_card_spec.js

describe('card', function() {
  describe('#isValid', function() {
    var payment, card;

    beforeEach(function() {
      payment = {validateCardNumber: jasmine.createSpy()};

      module(function($provide) {
        $provide.value('payment', payment);
      });

      inject(function($injector) {
        card = $injector.get('card');
      });
    });

    it('sets error when number is invalid', function() {
      spyOn(payment, 'validateCardNumber').andReturn(false);

      var c = new card();

      c.isValid();

      expect(c.errors.number).toEqual('invalid');
    });

    it('does nothing when number is valid', function() {
      spyOn(payment, 'validateCardNumber').andReturn(true);

      var c = new card();

      c.isValid();

      expect(c.errors.number).toBeUndefined();
    });
  });
});

1_payment.js

app.factory('payment', function($window) {
  return $window.jQuery.payment;
});

2_controller.js

app.controller('cardController', function($scope, card) {
  $scope.card = new card();

  $scope.submit = function() {
    $scope.card.isValid();
  };
});

3_form.html.erb

<form ng-controller="cardController" ng-submit="submit()">
  <input type="text" ng-model="card.number">
  <span ng-show="card.errors.number"><%= t('card.errors.invalid') %></span>
  <button>Submit</button>
</form>