custom jasmine matcher for index of 'selected' image

1-no-custom-matcher.js

describe("when no image is selected and previous is called", function(){
  beforeEach(function(){
    this.images.previous();
  });

  it("should select the last image", function(){
    var lastImage = imageList.length-1;
    var selectedImage = this.images.indexOf(this.images.selectedImage);
    expect(selectedImage).toBe(lastImage);
  });
});

2-with-custom-matcher.js

describe("when no image is selected and previous is called", function(){
  beforeEach(function(){
    this.images.previous();
  });

  it("should select the last image", function(){
    expect(this.images).toHaveSelectedImageAt(imageList.length-1);
  });
});

3-toHaveSelectedImageAt.js

beforeEach(function() {
  this.addMatchers({

    toHaveSelectedImageAt: function(expectedIndex) {
      var images = this.actual;
      var actualIndex = images.indexOf(images.selectedImage);
      this.actual = actualIndex;
      this.message = function(){
        return "Expected selected image index of " + actualIndex + " to be " + expectedIndex;
      };
      return (actualIndex == expectedIndex);;
    }

  });
});