Is there such a thing as an array/object hybrid?

I'm using an npm module called horseman which is basically PhantomJS for Node. When I console log the contents of a horseman object it has all sorts of data about the current page. Included in all this data is a list of all the resources of a page and their http status codes like this:

responses: [ 'http://tylertrotter.com/': 301,
  'http://www.tylertrotter.com/': 200,
  'http://www.tylertrotter.com/css/main.css': 200, 
  ...
]

It looks like an array/object hybrid and I've never encountered something like this before. Array.isArray(responses) yields true but responses.length tus up with 0.

What is this thing?