From the example:

    xhr.onload = function() {
      if(xhr.response)
        resolver.resolve(xhr.response)
      resolve.reject(new DOMError("JSONError"))
    }
    xhr.onloadend = function() { resolver.reject(new DOMError("NetworkError")) }
  })

If "resolve" in line 4 is "resolver", I think I understand this.

BTW: The idiom of "reject does nothing if resolver.resolve succeeded" wasn't obvious to me until the third reading; it might be reader-friendly to add a comment about this.