How do I read HTTP trailers?

How do I read HTTP trailers?

I am using $.ajax() to interrogate a server whose response incudes headers, response data and trailers. the response data are readily available via $.ajax().done(), as are the headers via jqXHR.getAllResponseHeaders() but I don't seem to find a way of reading the trailers.

Are they accessible?  Can anyone point me in the right direction?

Thank you
John