Question

What does the code below’s :layout do?

respond_with( @comment, :layout => !request.xhr? )

The code above is handling rendering of the different MIME type based on request data type (If my understanding is correct).
What is the :layout tag doing?

Answer 1

From the documentation for ActionController::Responder

respond_with also allow you to pass options that are forwarded to the
underlying render call. Those options are only applied success
scenarios. For instance, you can do the following in the create method
above:

So what the :layout => !request.xhr? it will pass this option down to the rendering. So if you had an HTML resource called as a regular request, it would use the standard layout, but an HTML request made as an XmlHttpRequest would not use a layout